Solar Panels for Detached Garage: Separate Meter Design Guide

A detached garage is often the best solar site on a residential property. The roof is simpler, the orientation is cleaner, and the structure is free of the plumbing vents, skylights, and dormers that fragment the main house roof. For homeowners with a tree-shaded primary residence, the garage is sometimes the only viable surface. The catch is everything that sits between the panels and the meter — the feeder, the trench, the subpanel, and the question of whether to use the existing utility account or add a second one.

This guide covers how to design solar for a detached garage or outbuilding from the meter back, including the three interconnection options, NEC rules that apply to feeders between buildings, voltage drop math for long runs, cost differences, and the permitting steps that change once a second structure is involved.

What this guide covers:

• The three ways to wire solar on a detached structure
• When a separate utility meter is worth it
• NEC 225, 250.32, 300.5, and 705.12 applied to detached buildings
• Feeder sizing and voltage drop tables for runs between 50 and 300 feet
• Cost comparison across the three interconnection strategies
• Permitting and inspection differences from a standard rooftop install
• A worked example for a 6 kW detached garage with EV charging

Why a Detached Structure Is Often the Best Solar Site

The case for a detached garage starts with the roof itself. A simple gable on a single-story garage, oriented south or east-west, gives a long unbroken plane with few obstructions. Compare that to a typical house roof with multiple hips, valleys, dormers, attic vents, plumbing stacks, and a chimney, and the garage usually offers more contiguous panel area per square foot of roof.

Several factors push homeowners toward the garage:

• Shading on the main house. Trees that mature over a 30-year-old home often cast morning or afternoon shadow on the primary roof. The garage, set apart from the lot center, frequently sits in better sun.
• Roof orientation. Many detached garages were built later than the house and oriented to the driveway rather than the lot, which sometimes happens to align with south, southeast, or east-west.
• Roof age and condition. A tile or slate house roof with 5 to 10 years of life left is a poor host for a 25-year solar system. A garage roof with simpler shingles is easier and cheaper to re-roof before installation.
• Clean roof plane. Garage roofs typically have no plumbing vents, no skylights, and few or no penetrations to design around.
• Future loads at the garage. EV chargers, workshop tools, heat pumps for ADU conversions, and battery storage all live closer to the garage than the main panel.

The trade-off is the wiring run. Every detached garage solar project pays a tax in trenching, conduit, conductor, and a subpanel that a roof-mounted system on the house avoids. The rest of this guide is about how to keep that tax small and how to choose the right interconnection method.

Three Ways to Wire Solar on a Detached Garage

There are three viable interconnection paths. Each has a use case, a cost profile, and a different relationship to the utility meter.

Option 1: Backfeed the Main House Service Panel via Feeder

The array, inverter, and combiner sit at the garage. A subpanel or junction at the garage joins the inverter output to the existing feeder that ran from the house to power the garage in the first place. Power flows back through that feeder to the main service panel, where solar production offsets house consumption and any excess exports through the single existing utility meter.

This is the default choice for 80 to 90 percent of residential detached garage projects. Net metering works exactly as it would for a roof-mounted system on the house. Permitting is standard. The only complication is whether the existing feeder is large enough to carry the new combined load and source.

Option 2: Dedicated Subpanel with Battery, Limited or No Backfeed

The garage gets a hybrid inverter and a battery. Critical loads at the garage — EV charger, workshop circuits, freezer, ADU lighting — run from the garage subpanel. Solar serves those loads first. Excess charges the battery. Any remaining production either backfeeds the main service through a small export-limited feeder or stays on-site if the utility prohibits export.

This option fits two situations. The first is off-grid or grid-edge sites where the utility connection to the garage is weak or expensive to upgrade. The second is jurisdictions with poor or zero export tariffs, where storing solar locally beats sending it back. Cable and trench requirements drop significantly because the feeder no longer carries continuous power flow.

Option 3: Separate Utility Meter and Account

The garage has its own service drop or lateral from the utility, its own meter base, and its own utility account. The solar array interconnects only to the garage account. Net metering applies only to that account.

This is the right choice in a small set of cases — accessory dwelling units (ADUs) with separate billing, farm buildings under agricultural rates, workshops billed to a business entity, or properties where the utility requires a separate service for any structure with a dwelling unit. The cost premium is significant because the homeowner pays for a second service drop, a second meter base, sometimes a second transformer, plus monthly fixed charges on two accounts.

Option 1 in Detail: Backfeed Through the Existing Main Service Panel

This is the path of least resistance, and the design problem reduces to four questions.

Is the Existing Feeder Big Enough?

Most older detached garages were wired with a 50 or 60 amp feeder pulling from a 15 or 20 amp double-pole breaker at the house panel. That feeder was sized for garage loads, not for solar backfeed plus loads. When the array goes in, the feeder carries:

1. The original garage loads going to the garage.
2. The solar export coming back to the house.

Continuous current rating governs. If the array exports more than the feeder can carry on a sunny day, the feeder overheats. The fix is either a feeder upgrade or an inverter export limit programmed to the feeder rating minus a safety margin.

For a 6 kW array at 240 V, the export current is roughly 25 amps continuous, plus the 125 percent NEC factor for continuous-duty calculations gives 31.25 amps of breaker rating. A 60 amp feeder handles this comfortably. A 100 amp feeder is the right call for 8 to 15 kW arrays.

Does the Main Panel Have Room for the Backfeed Breaker?

NEC 705.12(B)(3) sets the rules for backfeeding a busbar. The most common method is the 120 percent rule: the sum of the main breaker plus the solar backfeed breaker cannot exceed 120 percent of the busbar rating. For a 200 amp busbar with a 200 amp main, the maximum solar backfeed breaker is 40 amps, supporting roughly 7.6 kW of inverter output at 240 V.

Three options solve a 120 percent shortfall:

• Main breaker derate. Replace a 200 amp main with a 175 amp main if load calculations allow. This frees up backfeed capacity at the bottom of the busbar.
• Supply-side tap. Wire the solar conductors between the meter and the main breaker, bypassing the busbar entirely. This avoids the 120 percent rule but requires utility approval and a supply-side disconnect.
• Line-side tap. Same idea on the line side of the meter, which most utilities prohibit on residential services.

For most detached garage arrays in the 5 to 8 kW range, the 120 percent rule with a 40 to 60 amp backfeed breaker works without modification.

How Long Is the Run, and What Conductor Size Do You Need?

Voltage drop drives feeder sizing for any run over about 75 feet. NEC informational notes recommend 3 percent voltage drop on feeders and 2 percent on branch circuits, for 5 percent total. This is a recommendation, not a code requirement, but most jurisdictions and most solar designers honor it.

The feeder between house and garage carries continuous solar current in both directions over its lifetime, so it should be sized to the larger of the two: original garage load or solar backfeed. The table below shows minimum copper conductor sizes for a 60 amp feeder at 240 V single-phase under 3 percent voltage drop. Aluminum conductors require one or two sizes larger.

What Wiring Method Belongs in the Trench?

Three options dominate residential trenches:

• PVC conduit with THWN-2 conductors. Most common. 18 inch minimum cover under NEC 300.5. Allows pulling new conductors in the future without re-trenching.
• Direct-buried UF cable. 24 inch minimum cover. Cheaper materials. No future replacement without re-digging.
• Rigid metal conduit (RMC). 6 inch minimum cover. Uncommon for residential due to cost.

PVC with THWN-2 is the right answer in nearly every case because it lets you pull a larger conductor later if loads grow. The 6 inch difference in trench depth versus UF cable is rarely worth giving up future flexibility.

Option 2 in Detail: Subpanel with Battery, Limited Backfeed

This design puts a hybrid inverter at the garage and uses the battery as a buffer. The garage becomes a partial microgrid that drops to backfeed only when production exceeds local consumption and battery state of charge.

When This Design Makes Sense

• The existing feeder to the garage cannot be upgraded affordably (rocky terrain, paved driveway crossing, long run).
• The utility offers no net metering or has a low-value export tariff (zero export, avoided cost only, or feed-in-tariffs below retail).
• The homeowner wants resilience for EV charging or critical garage loads during grid outages.
• The garage hosts loads with a high coincidence factor with solar production, like daytime EV charging or workshop use.

Equipment Stack

A typical Option 2 build at the garage looks like this:

• Hybrid inverter rated for the array plus a battery channel (Sol-Ark, Tesla, Enphase, SolarEdge Energy Hub, FranklinWH).
• Battery sized to 10 to 30 kWh based on critical loads and outage duration target.
• Garage subpanel with critical loads downstream of the inverter.
• Existing feeder back to the main house, now carrying export only when battery is full and garage loads are low.
• Utility-required disconnect at the garage and at the main service.

A 6 kW array with a 13.5 kWh battery and a single hybrid inverter at the garage covers roughly 80 percent of an average household’s daytime consumption plus an overnight EV charge, when designed against real load data. For 15-minute interval data and load shaping, see the deep-dive on solar system sizing with 15-minute interval load data.

Cable Run Implications

Because the feeder carries less continuous current under Option 2, conductor size sometimes drops. A 50 amp feeder may be sufficient where Option 1 would have required 100 amps. Trench cost falls accordingly. This is the hidden value of the battery-at-the-garage approach in long-run scenarios.

Option 3 in Detail: Separate Utility Meter and Account

The separate meter approach is the rarest of the three but the right answer in specific cases.

When a Separate Meter Is Required or Preferred

• ADU rental. State or municipal code may require separate metering for any rented dwelling unit. Several California jurisdictions require this for new ADUs.
• Agricultural rate eligibility. A barn or farm building qualifying for agricultural electricity rates needs its own service to keep that rate.
• Business entity billing. A workshop operating as a business benefits from a separate utility account for tax and accounting clarity.
• Master-metered properties going solar. Multi-unit properties sometimes split into individual meters to qualify for residential net metering on each unit.

What the Utility Charges to Add a Meter

The separate meter cost stack is significant:

• Service drop or lateral extension. 800 to 4,000 dollars depending on overhead vs underground and whether a transformer upgrade is needed.
• Meter base and service entrance. 400 to 800 dollars in materials plus 600 to 1,200 dollars in labor.
• Trenching for the new lateral. 5 to 15 dollars per foot.
• Engineering and utility application fees. 200 to 1,000 dollars depending on the utility.
• Monthly fixed charges. 8 to 25 dollars per month per meter, often offsetting most of the solar production credit on a small ADU array.

Net Metering Treatment

This is where Option 3 gets brutal. In most jurisdictions, net metering applies per account. A separate ADU meter with solar credits the ADU only. Excess production at the ADU meter that exceeds ADU consumption banks at the avoided cost rate or expires at the end of the true-up year, depending on the tariff. The main house consumption gets no benefit unless the utility offers aggregate net metering across both meters on the same parcel — a feature available in California (NEM-A), Massachusetts (Class II), Maryland, and a handful of other states. The DSIRE database of net metering rules tracks aggregate and virtual net metering availability by state.

For an exhaustive country-by-country view of export tariff rules, see the global breakdown of grid export limitation rules by country.

Comparing the Three Approaches Side by Side

For most homeowners, Option 1 wins on cost, simplicity, and net metering treatment. Option 2 wins when the utility is hostile to export or when backup is part of the goal. Option 3 wins only when a separate utility account is required for legal, billing, or rate reasons.

NEC Rules That Govern Solar on a Detached Building

Five NEC articles do most of the work on a detached garage solar project. Anyone designing the system, pulling the permit, or inspecting the install should know what each one covers.

NEC 225: Outside Branch Circuits and Feeders

Article 225 governs conductors that run between buildings on the same property. Three rules matter for solar.

225.30 Number of Supplies. A building can have only one feeder or branch circuit supplying it, with limited exceptions. This means a detached garage cannot pull from both the main house panel and a separate utility service unless the design qualifies for an exception (battery backup is one such exception).

225.31 Disconnecting Means. A disconnect must be installed at the garage to disconnect all ungrounded conductors from the source. For solar, this is the AC disconnect at the garage end of the feeder, usually a fused disconnect or a backfed breaker in the garage subpanel.

225.32 Location. The disconnect must be installed at a readily accessible location either outside the garage or inside the garage nearest the point of entrance.

NEC 250.32: Grounding and Bonding at Separate Buildings

When a separate building is supplied by a feeder from another building, two grounding rules apply.

250.32(A). A grounding electrode (ground rod, plate, or Ufer) must be installed at the separate building. The grounding electrode is connected to the equipment grounding conductor in the feeder.

250.32(B). The neutral and the equipment grounding conductor must be kept separate at the separate building. The neutral bonds to ground only at the main service. At the garage subpanel, the neutral bus and the ground bus are isolated and only connect through the equipment grounding conductor in the feeder.

This is the most commonly violated rule on older detached garage installations. Pre-2008 NEC allowed bonding neutral to ground at the second building if no continuous metallic path existed between buildings. Current NEC removed this exception. Any garage rewire as part of a solar project must correct a bonded neutral at the subpanel.

NEC 300.5: Underground Installations

This article sets the minimum cover depths for underground wiring methods. The full code text is published by the NFPA as NFPA 70. The relevant lines for residential solar:

Add 6 inches for paths under driveways or paved areas. Conduit must be rated sunlight-resistant for portions exposed above grade and burial-rated for portions below grade.

NEC 705.12: Source Connections to a Premises Wiring System

Article 705 governs solar interconnection to the premises. The 120 percent rule under 705.12(B)(3)(2) is the most common path:

The sum of 125 percent of the inverter output current plus the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampere rating of the busbar.

Translated: a 200 amp busbar protected by a 200 amp main breaker accepts up to 40 amps of solar backfeed breaker, which corresponds to roughly 32 amps of continuous inverter output, or about 7.6 kW at 240 V.

For larger arrays, options include sum rule (705.12(B)(3)(3)), supply-side tap (705.12(A)), or main breaker derate.

NEC 690.13 and 690.15: PV System Disconnects

A readily accessible PV system disconnect is required. For roof-mounted arrays at the garage, the disconnect is typically at the subpanel or as a separate fused disconnect on the garage exterior wall.

For NEC 2026 changes affecting residential PV, see the NEC 2026 solar changes overview.

Feeder Sizing and Voltage Drop for the Run Between Buildings

Every detached garage solar project lives or dies by the feeder calculation. Get this right and the rest of the design follows.

The Voltage Drop Formula

For single-phase 240 V systems:

VD = (2 × K × I × D) / CM

Where:

• VD = voltage drop in volts
• K = 12.9 for copper, 21.2 for aluminum
• I = current in amps
• D = one-way distance in feet
• CM = circular mils of the conductor

Target VD ≤ 7.2 V for 3 percent on a 240 V circuit.

Feeder Size Tables for Common Distances

The table below shows minimum copper conductor size for a 60 amp continuous feeder at 240 V single-phase, holding voltage drop to 3 percent or less. These are sized for solar export current matching the feeder rating.

For a 100 amp feeder, step up two sizes from each value. For 200 amp, step up four sizes. For deeper guidance on conductor sizing, including ampacity and temperature corrections, see the dedicated guide to solar cable sizing calculation.

Voltage Drop Worked Example

A 7 kW array at the garage feeds a 60 amp subpanel back to the house, 175 feet one-way through PVC conduit using copper THWN-2.

Continuous solar export at 240 V: 7,000 / 240 = 29.2 A.

For 3 percent VD:

CM = (2 × 12.9 × 29.2 × 175) / 7.2
CM = 18,329 circular mils

The next standard size up is 4 AWG copper at 41,740 CM. But 4 AWG copper has a 70 amp ampacity at 75 °C, which is short of the 60 amp feeder breaker after derates. Step to 2 AWG copper (66,360 CM, 95 amp ampacity at 75 °C). Result: 2 AWG copper feeder, 60 amp breaker, voltage drop 0.83 percent.

A glossary refresher on wire gauge AWG covers the basics if these conversions are new.

Conduit Fill and Burial

For 2 AWG THWN-2, three current-carrying conductors plus an equipment grounding conductor fit comfortably in 1.25 inch PVC conduit per NEC Chapter 9, Table 1. See conduit fill for the full method. Trench depth: 18 inches PVC, plus 6 inches under any driveway crossing.

Sizing the Solar Array for a Garage Roof

Garage roofs come in three rough sizes. Knowing the array each one supports speeds early sizing decisions.

Single-Car Garage (250 to 320 sq ft roof)

A typical single-car detached garage measures about 12 by 22 feet, yielding 264 square feet of roof per pitched plane on a gable design. After fire setbacks (3 feet at the ridge under most state codes, 18 inches at the eave) the usable area drops to roughly 165 square feet. At 18 square feet per 440 W panel, this fits 9 panels, or about 4 kWp.

Two-Car Garage (450 to 550 sq ft per plane)

The two-car detached garage is the residential workhorse, typically 22 by 24 feet. A south-facing rear-pitched plane on a gable yields about 528 square feet. After setbacks: 380 square feet usable, or 21 panels at 440 W. That gives 9.2 kWp on a single plane. East-west designs split across two planes, giving 7 to 8 kWp total at slightly lower yield.

Three-Car or Workshop Garage (700 to 900 sq ft per plane)

A three-car or detached workshop with a 30 by 30 footprint yields about 900 square feet per plane on a gable. After setbacks: 700 square feet, supporting 38 panels or roughly 16.7 kWp. This is enough capacity to cover an above-average household plus EV charging plus battery charging on most days.

Pitch and Orientation Adjustments

The numbers above assume an unshaded south-facing 30 to 35 degree pitch in the Northern Hemisphere. Apply the following corrections:

• Due east or due west: 80 percent of due-south yield
• Northeast or northwest: 60 to 70 percent
• Pitch under 5 degrees (flat roof): 92 to 95 percent of optimal pitch yield, requires ballast or attachment design
• Pitch over 50 degrees: 92 to 95 percent of optimal yield

For a structured residential design walkthrough, the dedicated guide on how to design a residential solar system covers the full 12-step process including layout, stringing, and inverter sizing. Layout decisions are easier with solar design software that auto-detects roof planes from satellite imagery and applies setback rules.

Cost Breakdown: Where the Money Goes

A detached garage solar system costs more than a roof-mounted system on the house, and the premium falls into four cost buckets.

Trenching

The single largest cost driver. Pricing varies by soil and access:

A 150 foot trench across an open lawn with one driveway crossing typically lands at 1,200 to 2,500 dollars all in.

Feeder Conductor Upgrade

A typical existing 6 AWG aluminum feeder for a 50 amp subpanel costs about 2 dollars per foot in materials. Upgrading to 1/0 AWG aluminum for a 100 amp continuous-rated solar-capable feeder runs 5 to 8 dollars per foot. Over 150 feet, the conductor upgrade adds 450 to 900 dollars.

Subpanel and Disconnect at the Garage

A 100 amp NEMA 3R subpanel runs 150 to 350 dollars in materials. The required AC disconnect adds 100 to 250 dollars. Labor to mount, terminate, and label both: 400 to 800 dollars.

Permitting and Utility Fees

Local building and electrical permits add 200 to 600 dollars typically. A second utility meter adds 500 to 2,000 dollars in utility application fees, plus the cost of the new service drop or lateral if Option 3 is chosen.

Total Premium

For a detailed look at generation and financial tool outputs and ROI modeling, the financial side of garage solar is best evaluated against the household’s full load profile.

Permitting and Inspection Differences

A detached structure adds two complications to the permit process and one to inspection.

Building Permit

Most AHJs require a building permit any time roof penetrations or attachments are added to a structure. For the garage, this is typically the same submittal a roof-mounted system on the house would require: structural calculations or pre-engineered solution, layout drawings, attachment specifications.

Electrical Permit

Two permits sometimes apply, depending on the jurisdiction:

• Solar PV permit. Single-line diagram, AC and DC string calculations, label schedule, inverter and panel cut sheets.
• Feeder and subpanel permit. Trench section, conduit and conductor schedule, panel directory, grounding electrode connection at the garage.

Some jurisdictions combine these. Others require two separate submittals with two separate inspections. Confirm before submitting.

Inspections

Three inspection points to plan for:

1. Trench rough-in. Inspector verifies depth, conduit type, conductor size, and the absence of splices in the buried section. Schedule before backfilling.
2. Roof attachment and array rough. Standard solar inspection — flashing, attachment torque, conductor management, labels.
3. Final electrical and PV. Disconnect operation, grounding integrity, system labeling, anti-islanding test, IV curve or production verification.

Utility witness test or PTO is the final gate before energizing.

Utility Interconnection

The interconnection application is identical to a rooftop system in most utilities, with three variations:

• Site plan must show both buildings, the trench path, and the location of the array, inverter, garage subpanel, and main service.
• Single-line diagram includes the feeder with conductor size, breaker rating at both ends, and the disconnect at the garage.
• Net metering agreement specifies the meter the system feeds, which matters only when an Option 3 separate meter exists.

For a full primer on the agreements and procedural steps, the grid interconnection application glossary entry covers terminology and document flow.

Common Use Cases for Detached Garage Solar

Five scenarios drive most detached garage solar projects. Each shifts the design slightly.

EV Charging from the Same Array

The cleanest pairing in residential solar. A Level 2 EV charger draws 32 to 48 amps at 240 V, which matches well to a 6 to 10 kW array on a typical garage roof. The charger lives on the garage subpanel downstream of the inverter or hybrid inverter, so daytime charging pulls directly from solar production with minimal grid round-trip.

For load management, smart EV chargers (Wallbox, ChargePoint Home Flex, Tesla Wall Connector) can throttle to match available solar production. This avoids the inverter and feeder upsize that full-current EV charging would otherwise require. The dedicated solar EV charging integration guide covers the full design pattern.

Workshop with Heavy Power Tools

Table saws, planers, dust collectors, and air compressors draw locked-rotor currents 5 to 10 times running current at startup. Inverter-fed circuits handle this only if the inverter is sized for the surge or if a battery is in line to absorb the spike.

For pure backfeed designs (Option 1), the workshop loads run from the existing utility-fed feeder, and solar offsets consumption through the meter. For Option 2 designs with a battery, sizing the battery’s continuous and peak discharge ratings to the largest motor surge becomes the critical spec.

ADU with Separate Tenant Billing

This is the most common Option 3 scenario. The ADU has its own kitchen, its own bathroom, and often its own utility meter under municipal code. Solar on the ADU roof feeds only the ADU account.

Where the utility offers aggregate net metering across two meters on the same parcel, the math gets better. Where it does not, a battery on the ADU side is often a better investment than a larger array, because each kWh produced and consumed locally is worth retail rate, while excess exported to a per-account net meter often expires unused at year-end true-up.

Barn or Agricultural Building

Farm buildings on agricultural electricity rates almost always need a separate meter. State and utility rules vary, but the common pattern is that any structure billed at the agricultural rate must have its own service. Solar interconnects to the agricultural meter and offsets agricultural consumption only.

Net metering rules for agricultural accounts are usually less favorable than residential, with lower export credits and sometimes mandatory time-of-use schedules. Run the production-vs-rate math carefully before sizing the array.

Off-Grid Workshop or Cabin

A detached structure with no utility service is the natural Option 2 deployment. Solar feeds a battery and the load directly. No feeder. No utility interconnection. No PTO.

The design problem reduces to load profile, battery autonomy days, and seasonal sun. For 4-season cabins in northern climates, December solar is 25 to 35 percent of summer solar, and the system has to be sized to the worst month or supplemented with a generator.

Step-by-Step: Designing a Detached Garage Solar System

The design sequence below assumes Option 1 (backfeed main service), which is the most common path. Steps 3, 6, and 7 differ for Options 2 and 3 as noted.

Step 1: Survey the Site and Measure the Garage Roof

Walk the property. Measure the garage roof in two dimensions and record the pitch with a smartphone inclinometer. Measure azimuth from a south reference. Note all rooftop obstructions, fire setback boundaries, and the location of the existing electrical service entrance to the garage. Identify the trench path from the main house panel to the garage subpanel.

Record:

• Garage roof dimensions, pitch, azimuth
• Existing feeder size and breaker rating
• Existing garage subpanel size and breaker
• Trench path length, surface type, and obstacles
• Main service panel busbar rating, main breaker rating, and available spaces

Step 2: Estimate the Combined Load

Pull 12 months of utility bills. Add planned new loads:

• EV charger: 3,000 to 12,000 kWh per year per vehicle, depending on miles driven
• Heat pump for ADU: 4,000 to 8,000 kWh per year
• Workshop: 500 to 2,500 kWh per year for occasional use, more for full-time

Identify the peak month. This is the design point for sizing.

For deep dives on load shaping, see the residential solar load analysis with heat pumps and EVs post.

Step 3: Choose the Interconnection Strategy

Run the three options through a quick decision filter:

• Is there a separate utility account requirement? → Option 3.
• Is the existing feeder hard or expensive to upgrade, and does the homeowner want backup? → Option 2.
• Otherwise → Option 1.

Confirm the chosen approach with the utility before proceeding to design.

Step 4: Size the Array for the Garage Roof

Calculate target kWp:

Target kWp = Annual kWh ÷ Local Yield (kWh/kWp/year)

US local yields range from 1,100 (Pacific Northwest) to 1,800 (Arizona, Southern California). Apply a 10 to 15 percent design margin for derates and degradation. The NREL PVWatts calculator is the standard reference for ZIP-code-level yield factors.

Lay panels on the garage roof respecting setbacks. Confirm the resulting installable kWp meets the target. If short, evaluate adding panels on a second roof plane or exporting less of the load.

Step 5: Run a Detailed Shade Analysis

Even on a clean garage roof, neighboring trees, the main house, and chimneys cast shadows at certain hours. A high-resolution shade analysis identifies which panels see annual losses over 5 percent. These should be on dedicated MPPT inputs or microinverters.

Quality solar shadow analysis software generates the shading model from satellite imagery plus tree height inputs and produces hourly loss data per panel.

Step 6: Specify the Feeder

Calculate one-way distance. Run the voltage drop math for both directions of current flow. Select conductor size, breaker rating at both ends, and conduit size. Verify NEC 250.32 grounding at the garage and NEC 705.12 backfeed compliance at the main service.

For Option 2, the feeder spec is smaller because the battery buffers export. For Option 3, the feeder is replaced by a new utility service to the garage.

Step 7: Apply for Permits and Utility Interconnection

Submit to the AHJ:

• Site plan showing both buildings, trench, and array
• Single-line diagram with all conductor sizes, breaker ratings, and disconnects
• Structural calculations for the array attachment
• Panel and inverter cut sheets

Submit to the utility:

• Net metering application
• Single-line diagram
• Site plan
• Inverter spec sheet showing UL 1741 compliance

Step 8: Install, Inspect, and Commission

The build sequence:

1. Trench from house to garage
2. Pull conduit and conductor
3. Set garage subpanel and AC disconnect
4. Schedule trench rough-in inspection (before backfill)
5. Mount array, inverter, and conductor on garage roof
6. Schedule roof rough inspection
7. Final electrical inspection
8. Utility witness test or remote PTO

Commissioning verifies anti-islanding, ground fault protection, voltage and frequency settings, and production output against the design.

Real-World Example: 6 kW Detached Garage System with EV Charging

A 1,950 sq ft single-family home in Sacramento, California, has a tree-shaded south-facing house roof but a cleanly oriented two-car detached garage 130 feet from the main house. The homeowner wants to offset 100 percent of household consumption, charge a Tesla Model 3, and avoid significant remodeling of the main service.

Site Conditions

• Garage roof: 22 by 24 ft, south-pitched gable at 22 degree pitch
• Annual household consumption: 8,400 kWh
• Annual EV consumption added: 3,200 kWh
• Total target offset: 11,600 kWh
• Sacramento yield: 1,650 kWh/kWp/year
• Required system size: 11,600 / 1,650 = 7.0 kWp before margin, 7.7 kWp with 10 percent margin

Array Layout

• 18 panels at 440 W = 7.92 kWp installed
• Single south-facing plane, all unshaded
• 18 sq ft per panel × 18 = 324 sq ft, fits within 380 sq ft usable area

Equipment

• 18 × 440 W bifacial monocrystalline modules
• One 7.6 kW string inverter at the garage (DC/AC ratio 1.04)
• 60 amp PV system disconnect at garage exterior
• 100 amp NEMA 3R subpanel at garage interior
• 50 amp Level 2 EV charger on garage subpanel

Feeder Design

• One-way distance: 130 ft
• Continuous solar export: 7,600 / 240 = 31.7 A
• Continuous EV charge draw at garage: 50 × 0.8 = 40 A (worst case, charger only)
• Garage panel main breaker: 100 A
• Required feeder rating: 100 A continuous, sized for VD over 130 ft

Conductor size: 1/0 AWG aluminum THWN-2 in 1.5 inch PVC conduit, 18 inch trench. VD at full export: 1.4 percent. VD at full EV draw: 1.8 percent.

Main Service Backfeed Compliance

• Main panel busbar: 200 A
• Main breaker: 200 A
• Solar backfeed breaker: 40 A (matches 31.7 A continuous × 125 percent rounding)
• 705.12 check: 200 A + 40 A = 240 A ≤ 120 percent of 200 A = 240 A. Compliant.

Cost Breakdown

After the 30 percent residential clean energy tax credit (where eligible — see the DOE homeowner guide to the federal solar tax credit for current eligibility), net cost is 13,615 dollars. Annual production: roughly 13,070 kWh at 1,650 kWh/kWp. Payback at California’s NEM 3.0 rate structure with battery: 9 to 11 years. Without battery on NEM 3.0: 12 to 14 years.

For homeowners modeling these scenarios, the generation and financial tool handles tariff-specific export pricing, battery dispatch, and IRR.

Mistakes to Avoid

Five errors show up over and over in detached garage solar projects.

Undersizing the feeder for solar export. A feeder that worked for the original garage load may overheat under solar export, especially when garage loads run simultaneously. Always size the feeder to the larger of solar export or load, plus the 125 percent continuous-duty factor.

Forgetting NEC 250.32 grounding at the garage. A grounding electrode must be installed at the garage and bonded to the equipment grounding conductor in the feeder. The neutral must remain isolated from ground at the garage subpanel.

Choosing direct-buried UF cable to save money. UF cable saves 100 to 300 dollars in materials but locks the project into the existing conductor size. PVC conduit lets you pull a larger conductor in the future for an EV charger upgrade or system expansion.

Skipping the load coincidence check on Option 1. When solar export and garage loads run simultaneously through the same feeder, current can briefly exceed either side’s continuous rating. Calculate worst-case combined current and confirm the breaker on the garage end protects both directions.

Building Option 3 without checking aggregate net metering. A separate ADU meter is often the wrong call when the utility does not aggregate. Run the per-account net metering math before committing to a second meter.

For broader compliance reading, the NEC 2026 solar changes overview covers updates that may apply to projects permitted in 2026 and beyond.

Conclusion

A detached garage is the right home for solar more often than homeowners realize, especially when the main house roof is shaded, fragmented, or aging. The design problem is straightforward but unforgiving on details — feeder sizing, NEC 250.32 grounding, and net metering treatment all change once a second structure enters the picture.

Three concrete actions to take before designing:

• Run the three-option decision filter first. Confirm whether Option 1, 2, or 3 fits the project before any layout work begins. The interconnection choice drives everything downstream.
• Calculate the feeder before the array. Voltage drop, ampacity, and the 705.12 backfeed limit at the main panel together set the maximum array size for Option 1 designs. Do not size the array first and discover the feeder will not carry it.
• Verify net metering rules per account. Especially for Option 3 separate meter designs, confirm whether the utility aggregates across meters on the same parcel. The answer often flips the project from worth-doing to not-worth-doing.

Modern solar software handles detached structure design natively — separate roof planes, distinct subpanel schedules, feeder sizing tools, and single-line diagrams that include the trench, the disconnect, and the main service backfeed all in one project file.

Frequently Asked Questions

Can I install solar panels on a detached garage instead of the house?

Yes. A detached garage often makes a better solar site than the main house because the roof is simpler, less shaded, and free of plumbing vents and skylights. The array can feed the main service panel through a buried feeder, run a dedicated subpanel at the garage, or use a separate utility meter where the local utility allows it.

Do I need a separate utility meter for solar on a detached garage?

Usually no. Most detached garage solar systems backfeed the main house service panel through a feeder, so the existing utility meter measures net production and consumption. A separate meter is only needed when the garage has its own utility account, such as an ADU rental, a farm building under agricultural rates, or a workshop billed to a business.

How deep do I have to bury the cable from the house to a detached garage?

NEC 300.5 sets minimum cover depths based on wiring method. Direct-buried UF cable needs 24 inches of cover. PVC conduit needs 18 inches. Rigid metal conduit needs 6 inches. A residential branch circuit under 120 volts at 20 amps with GFCI protection can drop to 12 inches. Always confirm with the local AHJ because some jurisdictions require deeper trenches.

What size solar system can fit on a typical detached garage roof?

A two-car detached garage roof of about 400 to 500 square feet typically fits 12 to 18 panels at 440 watts, or roughly 5 to 8 kWp. A three-car garage with a clean ridgeline can support 25 to 35 panels, around 11 to 15 kWp. Single-car garages and small workshops fit 4 to 8 panels, between 1.7 and 3.5 kWp.

Does net metering work the same way for detached garage solar?

If the array backfeeds the main service, net metering operates exactly as it would for a roof-mounted system on the house, since both are behind the same revenue meter. If the garage has its own separate utility account and meter, net metering applies only to that account, and any excess production cannot offset consumption at the main house unless the utility offers aggregate or virtual net metering.

How much does it cost to add solar to a detached garage compared to the main house?

Expect a premium of 1,500 to 4,000 dollars over an equivalent rooftop system on the house. The premium covers trenching between buildings, the feeder cable upgrade, a dedicated subpanel or meter base at the garage, and additional permits. Long runs over 150 feet or rocky soil push trenching costs higher because larger conductors and deeper excavation are required.

Can I use the solar panels on my detached garage to charge an electric vehicle?

Yes, and the geometry is ideal because the EV charger and the array sit on the same building. The simplest design installs a Level 2 charger on the garage subpanel downstream of a hybrid inverter or AC-coupled battery. With load management, a 7 kW EV charger can be added to a 60 amp subpanel without upsizing the feeder.

Do I need a battery for a detached garage solar system?

A battery is optional. It is required only if the goal is backup power during outages or full self-consumption when net metering is unfavorable. For homes on a strong net metering tariff, a battery is rarely cost-effective on a detached garage build. For ADUs, agricultural sites, or off-grid workshops, a battery is often essential.