Off-Grid Solar Sizing in 5 Steps: Cabin to Home

Last updated: November 2026

An off-grid solar system must supply energy through the difficult season, start the connected loads and recover its battery after poor weather. A daily-energy calculation is the starting point, but it cannot prove that the system will meet every hour of demand.

This guide keeps the five-step sizing method and cabin, mid-size and whole-home examples. The examples are illustrative designs, not installed projects or equipment recommendations. Every selected load, efficiency and autonomy assumption is shown so you can replace it with site data.

SurgePV publishes this post and sells solar design software.

What Is an Off-Grid Solar System (And When It Makes Sense)

An off-grid solar system operates without relying on a utility supply. It commonly combines PV generation, battery storage and power electronics, and may include a generator or another energy source. An autonomous system still needs a plan for periods when generation is insufficient.

The PV array produces direct current, or DC. A charge controller regulates charging in a DC-coupled arrangement, while an inverter supplies alternating current, or AC, to compatible loads. Some systems combine several functions in an inverter-charger; others use separate devices. The selected architecture determines which ratings and controls must work together.

Off-grid can make sense at a remote site, a property without a practical grid connection, or a mobile installation with its own applicable rules. Compare a utility extension quote with the complete autonomous-system cost, including storage replacements, generator operation and service access. No universal cost per kilometre is assumed here.

A grid-connected home seeking outage backup has a different design brief. A hybrid or backup system may preserve utility service while supplying selected loads during an outage. The grid is neither free storage nor unlimited backup during a local outage, so compare the actual tariffs and operating modes.

Before sizing, agree what failure means. A seasonal cabin owner may accept postponing a visit, while a residence with critical equipment may require redundancy and an independent backup supply. That reliability requirement belongs in the brief before someone chooses battery capacity.

The European Commission’s PVGIS off-grid tool models energy flow using consumption patterns and solar data, including battery-full and battery-empty outcomes. It illustrates why chronological operation matters beyond annual generation. Source opened November 2026.

The 5-Step Off-Grid Solar System Sizing Calculator

Use the calculation as a screening worksheet, then validate the candidate system with chronological modeling and equipment checks. The five steps below are load, solar resource, array capacity, battery capacity, and power electronics.

Peak sun hours, or PSH, represents irradiation as equivalent hours at 1 kW/m². The delivery factor in the array calculation represents the chosen losses between rated PV output and the load boundary. In the worked examples it is an assumed 0.75, not a universal efficiency or a code requirement.

For battery autonomy, discharge-path efficiency covers losses from stored energy to the served load. It is not the full round-trip efficiency, which also includes charging losses. The examples use a selected 0.95 discharge-path value and 80% usable fraction solely to make the arithmetic visible.

Keep loss accounting explicit. If usable battery capacity is already specified after an operating reserve, do not reduce it by the same reserve again. If an energy model already includes battery cycling losses, do not multiply its delivered output by another battery-loss factor.

Inverter selection requires both continuous demand and the magnitude and duration of starting loads. Charge-controller selection requires separate PV input and battery output checks. Dividing array watts by battery volts estimates output current under simplified conditions; it is not a PV short-circuit-current calculation or a substitute for checking the standard.

Keep a versioned input sheet. Record which values were measured, taken from manufacturer documentation or selected as planning assumptions. That distinction makes later changes traceable when a customer adds a load or an equipment model becomes unavailable.

Step 1 , Build the Daily Load Worksheet

List every load, its energy use and when it runs. Use measured energy over representative periods for cycling equipment where possible; multiplying a nameplate running power by all hours of the day can overstate or understate actual use.

A suitably rated plug-in energy meter can help with accessible appliances. Electrical measurements requiring access to live equipment belong with qualified personnel and the appropriate procedure. Include inverter standby, communications, battery heating and any controls that remain active while the property is unoccupied.

The following table is an illustrative load schedule, not a typical-home survey. Refrigerator and freezer watts are assumed average power over their duty cycle. The heating use is also a selected assumption that must be replaced for the actual climate.

A 30 W continuous load uses 720 Wh per day or 262.8 kWh over 365 days. That arithmetic shows why small continuous loads deserve attention. Switching them off is only appropriate where it does not interrupt necessary controls, refrigeration, communications or protection.

Add a separate column for starting power, duration and likely coincidence. Energy in kWh does not tell you whether the inverter can start a pump. A short high-power event may contribute little daily energy yet determine an important equipment rating.

Build seasonal versions of the worksheet. A summer-only cabin and a year-round house have different design seasons. December is not universally the limiting month: southern-hemisphere winter, monsoon cloud, seasonal occupancy or irrigation demand can change the answer.

Also classify loads as essential, deferrable or optional. Document what the owner agrees to shed during low state of charge. The design should not silently rely on turning off heating or a critical appliance merely to make the energy balance work.

Step 2 , Find Your Worst-Month Peak Sun Hours

Find the season with the most difficult combination of load and available solar energy. A monthly average helps screen capacity, but it cannot describe the length of a cloudy spell or the probability of unmet demand.

The following resource values come from PVGIS 5.3 calls opened in November 2026. They use a south-facing plane at 30° tilt, calculated terrain horizon and the PVGIS-SARAH3 solar dataset for 2005–2023. Values are daily irradiation on that plane in kWh/m²/day, numerically equivalent to PSH at the reference irradiance.

Source: JRC PVGIS resource tool. Reproduce with the table coordinates, angle 30, aspect 0 and read H(i)_d in the monthly PVcalc output. The values do not include near-object shading specific to a proposed roof.

Compare all months, not just the two shown. Then combine them with seasonal loads. A winter resource minimum may not control a site used only in summer, while a large seasonal load may make another month more demanding despite better sunshine.

Tilt and orientation should be evaluated for the project objective. A winter-biased tilt can change seasonal production, but no latitude-plus-angle rule guarantees the best design. Snow, wind, available roof planes and shading also constrain the choice.

Use a documented horizon and near-obstruction assessment. Regional weather data cannot describe a tree next to the cabin or a chimney shadow across the array. SurgePV supports site shading analysis, while final resource assumptions still require site and model review.

For reliability assessment, use an off-grid simulation with the load profile, battery controls and available weather sequence. Review unmet energy and battery-empty periods instead of treating a positive annual energy balance as proof that the lights stay on every day.

Step 3 , Size the Solar Array

Divide the daily load by limiting-season PSH and a documented delivery factor to obtain an initial array size. This estimates an average energy balance; recovery after cloudy weather requires an additional check.

For an illustrative 12,000 Wh/day load, 3.5 PSH and a 0.75 delivery factor, the result is 12,000 / (3.5 × 0.75) = 4,571 W. With assumed 450 W modules, the arithmetic minimum is 11 modules, giving 4.95 kWp. That count still needs electrical and physical validation.

Losses should reflect the actual energy paths. Direct daytime consumption does not pass through the same battery cycle as energy stored for night use. Module temperature behaviour, wiring, power electronics and storage all need appropriate treatment rather than one chemistry-wide efficiency claim.

Use the selected module’s power temperature coefficient when evaluating temperature effects. Do not apply a universal coefficient to every crystalline module. Check whether the resource model already includes module temperature losses before adding another allowance.

Round module counts upward to meet the capacity requirement, then check string voltage, current and the controller’s operating range. An even module count is not a design requirement by itself. Two equal strings may be useful for a particular input arrangement, but the manuals determine what is acceptable.

Recovery is a separate energy requirement. If a battery has supplied several days of demand, the array must run the present load while replenishing the deficit. Specify the desired recovery period and evaluate whether available surplus can achieve it, or whether generator charging is part of the plan.

Additional PV can reduce deficits when sunlight is available, but it cannot guarantee supply through an extended period of little generation. Additional battery can bridge a deficit, but it cannot create the missing energy. Compare both options with the chronological model and the owner’s reliability requirement.

SurgePV supports array layout and energy-yield modeling. It does not replace the designer’s review of stand-alone system controls, battery compatibility or protection. Keep those checks alongside the layout before issuing a final bill of materials.

Step 4 , Size the Battery Bank (LiFePO4 vs Lead-Acid in 2026)

Battery capacity must cover the agreed load for the selected autonomy period within the permitted operating window. Check energy capacity and discharge power separately, including temperature, aging and reserve requirements.

For a 12 kWh/day AC load, two zero-solar days, an assumed 80% usable fraction and 95% discharge-path efficiency, nominal capacity is 24 / (0.80 × 0.95) = 31.58 kWh. Select a compatible bank at or above the required capacity after all applicable deratings; do not round down and still claim the same autonomy.

If a datasheet already specifies usable DC energy, divide the AC requirement by the relevant discharge-path efficiency without applying the same depth-of-discharge reduction again. If it specifies delivered AC energy, identify which losses and operating conditions are already included.

Choosing Autonomy

Autonomy is a design choice tied to acceptable interruption and backup availability. No city automatically needs a fixed number of days. Review weather sequences, seasonal use, generator access and the consequences of losing essential loads.

A larger bank may reduce generator starts but remain undercharged if the array cannot recover it. A smaller bank may be acceptable with planned load shedding or reliable backup. Compare these operating plans explicitly rather than describing one as a universally comfortable size.

Comparing Battery Chemistries

LiFePO4, also called LFP, is a lithium iron phosphate chemistry. Flooded lead-acid has different charging and maintenance requirements. Model-specific documentation should determine usable capacity, permissible current, temperatures and service expectations.

For example, Victron’s Lithium Smart manual allows charging within +5°C to +50°C. That is a specific product-family limit, not a universal LFP range. It shows why a discharge-temperature rating must not be reused as permission to charge a cold battery. Victron Lithium Smart operating instructions, opened November 2026.

Battery Voltage and Installation Constraints

Higher system voltage reduces current for the same power before losses. At an ideal 6 kW, 12 V implies 500 A while 48 V implies 125 A. These are arithmetic comparisons, not cable-size recommendations. Actual current changes with battery voltage and conversion losses.

Cable size, overcurrent protection, fault rating and isolation require the actual installation conditions and applicable standard. Do not select a conductor from power divided by nominal voltage alone. Battery fault current can be substantial even in a relatively small energy system.

Check the adopted electrical and fire requirements, equipment listing, location, clearances and emergency access with the responsible authority and designer. This guide does not assert a universal indoor battery-capacity limit. A bank above an arbitrary kWh threshold is not automatically made compliant by placing it in a separate room.

Step 5 , Size the Inverter and Charge Controller

Select power electronics from the actual load and source limits. Continuous power, starting performance, PV input ratings and battery charge current are separate checks.

Continuous and Starting Loads

Add the loads that may run simultaneously. Then assess starting events with the rest of the active load still connected. A pump’s starting requirement should not be compared with an inverter surge rating as though the rest of the house disappears during startup.

For illustration, if a pump requires 6 kW during startup while other loads draw 1.5 kW, the inverter must support the combined 7.5 kW for the relevant duration and power factor. This is a selected load example, not a generic motor multiplier. Obtain the equipment data or a suitable measurement.

Check the battery and battery-management system can supply the associated DC power. An inverter capable of the surge is insufficient if the battery current limit or protective device interrupts first. Verify the expected operating temperature and altitude derating where the manufacturer specifies them.

Charge-Controller Input and Output

A controller has limits on PV open-circuit voltage, PV short-circuit current and battery charging output. The Victron SmartSolar specification tables list these separately for each model. Victron MPPT technical specifications, opened November 2026.

As a rough power conversion example, a 5,000 W array divided by an assumed 56 V charging voltage gives about 89.3 A before losses and limiting. That is a battery-side estimate. It does not determine the array short-circuit current, which depends on module Isc and parallel-string configuration.

Check cold-condition string open-circuit voltage against the chosen controller’s limit and ensure operating voltage supports charging. Use the selected module coefficients and the applicable design conditions. Check the standard for required design factors; no universal NEC multiplier is supplied here.

Also verify maximum charging current accepted by the battery, including other chargers operating simultaneously. A generator charger and PV controller can both contribute current. Control coordination must keep the total within the battery system’s permitted limits.

AC- and DC-Coupled Architectures

DC-coupled and AC-coupled designs require different compatibility checks. A grid-following PV inverter cannot be assumed to operate autonomously simply because a battery is added. Identify which equipment forms the grid and how excess generation is controlled.

An inverter-charger does not necessarily include a solar charge controller, and product names may refer to different power ratings. Read the exact model documentation before combining inverter, PV and charging capacities in a specification table.

Generator integration needs its own review: generator output, charger demand, household demand, transfer arrangement and start/stop controls must work together. No universal generator runtime can be promised without modeling fuel, weather, load and dispatch assumptions.

Worked Example 1: 1.95 kWh/day Off-Grid Cabin

This is an illustrative seasonal cabin design, not a measured Colorado installation. The owner uses lights, refrigeration, a small pump and electronics, with cooking and space heating outside the electrical load schedule. An RV installation would require a separate assessment of mobile-system requirements.

Assume the served energy is expressed at a common load boundary, with DC-converter and inverter losses represented in the selected delivery factor. A mixed AC/DC design needs a more detailed split before final equipment selection.

At an assumed 4.2 PSH and 0.75 delivery factor, the array minimum is 1,950 / (4.2 × 0.75) = 619 W. Two assumed 400 W modules provide 800 W, subject to controller voltage and current checks. The selected PSH is a scenario input, not verified winter resource for Colorado.

For two days of autonomy, nominal battery energy is 1.95 × 2 / (0.80 × 0.95) = 5.13 kWh. A 5 kWh nominal bank falls below that calculation. Choose a compatible configuration meeting at least the calculated requirement after temperature, aging and reserve checks, or reduce the stated autonomy explicitly.

Do not select an inverter solely from the low daily energy. Record the pump and refrigerator starts, simultaneous demand and standby consumption. If some loads are DC, determine which bypass the inverter and how their converters are rated.

For a nominal 24 V design, check the actual charging voltage before estimating controller output current. Verify series and parallel battery arrangements are permitted by the manufacturer. A product sold as a server-rack battery is not automatically compatible with a chosen 24 V architecture.

The final quote should include mounting, protection, cabling, weatherproof enclosures, commissioning and any permits. A self-installed system does not make approvals unnecessary. Ask the local authority which electrical and building requirements apply before procurement.

Worked Example 2: 9 kWh/day Mid-Size Off-Grid Home

This illustrative mid-size home uses the following selected load schedule. It is not a documented Texas project, and no utility-extension quote or actual customer saving is claimed.

Round the planning demand to 9 kWh/day and assume 4.5 PSH with a 0.75 delivery factor. The screening array is 9,000 / (4.5 × 0.75) = 2,667 W. An illustrative eight-module arrangement at 450 W each gives 3.6 kWp and an average modeled delivery of 12.15 kWh/day under those assumptions.

That leaves 3.15 kWh/day above the selected load before any losses not included in the factor. Recovery from a substantial storage deficit may therefore take several favorable days. Validate this against actual weather rather than assuming the increased module count eliminates cloudy-period risk.

For 2.5 days of autonomy, battery capacity is 9 × 2.5 / (0.80 × 0.95) = 29.61 kWh nominal. A nominal 30 kWh candidate barely clears the arithmetic threshold and still needs aging, temperature and equipment checks. State separately if a larger reserve is required.

Use the peak-load schedule to select the inverter. The heat pump, pump, microwave and other loads must be assessed for coincidence and startup. An integrated charger specification needs to be checked independently from the inverter output rating.

For a nominal 48 V architecture, ensure the battery configuration, controllers and inverter share compatible operating ranges and communications. If parallel battery modules are proposed, review the permitted quantity, protection and current-sharing requirements.

Compare a complete off-grid quote with the actual utility-extension alternative, if available. Include recurring generator fuel, service and replacement assumptions. The system is not automatically cheaper simply because a hypothetical extension distance is large.

Worked Example 3: 25–35 kWh/day Whole-Home Off-Grid

This whole-home scenario distinguishes a 25 kWh/day base load from a 35 kWh/day difficult-season load. It is an illustrative design, not a California customer project, rebate approval or inspected installation.

Assume 3.5 PSH, a 0.75 array delivery factor, two zero-solar autonomy days, an 80% usable battery fraction and 95% discharge-path efficiency. All are selected inputs that require site and equipment validation.

If the 35 kWh load must be served through the difficult season, size against that requirement or explicitly define the load-shedding and backup plan. A design based on 25 kWh cannot claim the same autonomy at 35 kWh simply by selecting an inverter with more surge power.

Thirty assumed 470 W modules provide 14.1 kWp. Under the selected PSH and delivery factor, average daily delivery is 14.1 × 3.5 × 0.75 = 37.01 kWh. That leaves only about 2.01 kWh above the 35 kWh load for average recovery, showing why the arithmetic minimum is not a complete resilience design.

A 60 kWh nominal battery supplies 60 × 0.80 × 0.95 = 45.6 kWh at the load boundary in this simplified calculation. That is about 1.82 days at 25 kWh/day or 1.30 days at 35 kWh/day, not two days at either load. If that smaller bank is selected, disclose the resulting autonomy and dependence on backup.

The electric range, heating system and vehicle charger also need a simultaneous power schedule. Managed charging may reduce coincident demand, but the owner must accept the control strategy. An inverter rating alone cannot make the daily energy requirement disappear.

A generator can support extended deficits, subject to fuel availability, starting reliability, maintenance and permitted installation. Size it for the chosen combination of charging and active loads. Estimate runtime with a dispatch model; no annual hour figure is asserted here.

Final design review should cover protection, earthing or grounding arrangements, transfer controls, equipment compatibility, location and authority requirements. Use financial scenario modeling to compare the actual alternatives after the operating plan is defined.

Off-Grid Solar System Costs in 2026

Obtain a project-specific installed quote. The original market price ranges and battery-life assumptions were not supported by opened primary sources, so they are replaced here by a scope-based comparison method.

Compare the same reliability target. A quote with smaller storage and planned generator use may be valid, but it is not equivalent to one intended to run without a generator. Ask each supplier to identify expected unmet load, load-shedding rules and recovery behaviour.

The IRS states that the residential clean energy credit is unavailable for property placed in service after December 31, 2025. Do not deduct it from a new 2026 residential installation. IRS residential credit guidance, opened November 2026. Commercial and state incentives require their own eligibility review; none is assumed in these examples.

Avoid translating a cell-level manufacturing price into an installed battery quote. Enclosures, battery management, power electronics, freight, installation and warranty service can change the total substantially. Compare complete systems at the same usable-energy and power boundary.

Load reduction can reduce the required energy system, but savings are not necessarily proportional to reduced kWh. Some equipment and installation costs remain fixed. Identify the loads worth reducing, recalculate the system and request a revised quote rather than applying a universal cost-reduction percentage.

Present the lifecycle calculation with dated prices, replacement assumptions and discounting where relevant. Show a case with earlier replacement or more generator use. A single payback year is not enough to describe the cost of supplying an otherwise unconnected property.

Common Off-Grid Sizing Mistakes (and How to Avoid Them)

Common sizing errors mix energy, power and reliability or assume that a generic component specification applies to every product. Use the following checks before selecting equipment.

Sizing to annual energy alone: Compare seasonal demand with seasonal resource, then simulate the time sequence. A positive annual balance can coexist with long winter deficits.

Ignoring starting loads: Check simultaneous running demand plus the relevant startup event and duration. Do not infer motor startup from a universal multiplier or assume a surge rating applies indefinitely.

Rounding battery capacity down: If the formula requires more than a candidate bank provides, either increase capacity or state the reduced autonomy. Include operating reserve and derating once, with their basis documented.

Using universal state-of-charge cutoffs: Follow the selected battery, BMS and inverter instructions. Define load shedding and reserve settings through the actual system design; chemistry alone does not supply safe settings for every installation.

Skipping temperature limits: Check charging and discharging separately. A battery may be capable of discharge at a temperature where charging is restricted, and an enclosure heater can add to the winter load.

Confusing controller sides: PV open-circuit voltage and short-circuit current are input limits. Battery charge current is an output limit. Array watts divided by nominal battery volts cannot replace the input checks.

Assuming future loads are covered: Model the proposed heat pump, vehicle or appliance explicitly. Leave expansion provisions only where the equipment and installation allow them; additional battery modules may have manufacturer restrictions.

Also review failure recovery. The system must restart appropriately after a battery shutdown or generator fault, and the owner needs clear operating instructions. Commissioning should verify the agreed modes and record the final settings rather than relying only on a successful sunny-day demonstration.

When to Move From a Spreadsheet to Design Software

A spreadsheet is useful for documenting assumptions and screening sizes. Move to a more detailed model when the decision depends on hourly demand, multi-day weather, shading, dispatch controls or interactions between equipment limits.

PVGIS’s off-grid method reports missing energy and battery-state outcomes, which help assess the candidate design. Keep its default assumptions visible and check whether the load profile and controls represent the project. A model result is conditional on its inputs, not a guarantee of uninterrupted service.

Design documentation also needs a consistent module layout, equipment schedule and electrical review. If the roof cannot fit the screening array or a string configuration exceeds a controller limit, revise the energy model rather than carrying the old output into the proposal.

SurgePV’s published capabilities include roof modeling, array layout, shading analysis, energy-yield and financial modeling, electrical workflow support, bill-of-materials output and proposal generation. This guide does not claim a dedicated off-grid dispatch engine, automatic battery sizing or automatic code approval.

Ask any software provider to demonstrate the specific analysis you require. Check data inputs, losses, time resolution, export options and review responsibilities. A financial threshold such as project price alone cannot determine whether a spreadsheet is sufficient or whether a tool will pay for itself.

Keep the final calculation, manuals and assumptions with the customer documents. A future technician should be able to understand why the array and battery were selected and which loads the system was intended to serve.

Conclusion

Build a seasonal load schedule, verify the solar resource and calculate the initial array and storage requirements. Then check recovery, starting loads and the ratings of every connected device before selecting components.

Treat the three examples as worksheets to adapt, not shopping lists. The final design needs site data, product documentation, local approval and a defined backup or load-shedding strategy.

Bring your roof and load brief to a SurgePV project demo to explore the layout, yield and proposal portions of that process. Have the responsible off-grid designer validate storage operation, controls and electrical safety.

Frequently Asked Questions

How many solar panels do I need to run a house off grid?

Divide the design-season daily load by peak sun hours and a documented delivery factor, then divide by module wattage and round up. Validate recovery, shading and string limits before treating that preliminary count as a buildable design.

What size battery bank do I need for an off-grid solar system?

Size nominal storage from daily load, autonomy days, usable fraction and discharge-path efficiency. With selected inputs of 25 kWh/day, two days, 80% usable capacity and 95% discharge efficiency, the result is 65.79 kWh before further applicable derating.

How much does an off-grid solar system cost in 2026?

The cost requires a site-specific quote with a defined reliability target. Compare storage, power electronics, mounting, protection, labour, approvals and backup operation rather than using unsupported national price ranges.

Can an off-grid solar system power a whole house?

Yes, if its energy supply, storage and power ratings meet the agreed household demand. Chronological modeling and a backup or load-shedding plan are needed to assess extended low-solar periods.

How long do off-grid solar batteries last?

Battery life depends on the exact product, operating conditions and duty cycle. Use manufacturer cycle data and warranty terms, and distinguish them from an assumed calendar replacement date.

Should off-grid solar always be sized for December?

No, the limiting season depends on location, demand and occupancy. Check every month and weather sequence; southern-hemisphere winter or monsoon conditions can make another period more demanding.

How do I size an off-grid charge controller?

Check the controller’s PV voltage and current limits separately from its battery charging output. Use the module configuration, actual battery operating range and manufacturer instructions, and check the applicable standard for design factors.