Answer
Partial backup powers a critical-loads subpanel, usually the refrigerator, lights, router and medical devices, from one battery. Whole-home backup energizes the main panel, so HVAC decides the size: a 4.5 kW heat pump empties a 13.5 kWh battery in about three hours. Size by daily kWh, outage length and round-trip efficiency, not by nameplate capacity.
The average U.S. home bought about 29.6 kWh of electricity a day in 2022, according to the U.S. Energy Information Administration. A 10 kWh battery holds about a third of that. Run the air conditioner or heat pump from it and it drains in a few hours, while a refrigerator, lights and a router can run on it for most of a day. That gap between expectation and reality is why the partial-versus-whole-home backup decision trips up so many homeowners and installers.
Last updated: November 2026
Both backup modes work. Both fail when they are mis-sold or mis-sized. Partial backup is the sensible default for most homes because it covers the loads that matter during a short outage with fewer battery units. Whole-home backup is the right call when the load profile, outage frequency and budget line up. This guide shows how to check that with arithmetic you can repeat, not sales copy. For the full version with seasonal runtimes and installer workflows, see the companion whole-home vs partial backup solar guide.
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TL;DR
Partial backup powers 5 to 10 critical circuits, such as the refrigerator, lights and medical devices, with one 10 to 15 kWh battery. Whole-home backup covers all loads including HVAC, and a full day usually needs two to three battery units or more. Grid reliability, home size, budget and powered medical equipment should drive the decision, not marketing language about “energy freedom.”
In This Guide
- The exact circuits that belong on a critical-loads panel versus a whole-home bus
- How to size battery capacity for four common scenarios, from small partial backup to whole-home large houses
- Why HVAC is the single reason whole-home backup doubles or triples your storage requirement
- What drives the cost of each mode, and where the federal residential credit stands
- A decision matrix that weighs grid reliability, square footage, budget, and medical needs
- How solar installers should approach load calculations, inverter sizing, and expansion planning for each backup mode
- Where smart panels and vehicle-to-home charging fit into the backup spectrum
What Is Partial Backup, and What Is Whole-Home Backup?
Partial backup and whole-home backup are not marketing tiers. They are wiring architectures with different hardware, different costs, and different runtime outcomes.
How Partial Backup Works
Partial backup connects a battery inverter to a dedicated critical-loads subpanel. When the grid drops, an automatic transfer switch isolates that subpanel from the main service and feeds it from the battery. Everything not on the subpanel stays dark.
Typical coverage includes the refrigerator or freezer, a gas-furnace blower, select LED lighting circuits, the Wi-Fi router and modem, powered medical devices, a garage door opener, a well pump, and a handful of 120 V outlets. These are deliberately chosen because they keep the home safe and habitable without pulling the high-wattage loads that drain a small battery in minutes.
What partial backup excludes is just as important as what it includes. Central HVAC, electric ranges, electric dryers, EV chargers, and pool equipment stay on the main panel and do not receive battery power. That exclusion is the reason a 10 kWh battery can run critical loads for most of a day instead of a few hours.
The math is straightforward. A 10 kWh battery, derated to roughly 8.5 kWh of usable energy after 85% round-trip efficiency, feeding about 500 W of continuous critical load, delivers about 17 hours of runtime (8.5 kWh ÷ 0.5 kW). Add a coffee maker or microwave for a few minutes and the total drops slightly; add a space heater and it collapses.
How Whole-Home Backup Works
Whole-home backup ties the battery system to the main service panel through a whole-home transfer switch or a hybrid inverter with integrated switching. When the grid fails, the entire panel, every breaker, every circuit, receives power from the battery bank.
That sounds simple, but it creates a power-versus-energy problem. Tesla’s 2025 US datasheet for the Powerwall 3 lists up to 11.5 kW of continuous AC output per unit, 185 LRA load-start capability and 13.5 kWh of nominal energy. At a 10 kW household draw, that 13.5 kWh lasts about 80 minutes. The hardware can power the whole home; it cannot run it for long unless the energy capacity scales with the load. Compare the locked-rotor current of your compressor with the battery’s load-start rating before you promise it will start.
Coverage therefore includes all household loads: 240 V appliances, central heat pump or AC, general outlets, and everything else. The homeowner does not need to move breakers to a subpanel. The tradeoff is cost and physical space. Whole-home designs usually need more battery units, a gateway or transfer switch sized for the service, and sometimes a panel upgrade.
The Gray Area: Smart-Panel Backup
Smart electrical panels sit between partial and whole-home backup. They keep the full main panel in play, but use load shedding to drop non-essential circuits automatically when battery capacity runs low. For sizing logic, see the residential battery sizing guide.
A 15–20 kWh bank paired with a smart panel can behave like whole-home backup for short outages without buying a 30 kWh system. In one illustrative setup, the panel runs the heat pump for two hours, then locks it out and keeps lights and refrigeration going for the rest of the night. It is a middle path for homes that want flexibility but cannot justify three batteries. Ask the panel maker what load-shedding rules it supports.
Critical Loads vs. Whole-Home Loads: What Actually Stays On
The difference between partial and whole-home backup is visible at the breaker level. If you cannot name the circuits on your critical-loads panel, you are not ready to size a battery. The wattages below are planning ranges, so read the nameplate or use a plug-in meter on the real appliance.
Critical Loads List (Partial Backup)
| Load Category | Typical Wattage | Notes |
|---|---|---|
| Refrigerator / freezer | 100–200 W running; 600–800 W start | Highest priority on every critical panel |
| Gas-furnace blower | 300–800 W | Essential in cold climates; often overlooked |
| LED lighting (6–10 fixtures) | 60–120 W total | Circuits should be mapped room by room |
| Wi-Fi router / modem | 20–50 W | Communication lifeline during emergencies |
| Medical devices (CPAP, oxygen concentrator) | 40–300 W | Continuous; cannot be interrupted |
| Garage door opener | 400–600 W start | Safety and vehicle access |
| Well pump (½ HP) | 750–1,500 W start | Rural homes; requires 240 V circuit |
| Selected outlets (kitchen, bedroom) | 0–1,500 W intermittent | Phone chargers, small appliances |
Total simultaneous load for a well-mapped critical panel usually falls between 400 W and 1,200 W. That is why one battery works.
Whole-Home Loads That Break the Budget
| Load Category | Typical Wattage | Runtime Impact |
|---|---|---|
| Central AC or heat pump | 3,000–5,000 W | 3–4 hours of runtime per 13.5 kWh battery |
| Electric range | 2,000–4,000 W | Single meal can consume 2–3 kWh |
| Electric dryer | 2,500–4,000 W | Best excluded during outage unless oversized bank |
| EV Level 2 charger | 3,300–7,200 W | Usually isolated; V2H discussed later |
| Electric water heater | 3,000–4,500 W | Tank insulation buys hours; timer controls help |
These loads are not “bad.” They are simply energy-intensive. Running a central heat pump at 4 kW for six hours consumes 24 kWh. A single Powerwall 3 holds 13.5 kWh. The math is unforgiving.
Why the Distinction Matters for Installers
Incorrect wiring causes non-critical loads to back-feed the critical panel and drain the battery during normal grid-tied operation. That happens when a shared neutral or improperly separated bus allows phantom loads to pull from the backup circuit.
Customer expectation management matters just as much as wiring. Selling “whole-home” backup with only one battery installed sets up a gap between what the customer expects and what the system can run. Installers who show runtime by circuit in the solar proposal can close that gap before signing.
Battery Sizing Scenarios: From Small Partial to Whole-Home Large
Battery sizing starts with the load list, not the product brochure.
Sizing Table
The table shows planning scenarios built from this guide’s arithmetic. The durations are illustrative, not measured runtimes. Unit sizes come from the Tesla Powerwall 3 and Enphase IQ Battery 5P datasheets.
| Scenario | Battery Size | Battery Count | Backup Duration | Typical Home Profile |
|---|---|---|---|---|
| Small partial backup | 5–10 kWh | 1–2 (e.g., Enphase IQ Battery 5P, 5.0 kWh usable each) | 8–16 hours | Townhome, 1–2 residents, gas heat, no medical devices |
| Medium partial backup | 10–15 kWh | 1 (e.g., Tesla Powerwall 3) | 12–24 hours | Single-family, 3–4 residents, gas furnace, critical loads + some outlets |
| Whole-home small house | 15–27 kWh | 2 (e.g., 2× Powerwall 3, 27 kWh nominal) | 12–18 hours | 1,200–2,000 sq ft, heat pump, moderate usage |
| Whole-home large house | 20–30+ kWh | 2–3+ batteries | 18–36 hours | 2,500+ sq ft, all-electric appliances, pool, well pump |
The table makes one point clear: whole-home backup for a large, all-electric house requires at least double the storage of a medium partial backup. There is no software hack that changes the physics.
The EIA Baseline
The EIA reports that the average U.S. residential customer used 10,791 kWh in 2022, or about 899 kWh a month. That works out to roughly 29.6 kWh per day.
That baseline has a direct sizing implication. A single 13.5 kWh battery stores less than half a day’s average use. Whole-home backup for 24 hours takes about 30 kWh or more for an average household, and more for all-electric homes. Installers who quote one battery for “whole-home backup” are either assuming heavy load reduction or selling a runtime fantasy.
Round-Trip Efficiency and Depth of Discharge
Real-world capacity is never nameplate capacity. The National Laboratory of the Rockies (NLR, formerly NREL) Annual Technology Baseline 2024 assumes 85% round-trip efficiency for a representative 5 kW / 12.5 kWh residential system. Depth of discharge limits differ by product, so use the usable-capacity figure on the datasheet. Tesla lists 13.5 kWh as nominal energy for the Powerwall 3, while Enphase lists 5.0 kWh as usable capacity for each IQ Battery 5P.
A simple planning formula is:
Required Nominal Capacity = (Daily kWh × Backup Days Desired) ÷ (Round-Trip Efficiency × DoD)
For a home that needs 20 kWh per day, wants one day of backup, and uses 85% efficiency and an assumed 90% depth of discharge:
Required Nominal Capacity = 20 ÷ (0.85 × 0.90) = 20 ÷ 0.765 ≈ 26.1 kWh
That is two Powerwall 3 units at 27 kWh nominal. One is not enough.
- 5 kWh/day critical loads6.5 kWh
- 10 kWh/day13.1 kWh
- 20 kWh/day26.1 kWh
- 29.6 kWh/day (EIA average home)38.7 kWh about three Powerwall 3 units
Source: Illustrative example: 85% round-trip efficiency (NLR ATB 2024), assumed 90% depth of discharge, average-home load from EIA 2022 data, checked November 2026.
Key Takeaway
Capacity needed climbs in a straight line with daily kWh, and the average home sits near 39 kWh nominal for a full day. Cutting the load you back up is cheaper than adding batteries.
Why HVAC Is the Biggest Challenge for Whole-Home Backup
If you remove HVAC from the equation, whole-home backup gets much easier. Without heating or cooling loads, the remaining household draw is usually a small fraction of a heat pump’s. Add a heat pump, and the problem changes completely.
The Power vs. Energy Trap
Take a heat pump that draws 4.5 kW while running. That is an example, so read the nameplate on the real unit. One Powerwall 3 is rated up to 11.5 kW continuous in Tesla’s 2025 US datasheet, so power is not the constraint. Energy is: 13.5 kWh ÷ 4.5 kW is about three hours before efficiency losses. If the compressor cycles on and off, it stretches longer, but you will not get through a winter night.
Most homeowners assume “whole-home” means “everything runs normally for a full day.” It does not, unless the energy capacity scales with the load. Installers must clarify that power rating is a promise about what can run, not how long it will run.
Seasonal Runtime Variance
Runtime is not a single number. It changes with the weather. These are illustrative figures for a 20 kWh bank and a 4 kW cooling or heating load.
- Summer outage: AC at a 50% duty cycle uses about 2 kWh per hour, so a 20 kWh bank yields roughly 10 hours before losses.
- Winter outage: Electric resistance backup strips can add several kW on top of the heat pump, and runtime can collapse to a few hours unless the strips are locked out.
- Spring/fall outage: With little or no HVAC load, the same bank stretches much further, often past a day.
Installers should present runtime by season, not a single headline number. A battery that lasts a full day in October might last a few hours in January. PV design software that models seasonal load profiles, and shadow analysis that shows how shade can reduce midday recharge, makes this easy to demonstrate. For more on heat pump loads, see the heat pump and solar battery ROI guide.
Mitigation Strategies
Several tactics can stretch runtime without buying more batteries:
- Smart thermostats with load-shedding commands raise the setpoint during outages to reduce duty cycle.
- Locking out electric resistance heat during battery operation keeps the strip load from activating.
- Zoned HVAC lets the homeowner back up only the occupied zone, which cuts the load.
- Fuel-switching backup (a propane fireplace or wood stove as primary heat, with the battery running only the blower) is a low-cost winter strategy for homes that already have gas or wood heat.
Pro Tip
Before quoting whole-home backup, ask whether the resistance heat strips and the pool pump can be locked out during an outage. Those two decisions often change the battery count more than any choice of brand.
Cost Comparison: What Drives Partial vs. Whole-Home Cost
Whole-home backup costs more mainly because it needs more battery units, plus a larger gateway or transfer arrangement and more electrical labor. Installed prices vary by market and installer, so get written quotes with the battery count, gateway and labor itemized. For a payback comparison, SurgePV’s financial modeling tool can model solar generation and bill savings alongside the proposal.
What Does the Federal Credit Cover Now?
The IRS residential clean energy credit page says the credit equals 30% of the costs of new, qualified clean energy property and lists battery storage of at least 3 kWh as qualified. It also states that the credit is not available for property placed in service after December 31, 2025. Older pricing tables that show a 30% credit for a 2026 install are out of date for homeowner-owned systems, so ask a tax professional what applies to your project.
Component Cost Benchmark
The only battery cost data this guide cites is from the NLR Annual Technology Baseline, which lists component inputs for its representative residential system. Installation labor, permitting, gateway hardware and installer margin come on top, so these are not installed prices.
| Component (ATB 2024 representative 5 kW / 12.5 kWh system) | Published input (2022 USD) | Scaled to 12.5 kWh |
|---|---|---|
| Battery pack only | $283 per kWh | about $3,540 |
| Battery-based inverter | $183 per kWh (converted for this system) | about $2,290 |
| Hardware subtotal | $466 per kWh | about $5,830 |
- Battery pack only$283
- Battery-based inverter$183
- Hardware subtotal$466
Source: NLR Annual Technology Baseline 2024, residential battery storage; subtotal is the sum of the two published inputs, checked November 2026.
Key Takeaway
Hardware is only part of an installed quote. When a whole-home quote is far above a partial quote, look for the extra battery units, gateway and service work, then ask which of them your outage history justifies.
What Pushes the Price Up
- Battery count. Each added unit adds hardware and wiring.
- Gateway and transfer equipment. Whole-home designs need equipment sized for the main service.
- Service or panel upgrades. An old panel or a detached garage can add work.
- Labor and trenching. Long conduit runs and difficult mounting locations cost more.
- Local permitting. Rules and fees differ by jurisdiction. See the solar permit expediting guide.
The LBNL Tradeoff
Lawrence Berkeley National Laboratory’s report Bill Savings vs. Backup Power, published in October 2024, used the PRESTO model to test reserve settings across many locations and tariffs. In most cases the opportunity cost of holding storage in reserve outweighed the value of avoided interruptions. A higher reserve paid off only where grid reliability is very poor, the customer places a very high value on avoiding outages, and the tariff is a net billing or time-of-use rate that allows discharge to the grid but not charging from it.
For most homes with few outages, that points to partial backup with a smaller reserve.
Decision Matrix: Which Backup Mode Fits Your Home
The right backup mode is a function of four variables: how often the grid fails, how large the home is, how much money is available, and whether medical equipment requires power. The outage thresholds in the table are rules of thumb set by this guide, not published standards.
Four-Dimension Scoring
| Grid Reliability | Home Size | Budget | Medical Needs | Recommendation |
|---|---|---|---|---|
| Few outages (under 2/year) | Any | Tight | None | Partial backup; invest savings in more solar panels |
| Few outages | under 2,000 sq ft | Moderate | None | Medium partial with expansion path |
| Frequent outages (4+/year) | under 2,500 sq ft | Moderate | None | Whole-home small house; 15–20 kWh |
| Frequent outages | 2,500+ sq ft | Flexible | None | Whole-home large; 20–30+ kWh; consider smart panel |
| Any reliability | Any | Any | CPAP / oxygen / dialysis | Partial minimum; medical loads on dedicated circuit with automatic transfer |
| Poor grid + rural | Any | Tight | None | Partial + generator hybrid; battery for short outages, propane for extended |
| Poor grid + suburban | Any | Flexible | None | Whole-home, with a grid-services program where one is offered, to offset some cost |
The matrix is not a rulebook. It is a starting point for conversations that should include actual interval or bill data and a critical-loads worksheet.
Quick-Flow Logic
- Do you have powered medical equipment? Start with partial backup on a protected circuit; layer whole-home if budget allows.
- Is your annual outage count under two? Partial backup is the rational economic choice.
- Is your home all-electric (heat pump, range, dryer, water heater)? Whole-home requires 25+ kWh; consider whether fuel-switching or smart panels reduce that.
- Is your budget tight? Partial backup with one battery and a critical-loads panel.
- Do you pay time-of-use rates, or sell exports under net metering or net billing? Battery economics shift toward daily cycling, which pulls against holding a large backup reserve.
How Solar Installers Should Size Systems for Each Backup Mode
Good sizing separates professional installers from box-droppers. The workflow differs materially between partial and whole-home backup.
Partial Backup Sizing Workflow
- Load audit: Have the homeowner complete a critical-loads worksheet. Model number, wattage, and start-up surge for the refrigerator, furnace blower HP, and any medical device.
- Circuit mapping: Identify which breakers move to the critical-loads subpanel. Mark 240 V loads explicitly; well pumps need their own breaker slot.
- Inverter match: Ensure the battery inverter’s continuous output covers the sum of simultaneous critical loads plus 20% headroom. Enphase lists 3.84 kVA continuous for one IQ Battery 5P, and Tesla lists up to 11.5 kW continuous for one Powerwall 3.
- Runtime modeling: Present 4-hour, 12-hour, and 24-hour runtime estimates at average load. Never quote a single number.
- Expansion planning: Pre-wire conduit and panel space for a second battery. Confirm inverter and gateway compatibility now to avoid a full hardware swap later.
Whole-Home Backup Sizing Workflow
- Whole-home load profile: Download utility interval data (where available) or install a monitoring clamp for a year. Identify peak kW and average daily kWh by season.
- HVAC lockout strategy: Decide whether heat-pump resistance strips are excluded from battery backup via relay or smart panel. This one decision can double winter runtime.
- Seasonal scenario modeling: Model worst-case runtime: summer afternoon with AC, winter evening with heat pump. Show the homeowner both numbers.
- Inverter and battery count: Match peak household load to total battery continuous output. Size energy capacity to the longest expected outage, not the average.
- Transfer-switch coordination: Whole-home requires either a hybrid inverter with integrated automatic transfer switch or a separate ATS rated for the main service ampacity. Size the switch to the service rating and follow the manufacturer instructions and local code.
Documentation to Leave with the Customer
Every installation should include:
- Critical-loads panel diagram or whole-home single-line diagram
- Seasonal runtime estimate sheet
- Warranty terms from the datasheet or warranty document (Tesla lists 10 years for Powerwall 3; Enphase lists a 15-year limited warranty for IQ Battery 5P)
- Expansion path and compatible battery SKUs
Clear documentation prevents the “why didn’t my battery last all night?” call. A solar proposal that carries the circuit list and assumptions makes this packet easier to assemble.
Smart Panels, V2H, and Grid Services
Backup architecture is not static. Smart panels, bidirectional EV charging and grid-services programs are creating new options between partial and whole-home backup.
Smart Panels as the Middle Path
A smart panel replaces the standard electrical panel with a circuit-level hub. Paired with a 15 kWh system, dynamic load shedding can let the home behave like whole-home backup for part of an outage by dropping non-essential circuits as state of charge falls. Panel pricing is not published in a single place, so get it as a line item in the quote.
Vehicle-to-Home (V2H) Charging
Some bidirectional EVs can feed power back to a home through compatible charging equipment. An EV battery is much larger than a stationary home battery, but compatibility is the catch. V2H needs a compatible vehicle, charger and gateway, and the vehicle and charger makers publish the output ratings. Treat it as a complement for whole-home aspirants who already own an eligible EV, not a battery replacement.
Virtual Power Plants and Grid Services
A virtual power plant program pays battery owners to discharge during grid events. Enphase’s ConnectedSolutions page lists average payments of $275 per kW in Massachusetts and $225 per kW in Rhode Island, with utilities calling up to 60 events a year from June through September. It also says the battery is set to Full Backup from 12 hours before each event until it ends. Programs, payments and eligibility differ by utility, so read the terms before counting the income.
Partial vs. Whole-Home Backup at a Glance
| Question | Partial backup | Whole-home backup |
|---|---|---|
| What stays on | Critical-loads subpanel | Every circuit on the main panel |
| Typical storage | 5–15 kWh | 20–30+ kWh |
| Biggest runtime driver | Refrigerator, router, medical loads | Heat pump, AC, electric range, dryer |
| Wiring | Subpanel and transfer switch | Main-panel transfer switch or hybrid inverter |
| Best fit | Few outages, tight budget, gas heat | Frequent outages, large all-electric homes |
If a contractor proposes one battery for whole-home backup, the arithmetic above gives you the numbers to push back. If a contractor insists on three batteries for a gas-heated townhome with no medical devices, the same numbers let you question that scope. The summary does not replace the load audit and seasonal modeling described earlier. It is a sanity check before you reach the sizing table.
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Conclusion
Partial backup is the right starting point for most U.S. homes. It keeps the lights on, the refrigerator cold and medical devices running with fewer battery units than whole-home coverage. Whole-home backup becomes rational when grid reliability is poor, the home is all-electric, or the budget can absorb 20–30+ kWh without crowding out other solar investments.
Start with a critical-loads audit, model runtime by season, and leave a clear expansion path. The best backup system matches real consumption to real capacity, with room to grow. When you are ready to put that into a customer proposal, book a SurgePV demo or read how solar design software handles layout and shading inputs.
Frequently Asked Questions
How many kWh do I need for whole-home backup?
Most homes need roughly 20 to 40 kWh of nominal storage for a full day of whole-home backup, depending on daily use. EIA data for 2022 puts the average U.S. home at about 29.6 kWh per day, so one 13.5 kWh battery covers less than half of that.
What is partial home backup solar?
Partial backup powers only designated critical circuits, such as the refrigerator, lights, router and medical devices, through a subpanel. At roughly 500 W of critical load, a 10 kWh battery with 85% round-trip efficiency runs about 17 hours.
What is the difference between whole-home and partial backup?
Partial backup serves a critical-loads subpanel, while whole-home backup serves the main panel including HVAC. The difference in storage comes almost entirely from heating and cooling loads, which draw several kilowatts.
How long will a Powerwall 3 run a house?
It depends on the load, not the unit. Tesla lists 13.5 kWh of nominal energy, so a 500 W critical load can run for roughly a day, while a 4.5 kW heat pump uses it up in about three hours.
Can a partial backup system be expanded to whole-home later?
Often yes, if the original design left room to grow. Plan conduit, gateway and inverter compatibility, and panel space for a transfer switch during the first install, and ask the manufacturer how many units one system supports.
Is a battery worth it if outages are rare?
Backup alone rarely pays back when outages are rare and short. A 2024 Berkeley Lab study found that in most circumstances customers should keep low reserves and prioritize bill savings over backup power.
Where this fits
This article is part of SurgePV's Energy Storage hub, which works through the topic from first principles to the decisions a project team actually has to make.
