Whole-Home vs Partial Backup Solar: kWh Sizing Guide

The gap between partial and whole-home backup is wider than most sales presentations suggest. The average U.S. home bought about 29.6 kWh of electricity a day in 2022, per the U.S. Energy Information Administration, so a battery that holds a third of that cannot run everything for long. Grid reliability, home size, budget and medical equipment needs should drive the decision, not marketing about “energy freedom.”

Last updated: November 2026

If you are a homeowner evaluating backup options or an installer sizing a system for a client, this is more than adding one more battery. The wiring topology, inverter selection, transfer-switch rating and load calculations change. Getting it wrong produces a familiar post-install complaint: a customer who expected the air conditioner to run through a blackout and discovered it was never on the backup circuit. See the inverter sizing guide for related design detail.

This guide breaks down the circuits that belong on a critical-loads panel versus a whole-home bus, shows how to size battery capacity for four common scenarios, explains why HVAC is the single reason whole-home backup doubles or triples your storage requirement, and gives you a decision matrix. We also cover how solar design software can help installers model these scenarios before anyone signs a contract.

In this guide, you will learn:

• 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 weights 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 isolates critical circuits onto a sub-panel powered by one 10–15 kWh battery, while whole-home backup ties 20–30+ kWh to the main panel so every circuit stays energized. These are fundamentally different electrical architectures with different hardware, wiring topology, and cost expectations, not adjustable points on a single continuum.

Understanding the distinction before the site visit prevents an expensive mistake in residential storage: selling a whole-home promise with partial-home hardware.

How Partial Backup Works

Partial backup isolates a subset of household circuits onto a dedicated critical-loads subpanel. The battery inverter feeds only that subpanel through a dedicated output. When the grid drops, an automatic transfer switch disconnects the critical panel from the main service and connects it to the inverter. Everything else in the main panel goes dark.

Typical coverage includes the refrigerator or freezer, the gas-furnace blower, select LED lighting circuits, the Wi-Fi router and modem, powered medical devices, the garage door opener, a well pump if present, and a handful of 120 V outlets in the kitchen and bedrooms. What partial backup deliberately excludes is just as important: central HVAC, electric range, electric dryer, EV charger, and pool equipment. Those loads draw too much power for a single battery to sustain, and trying to include them drains the bank in minutes.

Runtime math is straightforward. A 10 kWh battery powering roughly 500 W of critical loads runs about 17 hours at 85% round-trip efficiency (10 kWh × 0.85 ÷ 0.5 kW). That is enough to get most households through a single overnight outage. If the outage stretches into a second day, solar production can recharge the bank, weather permitting.

How Whole-Home Backup Works

Whole-home backup ties the battery to the main service panel so every circuit in the house stays energized during an outage. This requires either a hybrid inverter with an integrated whole-home transfer switch or a separate automatic transfer switch rated for the main service ampacity.

Coverage is comprehensive: all household loads including 240 V appliances, central heat pump or air conditioning, and general outlets. But there is a critical distinction between power and energy. Tesla’s 2025 US datasheet lists up to 11.5 kW of continuous AC output per Powerwall 3, 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. “Whole-home” is a power-rating promise, not a runtime promise, unless the system is paired with 20–30+ kWh of storage. Compare the compressor’s locked-rotor current with the battery’s load-start rating before you promise it will start.

The Gray Area: Smart-Panel Backup

Smart electrical panels offer a middle path. They monitor each circuit and can shed non-essential loads automatically during an outage. A 15–20 kWh bank paired with a smart panel can behave like whole-home backup for short outages: the panel keeps the refrigerator and lights on, drops the dryer and pool pump, and rotates priority based on user settings. It can avoid the jump to a second or third battery for homes that only need whole-home coverage for a few hours at a time. Ask the panel maker which load-shedding rules it supports, and get its price as a line item in the quote.

Critical Loads vs. Whole-Home Loads: What Actually Stays On

Partial backup typically covers the refrigerator, lights, gas furnace blower, Wi-Fi and medical devices, a load usually in the range of a few hundred watts to about 1.2 kW. Whole-home backup adds HVAC, electric range, dryer and EV charger, which draw several kW each. Specific circuits either stay energized or go dark based on the mode selected, and homeowners and installers need to agree on that list before any hardware is ordered. The wattages in the tables below are planning ranges, so read the nameplate or use a plug-in meter on the real appliance.

Critical Loads List (Partial Backup)

The total continuous load for these circuits typically falls between 400 W and 1,200 W. Start-up surges, especially from refrigerator compressors and well pumps, require the inverter to handle brief spikes, but the sustained draw is modest. That is why a single 10–15 kWh battery works.

Whole-Home Loads That Break the Budget

These loads are large and continuous. A central heat pump drawing 4 kW will empty a Powerwall 3 in three hours. An electric range boiling water for pasta pulls 3 kW for twenty minutes. Individually, these moments seem small. Together, they explain why whole-home backup requires two to three times the storage of partial backup.

Why the Distinction Matters for Installers

Incorrect wiring can leave non-critical loads on the critical panel and drain the battery during normal grid-tied operation. The battery then discharges quietly overnight, and the customer finds a depleted bank during the first outage. Follow the manufacturer’s installation manual and local code when separating circuits.

Installers should document exactly which circuits are on the backup panel, show runtime estimates by season, and have the customer sign off on the list. A solar proposal can carry the circuit list and runtime assumptions, so there is less ambiguity about what stays on.

Battery Sizing Scenarios: From Small Partial to Whole-Home Large

Small partial backup needs 5–10 kWh for 8–16 hours of critical loads. Medium partial uses 10–15 kWh for 12–24 hours. Whole-home small houses need 15–20 kWh, and large all-electric homes need 20–30+ kWh. Sizing should start from actual daily consumption, which averaged about 29.6 kWh for U.S. households in 2022.

Sizing a battery system without modeling actual loads produces either an undersized bank that dies in two hours or an oversized system that wastes budget. The four planning scenarios below give you starting points based on home profile. The durations are illustrative arithmetic, not measured runtimes.

Sizing Table

Small partial backup suits townhomes and condos with gas heat, where the critical load is under 500 W continuous. One or two Enphase IQ Battery 5P units (5.0 kWh usable each, per Enphase’s datasheet) can keep the essentials running through an overnight outage. Medium partial backup is the sweet spot for most single-family homes: one Powerwall 3 or equivalent covers critical loads plus a few convenience outlets, giving the household a full day of autonomy.

Whole-home small house requires stepping up to two batteries. A 1,500 sq ft home with a heat pump can pull 3–5 kW intermittently. Two Powerwall 3 units deliver 27 kWh nominal, which translates to roughly 20 kWh usable after efficiency and depth-of-discharge losses. That covers 12–18 hours if the occupants are careful. Whole-home large house is a different category entirely. A large all-electric home with a pool and well pump can use well above the national average on a heavy day. Two batteries are not enough for 24 hours, so plan for three or more and check the design against the home’s own interval data.

The EIA Baseline

The U.S. Energy Information Administration reports that the average residential customer used 10,791 kWh in 2022, or about 899 kWh per month. That is roughly 29.6 kWh per day, and it is the anchor point every sizing conversation should start from. A single 13.5 kWh battery stores less than half of it. Whole-home backup for a full 24 hours takes about 30 kWh or more of nominal storage for an average household, and more if the home is above average or all-electric.

Round-Trip Efficiency and Depth of Discharge

The National Laboratory of the Rockies (NLR, formerly NREL) Annual Technology Baseline 2024 uses a representative residential system of 5 kW / 12.5 kWh with 85% round-trip efficiency. 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, and Enphase lists 5.0 kWh usable 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 using 29.6 kWh per day that wants one day of whole-home backup, with an assumed 90% depth of discharge:

Required Nominal Capacity = 29.6 ÷ (0.85 × 0.90) ≈ 38.7 kWh

That is three Powerwall 3 units (40.5 kWh nominal). Installers who quote one battery for “whole-home backup” are either assuming heavy load shedding or setting the customer up for disappointment.

Why HVAC Is the Biggest Challenge for Whole-Home Backup

A heat pump drawing 4.5 kW (an example, so read the real nameplate) can drain a 13.5 kWh battery in about three hours before losses. Without HVAC, the remaining household load is usually a small fraction of that, which is why HVAC sets the battery count more than anything else.

The Power vs. Energy Trap

One Powerwall 3 is rated up to 11.5 kW continuous in Tesla’s 2025 US datasheet, so a 4.5 kW heat pump is within its power rating. The limit is energy: 13.5 kWh ÷ 4.5 kW is about three hours. If the compressor cycles on and off, you might stretch that to five or six hours. You will not get through the night.

Most homeowners interpret “whole-home backup” as “everything runs normally.” Installers have to correct that expectation explicitly. Whole-home backup means every circuit is energized and every appliance runs for a limited time. Runtime is a function of stored energy, not inverter power.

Seasonal Runtime Variance

Runtime estimates that ignore season mislead customers. 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 the 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 customer who sees “24-hour backup” on the proposal and discovers it means a full day in October and a few hours in January will feel misled. Solar design software can build seasonal scenarios from interval data or estimated load profiles, and the battery size calculator gives a quick first estimate.

Mitigation Strategies

Several tactics can stretch runtime without adding batteries. Smart thermostats with load-shedding commands can raise the setpoint during an outage and cut run time. Locking out electric resistance heat during battery operation keeps the strip load from activating. Zoned HVAC lets the backup system serve only the occupied zone. Fuel-switching backup, such as a propane fireplace or wood stove as primary heat with the battery running only the blower, takes most of the heating load off the battery.

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 ask for written quotes that itemize the battery count, gateway and labor. A homeowner cannot judge a quote without knowing which of those items drives the difference.

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 it 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 price tables that show a 30% credit on 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 comes 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.

The scaled column and the subtotal are arithmetic on the published inputs. For a payback comparison, the generation and financial tool models solar generation and bill savings alongside the proposal. See also the battery storage payback calculator guide.

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 U.S. households, cycling the battery daily and accepting shorter backup runtime produces better financial returns.

Decision Matrix: Which Backup Mode Fits Your Home

The right backup mode depends on outage frequency, home size, budget, and medical needs. Homes with few outages and tight budgets should choose partial backup. Frequent outages, all-electric homes, or medical equipment requirements justify whole-home backup or a smart-panel middle path. The outage thresholds in the table are rules of thumb set by this guide, not published standards.

Four-Dimension Scoring

Homes with few outages and tight budgets should not buy whole-home backup. The payback is weak. The rational choice is partial backup with a single battery, redirecting the saved capital toward additional solar panels that produce value every day.

Homes with frequent outages and moderate budgets face a harder call. A 2,000 sq ft home in a storm-prone area with a gas furnace can get by on 10–15 kWh of partial backup for most events. But if outages regularly last 24 hours or more, the comfort of whole-home backup, even with limited runtime, may justify the extra cost.

Medical needs override everything else. A CPAP machine or oxygen concentrator cannot tolerate even a brief interruption. The correct design is a dedicated critical circuit with automatic transfer, isolated from discretionary loads. Whole-home backup can be layered on top if the budget allows, but the medical circuit must be protected first.

Quick-Flow Logic

1. Do you have powered medical equipment? Start with partial backup on a protected circuit; layer whole-home if budget allows.
2. Is your annual outage count under two? Partial backup is the rational economic choice.
3. 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.
4. Is your budget tight? Partial backup with one battery and a critical-loads panel.
5. 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

Installers should decide backup mode during the initial load audit, before module stringing or inverter selection. Partial backup requires a critical-loads worksheet, circuit mapping, and inverter matching with headroom. Whole-home backup needs a year of interval data, an HVAC lockout strategy, seasonal modeling, and a transfer switch rated for the main service. Installers who treat battery backup as an afterthought produce the worst customer outcomes.

Partial Backup Sizing Workflow

1. Load audit. Have the homeowner complete a critical-loads worksheet listing refrigerator model, furnace blower horsepower, and medical device wattage. Do not guess.
2. Circuit mapping. Identify which breakers move to the critical-loads subpanel. Mark 240 V loads explicitly; they require special handling.
3. Inverter match. Ensure the battery inverter’s continuous output covers the sum of simultaneous critical loads plus 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.
4. Runtime modeling. Present 4-hour, 12-hour, and 24-hour runtime estimates at average load. Never quote a single number.
5. Expansion planning. Pre-wire conduit and panel space for a second battery. Confirm inverter compatibility so the customer can upgrade without replacing the gateway.

Whole-Home Backup Sizing Workflow

1. 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. Estimated load profiles are thin evidence for whole-home design.
2. HVAC lockout strategy. Decide whether heat-pump resistance strips are excluded from battery backup via relay or smart panel. This decision alone can double runtime.
3. Seasonal scenario modeling. Model worst-case runtime: summer afternoon with AC running, winter evening with heat pump at full load.
4. Inverter and battery count. Match peak household load to total battery continuous output. Size energy capacity to the longest expected outage, not the average.
5. Transfer-switch coordination. Whole-home requires either a hybrid inverter with integrated ATS or a separate automatic transfer switch rated for the main service ampacity. Follow the manufacturer instructions and local code.

Documentation to Leave with the Customer

Every installation should include a critical-loads panel diagram or whole-home single-line diagram, a seasonal runtime estimate sheet, the warranty terms from the datasheet (Tesla lists 10 years for the Powerwall 3, and Enphase lists a 15-year limited warranty for the IQ Battery 5P), and a written expansion path listing compatible battery models. A solar proposal can carry the circuit list and assumptions into the customer packet.

Smart Panels, V2H, and the Future of Backup Scope

Smart panels, bidirectional EV charging and grid-services programs are creating options between partial and whole-home backup. Each one has limits that the maker or utility publishes.

Smart Panels as the Middle Path

A smart panel installs at the main service and provides circuit-level control. During an outage it can shed non-essential loads, such as the pool pump first, then the dryer, then the water heater, while keeping essentials running. Paired with a 15 kWh system, that can let the home behave like whole-home backup for part of an outage because the battery never sees the full simultaneous load. Panel pricing is not published in one 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 V2H is not a direct replacement yet. Compatibility is limited, utility interconnection rules vary by state, and the vehicle may not be fully charged when an outage begins. The vehicle and charger makers publish the output ratings, so use those, not a rule of thumb.

Virtual Power Plants

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.

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.

For installers, the most common source of dissatisfaction in residential storage is the gap between what was promised and what the hardware can deliver. Close it by documenting circuit maps, showing seasonal runtime estimates, and getting written sign-off before the install. Use solar PV design software to model the array side with actual load data, and book a SurgePV demo to see how that carries into a proposal.

Homeowners should treat battery backup as insurance, not an investment. If your grid is stable and outages are brief, buy partial backup and consider adding panels. If your grid is unreliable or your household depends on powered medical equipment, whole-home backup may be worth the premium, as long as you size it honestly.

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. 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 through a subpanel, such as the refrigerator, lights, router and medical devices. A single 10 to 15 kWh battery can often carry those loads for most of a day.

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. Heating and cooling loads are what push whole-home systems to 20 to 30 kWh or more.

How long will a 10 kWh battery run a house?

It depends on the load. At about 500 W of critical loads, a 10 kWh battery at 85% round-trip efficiency runs roughly 17 hours, while a 4.5 kW heat pump would use it up in under two.

Does whole-home backup need a different inverter or transfer switch?

Usually yes. Whole-home backup typically needs a hybrid inverter with an integrated transfer switch or a separate automatic transfer switch rated for the main service, while partial backup uses a dedicated critical-loads subpanel.

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 in the first install, and ask the manufacturer how many units one system supports.

Is the 30% federal tax credit still available for a home battery?

Not for property placed in service after December 31, 2025, according to the IRS residential clean energy credit page. Ask a tax professional what applies to your project before you count on a credit.