Whole house battery backup: sizing an installed system, with or without solar

An installed home battery is the class that makes an outage invisible: the lights do not flicker, the air conditioner keeps running, and nobody carries anything to the basement. It is also the class with the widest gap between the marketing and the arithmetic, because “whole house” means whatever your house draws, and that is a number on your utility bill, not on the battery. Size from the bill and the biggest motor, decide what “whole” means for you, and treat the installer’s quote as a document to check, not a decision already made.

Start from the bill, not the brochure

Your utility bill lists kilowatt-hours per month. Divide by 30 and you have an average day. EIA’s figure for the average American household in 2023 is 10,260 kWh a year, about 28 kWh a day, but yours could be half or double that depending on climate, heating fuel and square footage. That daily figure is the “whole house” a battery would have to carry, and against it one 13.5 kWh Powerwall 3 or 15 kWh aPower 2 is about half a day. Two units are a day. Every installer’s proposal is, underneath, a choice about how many of those days you are buying.

Then decide what “whole” means

Almost nobody needs the whole house through an outage. The home battery backup page adds the essentials to about 2.5 kWh a day at the outlet; add a well pump at a heavy duty cycle and it is roughly 5.3 kWh; add central air on a hot day and it climbs fast. Installers handle this with a critical-loads panel, a subpanel carrying the circuits the battery backs, or with a smart gateway that sheds the big loads as the battery drains. A battery sized for a critical-loads list lasts days; the same battery asked to run everything lasts hours. Deciding the list is the single most valuable hour of the project, and the sizing calculator is built to do it appliance by appliance.

The second number: the biggest motor

Capacity says how long. Start-up capability says whether the air conditioner and the well pump run at all. Franklin’s 1/2 HP well pump motor draws about 32 A locked rotor at 230 V; a central air conditioner’s compressor is typically larger. Tesla’s Powerwall 3 datasheet quotes a 185 A locked-rotor start capability and up to 11.5 kW continuous; FranklinWH’s aPower 2 quotes the same 185 A, 10 kW continuous and 15 kW for ten seconds. Those figures are why an installed battery handles loads that no portable unit can, and the installer’s job includes checking them against your equipment’s nameplate, sometimes adding a soft starter to the compressor. The physics is in running versus surge watts.

What the two datasheets state

Figure (makers’ datasheets, read September 10, 2026)	Tesla Powerwall 3	FranklinWH aPower 2
Energy per unit	13.5 kWh	15 kWh
Continuous output	up to 11.5 kW	10 kW
Peak	185 A locked rotor	15 kW for 10 seconds, 185 A locked rotor
Chemistry	Lithium-ion, LFP	LFP
Safety listings stated	UL 9540	UL 9540, UL 9540A, UL 1973, UL 1741
Warranty	10 years	15 years or 60 MWh throughput
Solar	Built-in solar inverter, up to 20 kW DC	AC-coupled; pairs with any solar inverter through the aGate
Scales to	Multiple units	15 units per aGate

Two things to notice. The start capability is identical, so for a well pump or an air conditioner either works. And the solar architecture is the real difference: Powerwall 3 contains a solar inverter, which suits a new solar installation; aPower 2 is AC-coupled, which suits a house that already has solar.

Without solar

A common question, and the answer is simpler than the search results make it. An installed battery charges from the grid, holds its charge, and takes over when the grid fails; solar is not required for any of that. What solar changes is the outage that outlasts the battery: with panels, a sunny day refills it and a week is survivable; without panels, the battery is finite, and the honest plan for the long tail is either a second unit or a generator input, which several systems support. Some utilities also offer time-of-use tariffs that make a grid-charged battery pay for itself between outages; the installer will know, and so will the DSIRE database for your state.

Battery or generator, once more

If your outages are long and your house has a gas line, read the battery versus standby generator page before choosing. The strongest setup for a week-long outage is often both.

What to ask the installer

• Which loads are on the backed panel, and which are shed first. Get it in writing.
• The start-up check for the air conditioner and any pump, against the nameplate, and whether a soft starter is quoted.
• The gateway or backup switch model and its listing; the transfer switch page explains what it does.
• Round-trip efficiency and the warranty’s throughput limit, both of which decide how much of the 15 kWh you actually get over 15 years.
• Permit and inspection, itemised.

Incentives

The federal residential clean energy credit, which covered batteries of 3 kWh or more at 30%, ended for systems placed in service after December 31, 2025. Some states and utilities still pay for storage, particularly where it can be dispatched to the grid; DSIRE at dsireusa.org lists them. We do not compute anyone’s credit, and nothing here is tax advice.