Guides · Checked October 13, 2026
Sizing a Home Battery Correctly
Why a battery must pass three tests (energy, continuous power and surge), how a critical-load list sets each one, and when partial-home backup beats buying more kWh.
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Short answer
- A battery has to pass three tests, not one: enough kWh for the outage, enough continuous kW for everything running at once, and enough surge for the largest motor to start.
- Essentials are small; air conditioning is not. In our example, a refrigerator, lights, Wi-Fi, a sump pump and a gas furnace blower use about 5.1 kWh a day and peak near 3.3 kW. Adding a 3-ton central air conditioner raises that to about 18 kWh a day and a peak near 12 kW.
- One 13.5 kWh battery covers the essentials list for about 2.3 days without solar, but less than one day once central AC is included.
- Partial-home backup is usually the economic answer. A dedicated backup panel limits the load to what matters, so one battery can do the job of two or three.
Three numbers that size a battery
Energy (kWh) is how long the battery lasts: the sum of each load’s watts times the hours it actually runs. Continuous power (kW) is how much can run at once, limited by the inverter. Surge is the brief spike when a compressor or pump motor starts, often 2 to 3 times its running draw. Batteries list it as peak kW for a few seconds or as locked-rotor amps (LRA). Powerwall 3 and FranklinWH’s aPower 2 are both rated for 185 A LRA (Tesla Powerwall 3 datasheet; FranklinWH aPower 2 datasheet).
The average U.S. home bought 10,791 kWh of electricity in 2022, about 29.6 kWh a day (EIA household electricity use). Whole-home figures like that overstate what most backup systems need to carry, because an outage is when electric dryers, ranges and EV chargers can wait.
Build the load list first
Daily energy by load: three backup scenarios
kWh per day; running watts × duty cycle × hours in use
Swipe to see the full chart →
The same list gives the power test. Adding up running watts gives about 2.0 kW continuous for the essentials list and 5.0 kW once the air conditioner is included. Then add the largest single start-up spike on top: about 3.3 kW and 11.9 kW peak. With an electric water heater, range and well pump added, the “most of the home” case reaches about 14.0 kW continuous and 22 kWh a day.
Battery power limits vs. scenario peaks
Continuous and short-term peak kW per unit, from manufacturer datasheets
Swipe to see the full chart →
From load list to battery size
Multiply daily kWh by the backup days needed, then add a margin. Our Battery Size Calculator uses a 12.5% sizing reserve, the midpoint of a 10–15% range, to cover temperature, aging and estimation error. Applied here:
- Essentials with gas heat: 5.1 kWh × 1.125 = 5.8 kWh per day. One 13.5 kWh battery covers about 2.3 days; power needs fit within almost any single unit.
- Essentials plus central AC: 17.6 kWh × 1.125 = 19.8 kWh per day, so two 13.5 kWh units for one full day. The 12 kW peak also rules out the smaller inverters unless a soft starter is fitted to the compressor.
- Most of the home: about 25 kWh per day. That usually means three or more units, or solar to recharge daily, or load management that sheds the water heater and range.
Solar changes the math because it can refill part of the battery each day. Programs change it too. A battery enrolled in a virtual power plant (VPP) is usually paid for its kW during events, so a bigger inverter and more kWh can raise program pay. A higher backup reserve, meanwhile, keeps energy for outages but leaves less to sell as grid flexibility; see How VPP Programs Pay.
Backup panel
Moving critical circuits to a backup panel keeps the load inside one battery’s limits and avoids paying for kWh that only an outage-time dryer would use.
Load management
Whole-home backup needs either several units or smart breakers and load controllers that shed big loads automatically when the battery is the only source.
Check the LRA
Compare a compressor or well pump’s nameplate LRA with the battery’s rating. Soft starters reduce start-up current and can let a smaller system start a large AC unit.
Size it with your own loads
The Battery Size Calculator builds the energy, continuous and peak numbers from a load list and ZIP code. The Backup Runtime Calculator shows hours of backup for a chosen battery.
Method and sources
Load figures are the national planning defaults used in our calculators, each sourced to a manufacturer or sizing guide. Daily kWh = running watts × duty cycle × daily hours in use. Peak = total running watts plus the largest single start-up increment, assuming motors do not start at the same moment.
Battery figures are per unit from manufacturer datasheets; Enphase lists kVA, treated here as kW. The 12.5% sizing reserve follows our Battery Size Calculator. This is a planning estimate, not an electrical design; a licensed installer must confirm loads and code requirements.
- EIA household electricity use
- EnergySage backup-load guide
- EnergySage household watts
- Generac generator sizing worksheet
- Furnace blower electrical load
- HVAC generator sizing reference
- Enphase IQ Battery 5P datasheet
- sonnenCore+ technical specifications
- Tesla Powerwall 2 datasheet
- LG enblock S datasheet
- FranklinWH aPower 2 datasheet
- Tesla Powerwall 3 datasheet
Related
Figures are published specifications, prices and program terms checked October 13, 2026. Modeled results are illustrations, not quotes or engineering designs. Program payments are gross, not net profit. Confirm with a licensed installer and the program administrator before buying or enrolling. ← All Guides