Disclosure: some links on this page go to brand or retailer sites. Some may be affiliate links, and BatteryPlaces may earn a commission at no extra cost to you. This does not change what we recommend. How this works.
How to size a home backup battery
Two numbers decide whether a home battery will carry you through an outage: how much energy it stores (kWh) and how much power it can deliver at once (kW). Here is how to work out both.
First decide what the battery should run. That choice is covered in Whole-home vs critical-loads backup. How the parts connect is in How home backup works.
kWh vs kW, in one minute
- kWh (energy) is how much the battery holds, like the size of a fuel tank. It decides how long you can run things.
- kW (power) is how much it can deliver at one moment, like the width of the fuel hose. It decides what you can run at the same time.
A battery can have plenty of kWh and still trip if you switch on too many big loads at once, or have high kW and run out quickly. You need enough of both.
Step 1: List the loads you want to back up
Write down each device and its wattage from its label or manual. Typical backup lists include the refrigerator, some lights, internet router, phone chargers, a furnace fan or well pump, and medical devices. Big loads such as central air, electric ranges, electric water heaters and EV chargers quickly multiply the battery size you need.
Step 2: Estimate daily energy (kWh)
For each device: watts × hours used per day ÷ 1,000 = kWh per day. Add them up. Devices that cycle on and off, like fridges and pumps, run for only part of each hour, so use the hours they actually run, or measure with a plug-in meter.
For context, the U.S. Energy Information Administration (EIA) reports that the average U.S. residential utility customer bought 10,791 kWh of electricity in 2022, about 899 kWh per month (roughly 30 kWh per day). Backing up a whole average home therefore takes far more storage than backing up a few essentials. Your own bill shows your monthly kWh.
Step 3: Find the peak power (kW) and surge
Add the watts of everything that could run at the same time. That total must fit within the battery system's continuous output rating. Motors (well pumps, compressors, AC units) briefly draw several times their running power when they start, so also check the system's surge or peak rating against your largest motor. Your installer can measure or look up starting current.
Step 4: Decide how long you need backup
Multiply daily kWh by the number of days (or fraction of a day) you want to cover. This is sometimes called days of autonomy. Many people plan for a typical outage in their area; people with medical needs or long rural outages plan for more. Solar panels can recharge the battery during the day and stretch backup time; use PVWatts to estimate output for your roof, and remember winter and storms reduce it.
Step 5: Use usable capacity, not just the headline number
Check whether a battery's rating is usable energy or total energy, and whether it is measured on the AC side (after the inverter) or DC side. Manufacturer datasheets usually list both. Compare products using the same kind of number, and leave some margin for losses, ageing and cold weather.
Worked example
Example only. This is a made-up household to show the method. The wattages and hours are illustrative round numbers, not measurements of any appliance. Use your own labels or a meter.
| Load | Running watts (example) | Hours per day (example) | kWh per day |
|---|---|---|---|
| Refrigerator (runs about a third of the time) | 150 W | 8 h of actual running | 1.2 |
| LED lights | 100 W total | 5 h | 0.5 |
| Internet router and modem | 20 W | 24 h | 0.48 |
| Phones and laptop | 60 W | 3 h | 0.18 |
| Furnace blower | 500 W | 6 h | 3.0 |
| Well pump | 1,000 W | 1 h | 1.0 |
| Total | about 6.4 kWh/day |
Energy: about 6.4 kWh per day. For two days without sun or grid, that is about 12.8 kWh, plus margin. Adding 20% margin gives roughly 15 kWh of usable storage.
Power: if the fridge, lights, router, chargers, furnace blower and well pump all ran at once, that is about 1.8 kW of running load. The well pump's starting surge would be the main thing to check against the system's surge rating.
Result for this example: a system with about 15 kWh of usable storage and a few kW of continuous output, with enough surge for the pump, would cover these loads for about two days. Adding central air or electric heat would change the answer completely.
Home backup battery calculator
Pre-filled with the made-up worked example above, so the result matches it (about 15 kWh). Replace the rows with your own loads. Runs in your browser; nothing is sent anywhere. Compare the result with a battery's usable capacity.
| Device | Watts | Hours per day | How many | Wh per day |
|---|
Common sizing mistakes
- Sizing only for kWh and forgetting motor surge.
- Using a battery's total capacity instead of usable capacity.
- Assuming summer solar output in winter.
- Forgetting small always-on loads that add up over 24 hours.
Money and next steps
The federal 25D residential clean energy credit does not apply to systems installed after December 31, 2025, according to the IRS. Check state and utility programs instead: see Home battery incentives in 2026. Then get more than one quote from licensed installers, and share your load list so the quotes are comparable.
Common questions
Is more battery always better?
Not always. A bigger battery costs more and takes longer to recharge. The goal is enough energy for the loads and hours that matter to you, with some margin. A smaller battery that covers your essentials is often a better fit than a large one sized for everything.
Why does the inverter power rating matter if I have enough kWh?
Energy (kWh) tells you how long you can run. Power (kW) tells you how much you can run at once. If too many things start together, such as a well pump and a microwave, the system can hit its power limit even when the battery is full.
Can solar recharge the battery during an outage?
It can if the system is designed for it. Many grid-tied solar systems shut off during outages for safety unless they are paired with a battery and equipment that can isolate the home from the grid. Ask your installer how your system behaves in an outage.
What should I bring to an installer?
- Your last 12 months of electric bills (kWh per month).
- A list of the circuits and appliances you want backed up.
- How long your typical outages last.
- Whether you have, or plan to add, solar.
See also whole-home vs critical-loads backup and home battery incentives.
Home battery installs need a licensed electrician, permits and usually utility approval. General information, not professional advice.