How much battery do I need for my home?

One formula, four numbers. Do it by hand or use the calculator below.

This is arithmetic, not a test. The efficiency figure is an assumption you can change, not a measured value for any product.

The formula

Battery size (kWh) = essential loads (kWh per day) × days of backup ÷ inverter efficiency

  1. List your essential loads. Fridge, freezer, lights, internet, a furnace fan, a well pump, medical devices. Use the watts on each label, or the device-by-device calculator.
  2. Convert to kWh per day. Watts × hours each runs per day ÷ 1,000. Add them up. Fridges and pumps cycle on and off, so count the hours they actually run.
  3. Pick your days. How long are outages where you live, and can you recharge from solar in between?
  4. Divide by efficiency. Some energy is lost turning battery power into household AC. We assume 85% (the same figure our other calculators use). Treat 85–90% as a planning range, not a spec.

Worked example: 500 W of essential loads for 20 hours a day is 10 kWh per day. For 2 days that is 20 kWh. Divide by 0.85: about 23.5 kWh of battery.

A note on ratings: Tesla lists Powerwall 3 at “13.5 kWh AC” and FranklinWH lists aPower 2 at “15 kWh AC”, an energy figure on the AC side. Enphase lists “usable capacity.” Power-station brands usually list battery capacity. Compare like with like, and when in doubt keep the efficiency factor in.

Output matters too

Storage (kWh) sets how long you last. Output (kW) sets what can run at once. Add the watts of everything that could run together, and check the surge or LRA (locked-rotor amps) rating if you have a well pump or air conditioner. Loads like central air, well pumps, dryers and ranges often run on 240 V, so the system must list 120/240 V output.

Home battery calculator

Example numbers are made up to show the method. Runs in your browser; nothing is sent anywhere. Matches use brand-listed maximum expansion from our comparison table.

Next: compare systems that size up to your number in home backup power, or read whole-home vs critical loads.

Frequently asked questions

How many kWh does a house need for backup?

It depends on what you back up. Multiply your essential loads in kWh per day by the days you want, then divide by inverter efficiency (we assume 85%). 10 kWh a day for 2 days is about 23.5 kWh.

What inverter efficiency should I use?

We use 85% as a planning assumption. It is not a measured figure for any product. Some installed batteries list round-trip efficiency on their datasheets, for example 89% for Tesla Powerwall 3 and 90% for Enphase IQ Battery 10C and FranklinWH aPower 2.

When do I need more than a regular power station?

Roughly when you need more than about 3 kWh between recharges, or 240 V for loads like a well pump or central air.

Sources