Independent guide. Some links may be affiliate links, and we may earn a commission at no cost to you. How we make money.

Power outage battery sizing calculator

Tick what you want to keep running, adjust the hours per day, and choose how many days. Typical wattages are filled in. Check your appliance's label for a more exact figure.

Use	Item	Running watts	Hours / day	Qty

Days without power: Solar panels for recharging (watts, 0 if none): Season:

Hardwired and 240-volt items. Furnaces, well pumps and many sump pumps can't simply plug into a power station. They need an inlet or transfer switch installed by a licensed electrician, with a permit. Never plug a generator or power station into a wall outlet to "backfeed" the house. It can kill line workers and start fires.

How the numbers work

Energy (watt-hours, Wh) is how long the battery lasts: watts × hours. A 100-watt item for 10 hours uses 1,000 Wh, which is 1 kWh. Fridges and freezers cycle on and off, so we use their average draw, not their peak.

Output (watts, W) is how much the battery can power at once. Add up everything that might run at the same moment. Motors (fridges, pumps, furnace blowers) briefly draw two to three times their running watts when they start, so the station's surge rating matters too.

Losses. Turning battery power into household AC loses roughly 10 to 15%. We divide by 0.85 to allow for that, and suggest a little headroom so you aren't running the battery to empty.

Typical wattages are general estimates from common appliance ratings, not measurements of your home. A plug-in watt meter (about $20 to $30) will tell you what your own fridge really uses.