Home Battery Backup Calculator
Check which appliances you want to keep running, enter your battery size, and see how many hours of power you get - with a side-by-side comparison of popular home batteries. All math runs in your browser. Nothing uploaded.
Batteries needed to reach target ?
Brand comparison at this load
Estimated installed cost shown. Actual prices vary by installer, region, and incentives. Hours shown per single battery unit.
Learn more: nameplate kWh is not the whole story
How backup hours are worked out
The calculator adds up each appliance's watts times its hours of use to get daily watt-hours, then divides by 24 for an average load. Backup hours are usable watt-hours divided by that average load. A home using 3,600 Wh a day averages 150 W, and 9 kWh of usable energy then lasts 9,000 / 150 = 60 hours, a little under two and a half days.
Appliances change the total quickly. A 1,000 W microwave used for half an hour is 500 Wh, which is 14% of that 3,600 Wh day, and a 750 W well pump running an hour adds another 750 Wh.
Usable capacity is lower than nameplate
The depth of discharge setting is the share of capacity you allow the battery to use. In the brand comparison the calculator also applies each model's own usable factor to the nameplate size, so the two combine. At 90% depth, a Tesla Powerwall 3 (13.5 kWh at a factor of 1.00) gives 12.15 kWh and about 81 hours at 150 W. A Sonnen eco (10 kWh at 0.90) gives 8.1 kWh and 54 hours.
The brand specifications and costs in that table are rough figures built into the calculator. Check them against the manufacturer's data sheet before you buy.
What the hours do not tell you
The calculation spreads the load evenly over the day and ignores inverter and round-trip losses, so real runtime will be shorter. It also does not compare the batteries' power rating with your peak load. A microwave, a well pump and a refrigerator starting together can exceed what one battery can deliver at once, even if the energy is enough.
FAQ
Why do two 10 kWh batteries give different backup times?
Their usable factors differ. In the calculator a battery with a 0.96 factor delivers more real energy than one with 0.90 at the same nameplate size and depth of discharge.
Which appliances should I back up first?
Start with what must run through the day, such as the refrigerator, medical equipment, a CPAP machine and the router. High-wattage items like a microwave draw a lot for a short time, so they add less to total daily energy than their wattage suggests.
How many batteries do I need for three days?
Divide the energy for 72 hours of average load by one battery's usable energy and round up. For 150 W that is 10,800 Wh, so one 13.5 kWh battery at 90% depth (12,150 Wh) covers it.