Most backup calculators stop at a single wattage box. This one walks through your appliances, then accounts for battery depth-of-discharge and inverter loss, so the result is a number you can actually build a system around.
Interactive Tool
Battery Bank Sizing Calculator
Build your load appliance by appliance and get a buildable kWh figure, not a vague guess.
Battery Bank Sizing Calculator
v1.0 / kWh estimator
Step 1 — Build your load
Quick-add a profile:
Appliance
Watts
Hours/day
Step 2 — System settings
Depth-of-discharge you can safely use.
How long the bank runs with no recharge.
Energy lost converting DC to AC.
Headroom for loads you forgot.
Recommended battery bank
0.0 kWh usable
Daily load
0.0 kWh
Raw daily consumption0.0 kWh
After inverter loss0.0 kWh
× days of autonomy1×
÷ usable depth85%
× safety reserve20%
Suggested nominal capacity0.0 kWh
Add appliances above to see your recommended bank size.
Estimate only. Real-world results vary with temperature, battery age, and load behavior. Consult a licensed electrician or installer before purchasing or wiring a battery system.
How to read your result
The headline number is usable kWh: the energy you can actually pull from the bank after depth-of-discharge is accounted for. The breakdown panel shows every step, from raw daily consumption through inverter loss, days of autonomy, and the safety reserve.
Motor appliances like fridges and well pumps do not run continuously. The presets apply a realistic duty cycle, so the estimate is not wildly oversized the way a naive watts-times-hours calculation would be.
Frequently asked questions
How many kWh do I need to back up my house?
It depends entirely on which circuits you want to keep alive and for how long. Essential-only backup (fridge, lights, internet, a well pump) often lands between 5 and 15 kWh for a full day. Whole-home backup runs higher. Build your real load above for a figure specific to your home.
Why is depth-of-discharge in the math?
You cannot safely drain a battery to zero. LiFePO4 banks tolerate deep cycling and commonly use 80 to 90 percent of rated capacity. Lead-acid is closer to 50 percent. The calculator divides by this figure so the bank you buy actually delivers the energy you need.
Should I add a safety reserve?
Yes. A 20 percent reserve covers loads you forgot, colder weather that reduces capacity, and battery aging over time. It is cheaper to size up slightly now than to discover the bank is short during an actual outage.
Is this estimate accurate enough to buy from?
It is a solid planning estimate. Real-world results shift with temperature, battery age, and how appliances actually cycle. Treat the result as a target range, and have a licensed electrician or installer confirm the design before you purchase or wire anything.