Battery Backup Calculator
Free battery tool
Battery Backup Calculator
Enter your battery specs and load — see exactly how long it lasts, with real depth-of-discharge and efficiency losses applied.
Battery specs
Your load
Sum of everything running simultaneously on the battery.Estimates for planning only. Real backup time varies with temperature, battery age, and load fluctuations.
Frequently Asked Questions
How long will a 200Ah battery run a refrigerator?
A 200Ah 12V LiFePO4 battery runs a 150W refrigerator for approximately 13 hours. The math: 200Ah × 12V × 0.90 (DoD) × 0.95 (inverter efficiency) = 2,052 usable Wh, divided by 150W = 13.7 hours. A lead-acid 200Ah battery lasts about 7 hours because its safe depth of discharge is only 50 percent. Enter your exact battery and load above for a precise calculation.
How many batteries do I need to backup my house for 8 hours?
It depends on your load. A basic essentials load (fridge, lights, router, phone charging) draws about 300 watts. For 8 hours, you need 2,400 Wh usable — which is two 100Ah 12V LiFePO4 batteries or one 200Ah. A full house load including AC and water heater might draw 3,000 watts, requiring 24,000 Wh — a 48V battery bank with 500Ah or more. The calculator above sizes the battery bank for any load and duration.
Will a 100Ah battery last through the night?
A 100Ah 12V LiFePO4 battery delivers about 1,026 usable Wh. Running LED lights (40W), a router (15W), and phone charging (10W) — 65W total — it lasts about 15 hours, easily covering a full night. Add a refrigerator (150W) and runtime drops to about 4.5 hours. See our 100Ah runtime guide for a full appliance breakdown.
Why does my battery run out faster than the calculator says?
The most common reasons are: (1) the actual wattage of your appliances is higher than the label states — especially refrigerators and ACs that cycle on and off at varying power levels; (2) your battery is older and has lost capacity from degradation; (3) cold temperatures are reducing the battery’s effective capacity (LiFePO4 loses 15 to 20 percent at 0°C); (4) your inverter’s standby consumption (5 to 25W) is not accounted for in the load calculation.
Is one big battery better or two smaller batteries?
For most home setups, two smaller batteries in parallel are more practical than one large unit. They are easier to transport and install, provide redundancy if one fails, and can be expanded by adding a third later. The total capacity and runtime are identical — two 100Ah batteries in parallel equal one 200Ah battery electrically. Just ensure all batteries are the same chemistry, voltage, capacity, and ideally the same age.
Lead acid vs lithium for home battery backup — which lasts longer?
LiFePO4 (lithium iron phosphate) lasts dramatically longer in both runtime and lifespan. A 200Ah LiFePO4 delivers nearly twice the usable energy per cycle (90% DoD vs 50%) and lasts 4,000 to 6,000 cycles versus 300 to 500 for lead-acid. Over 10 years, LiFePO4 costs less per kWh delivered despite the higher purchase price. The calculator above shows the runtime difference side by side when you switch the chemistry dropdown.
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