Battery Backup Runtime Calculator
Estimate how long your battery backup will power your devices.
Batteries are ideal for small loads. For longer runtimes or larger appliances, compare with a generator or use the Generator Size Calculator.
Calculate battery runtime
7.5 hours
After 25% inverter loss · 1,000 Wh battery powering 100 W load
Quick tips
- Match battery voltage to your inverter input (typically 12V for small systems, 24V or 48V for larger).
- Depth of discharge matters: lead-acid should not drop below 50%, lithium (LiFePO4) tolerates 80–90%.
- Add a 20–30% efficiency loss for inverter conversion (DC to AC) in your runtime estimate.
- Cold temperatures cut usable capacity by 15–25%, derate if storing batteries in a garage or shed.
Practical note
Runtime estimates assume the listed load runs continuously at steady draw. Real appliances cycle on and off (fridges, fans, laptops), so actual runtime is often 20–40% longer than the worst-case number shown. For critical equipment, always size with a safety margin and test the system end-to-end before relying on it in an emergency.
FAQ
How long will a battery run my fridge?
A typical 100 Ah 12V battery (1,200 Wh) running a 150-watt fridge that cycles at about 40 percent duty would last roughly 16 to 20 hours, assuming you don't drain the battery below 50 percent. For lithium batteries you can drain deeper (down to 10 to 20 percent) so usable runtime is longer for the same nameplate capacity. Enter your battery's Wh and the fridge's average draw to get a specific number.
What's the difference between Ah and Wh when sizing?
Wh (watt-hours) is what matters for runtime, it directly measures total energy. Ah (amp-hours) only tells you current capacity at a given voltage. To convert: Wh = Ah × V. A 100 Ah 12V battery is 1,200 Wh. A 100 Ah 24V battery is 2,400 Wh. Always compare batteries by Wh when planning runtime for devices rated in watts.
Should I factor in inverter efficiency losses?
Yes. Inverters lose 10 to 15 percent converting DC battery power to AC. A 1,200 Wh battery through an 85 percent efficient inverter delivers about 1,020 Wh of usable AC power. The calculator uses an assumed efficiency you can adjust. For DC-powered loads (phones, 12V LED lights) inverter loss doesn't apply.
Is it safe to drain a battery all the way down?
No for most battery chemistries. Deep-cycle lead-acid should not drop below 50 percent state of charge or it damages the plates and shortens life. AGM can go to 30 percent. Lithium iron phosphate (LiFePO4) can safely discharge to 10 to 20 percent. The calculator's usable-capacity multiplier accounts for this, don't plan your runtime off 100 percent of nameplate capacity unless you're on lithium.
How many batteries do I need for whole-home backup?
Whole-home backup (refrigerator, lights, communications, small HVAC) typically needs 5 to 10 kWh of usable battery, which is 3 to 8 typical 100 Ah lithium batteries wired in parallel. A Tesla Powerwall is 13.5 kWh for context. For essentials-only (fridge, lights, laptop), 2 to 3 kWh, roughly one or two batteries, gets you 8 to 12 hours.
Battery vs generator: what lasts longer?
Battery systems are silent and efficient for small loads, but limited by stored energy. Generators run longer with fuel.
- Battery: quiet, instant, limited runtime
- Generator: longer runtime, requires fuel
- Best setup: combine both