Estimate battery runtime hours for a backup or off-grid load.
Appliance runtime (this bank)
Energy breakdown
Usage Tip
Do not discharge lead-acid batteries below 50 percent; lithium tolerates deeper cycles, so size capacity to the chemistry.
usable Wh = Wh × efficiency ÷ 100
runtime = usable Wh ÷ load
The result is approximate runtime.
How long will my battery last — and what size do I need?
Battery runtime comes down to usable energy divided by the load. A battery stores energy in watt-hours: amp-hours times voltage. But you never get all of it — you only safely use part of the capacity (the depth of discharge), and an inverter loses some converting DC to AC. So the real runtime is the battery watt-hours, times the depth of discharge, times the inverter efficiency, divided by the load in watts. This calculator runs it both ways: tell it your battery and it gives the runtime, or tell it the runtime you need and it sizes the battery bank.
Battery runtime chart
Approximate runtime from a single 12V 100Ah battery (about 1,000 usable watt-hours after depth of discharge and inverter losses):
| Load | Runtime (100Ah battery) |
|---|---|
| 50 W | about 20 hrs |
| 100 W | about 10 hrs |
| 200 W | about 5 hrs |
| 500 W | about 2 hrs |
Watts vs amps
These trip everyone up. Watts measure power; amps measure current; volts tie them together: watts equal volts times amps. Batteries are rated in amp-hours at a voltage, while appliances are rated in watts, so you convert through watt-hours. A 100Ah battery at 12V holds 1,200 watt-hours. A 100W load draws about 8.3 amps at 12V (100 divided by 12). Always compare on watt-hours, not amp-hours, unless the voltages match.
Depth of discharge by battery type
How deeply you can drain a battery without harming it varies hugely by chemistry, and it directly changes usable capacity:
| Battery type | Recommended depth of discharge |
|---|---|
| Lead acid (flooded) | 50% |
| AGM | 50 to 60% |
| Gel | 50 to 60% |
| Lithium ion | 80 to 85% |
| LiFePO4 | 80 to 90% |
This is why a 100Ah lithium battery often outlasts a 100Ah lead-acid one: you can use far more of it.
Battery bank examples
Wiring batteries changes voltage and capacity, but the total energy is always the battery count times each battery's watt-hours:
| Bank | Result |
|---|---|
| 2 x 100Ah, 12V, parallel | 12V, 200Ah = 2,400 Wh |
| 2 x 100Ah, 12V, series | 24V, 100Ah = 2,400 Wh |
| 4 x 100Ah, 12V, series-parallel | 24V, 200Ah = 4,800 Wh |
Series adds voltage; parallel adds capacity; series-parallel does both.
Inverter efficiency
If you run AC appliances, the inverter that converts the battery's DC to household AC wastes 5 to 20 percent of the energy as heat. A good pure sine inverter runs around 90 to 95 percent efficient; cheaper units nearer 80. That loss is real runtime, so this calculator includes it. DC loads (12V lights, fans) skip the inverter and avoid the loss.
Battery and solar
Many battery questions are really solar questions. In an off-grid or backup solar setup, the battery bank stores what the panels make during the day to run loads at night. Size the bank for the runtime you need between charges (often a full day or two of autonomy), then size the panels to refill that bank during available sun hours. Use this calculator for the battery side, then match panels and a charge controller to keep it topped up.
Frequently asked questions
How do I calculate battery runtime?
Battery watt-hours times depth of discharge times inverter efficiency, divided by the load in watts. A 1,200 Wh battery at 50% DoD and 90% inverter running a 100W load lasts about 5.4 hours.
What battery do I need to run something for 8 hours?
Multiply the load by 8 to get watt-hours, divide by the inverter efficiency and the depth of discharge, then size the bank to that. The sizing mode above does this for you.
How many watt-hours in a 100Ah battery?
1,200 Wh at 12V (100 times 12). Usable is less after depth of discharge.
Why does my battery die faster than the math says?
Depth of discharge and inverter losses, plus cold temperatures and age. Lead-acid also loses capacity at high discharge rates.
Series or parallel?
Series raises voltage (same Ah); parallel raises capacity (same voltage). Total energy is the same either way.
Can I run AC appliances off a battery?
Yes, with an inverter, but factor in its efficiency. DC appliances skip the inverter and last longer per watt-hour.
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Coming soon: Solar Battery, Generator Sizing, Power Consumption and Electrical Load calculators
Estimates are for planning. Real runtime depends on battery age, temperature, discharge rate, and inverter quality; lead-acid in particular delivers less at high loads (Peukert effect). Size with a margin and never routinely discharge below the recommended depth for your battery type.