Battery runtime by capacity and load
These are calculated planning estimates using 90% depth of discharge, 92% system efficiency and a 10% reserve—the same defaults as the calculator.
| Battery capacity | 25W | 50W | 100W | 250W | 500W | 1,000W* |
|---|---|---|---|---|---|---|
| 256Wh | 7h 38m | 3h 49m | 1h 54m | 46m | 23m | 11m |
| 512Wh | 15h 16m | 7h 38m | 3h 49m | 1h 32m | 46m | 23m |
| 100Ah at 12.8V 1,280Wh | 38h 9m | 19h 5m | 9h 32m | 3h 49m | 1h 54m | 57m |
| 200Ah at 12.8V 2,560Wh | 3d 4h 19m | 38h 9m | 19h 5m | 7h 38m | 3h 49m | 1h 54m |
Calculated, not measured. Values assume a steady load. *A runtime figure does not mean the battery, BMS, inverter or power station can safely supply 1,000W; verify continuous and surge ratings.
Start with the use case that matches yours
Starlink Mini
Compare calculator estimates with the dedicated page’s source-backed owner observations and published load range.
Trolling motor
Estimate from motor current and battery amp-hours, where speed and water conditions matter.
12V battery systems
Use the dedicated 12V tool for watts or amps and system-specific guidance.
100Ah LiFePO4
See focused 100Ah examples without turning this broad calculator into a duplicate guide.
Other common setups: For laptops, workstations, routers and Wi-Fi, enter measured average watts when possible. For a cycling 12V fridge, use average power over a representative period. For a portable power station, select rated watt-hours and match efficiency to its AC or DC output.
Battery runtime formula
Watt-hours (Wh) measure stored energy. When a battery is rated in amp-hours, nominal energy is Wh = V × Ah.
Runtime (hours) = nominal Wh × depth-of-discharge fraction × efficiency fraction × (1 − reserve fraction) ÷ load W
Depth of discharge limits how much nominal energy is used. Efficiency accounts for conversion losses; for an AC appliance this normally includes the inverter. The reserve then leaves a planning margin. An equivalent load-side view is to divide usable battery energy by battery-side draw, where battery-side W = appliance W ÷ efficiency.
See the full PowerUseLab methodology for the broader calculation policy and limitations.
Assumptions and limitations
- Use average load. Startup surges, cycling appliances and inverter idle draw can make a nameplate watt figure misleading.
- Use manufacturer limits. Battery-management cutoffs, allowed depth of discharge and continuous output ratings take priority over a calculated estimate.
- Allow for conditions. Temperature, battery age, wiring loss and discharge rate can change available energy. Lead-acid capacity is especially sensitive to high discharge rates.
- Match the power path. AC inverter losses and direct-DC conversion losses differ; enter an efficiency appropriate to the actual setup.
Battery runtime FAQ
How do I calculate how long a battery will last?
Convert the battery to watt-hours, apply usable depth of discharge, efficiency and reserve, then divide by the average load in watts. The calculator performs each step above.
How long will a 100Ah battery last?
It depends on voltage, load and usable capacity. Under this page’s defaults, a 12.8V 100Ah battery running a steady 100W load is estimated at 9 hr 32 min. Use the 100Ah LiFePO4 runtime page for focused examples.
Why is real-world runtime lower than Wh ÷ watts?
Rated Wh is nominal energy. Depth-of-discharge limits, conversion loss, standby draw, temperature, battery age and a variable load can all reduce actual runtime.
Does LiFePO4 last longer than lead-acid?
Not automatically for the same usable watt-hours and load. In practice, manufacturer-approved depth of discharge and discharge-rate behavior can make the usable share different. Compare using each battery’s specifications.
How does inverter efficiency affect runtime?
An inverter draws more power from the battery than the AC appliance receives. At 92% efficiency, a 100W appliance represents about 108.7W of battery-side draw before other unmodeled losses.
Can I calculate power-station runtime from Wh?
Yes. Select “Rated watt-hours (Wh),” enter the power station’s rated energy and use an efficiency appropriate to its AC or DC output. Also verify that its output rating supports the load.