Battery Runtime Calculator - 12V, Wh, Ah & Load Watts
Use this when you know the battery capacity and the wattage of the device you want to run. It works for 12V battery runtime, power station runtime checks, watt-hours, amp-hours, usable capacity, and inverter-loss planning.
Last updated: August 2026
Buying and sizing notes
- If you are comparing batteries, compare watt-hours, not just amp-hours, unless voltage is the same.
Battery runtime reality check
Battery runtime estimates are most useful when you think in watt-hours and continuous watts. Real-world runtime can change when equipment cycles, surges, or loses energy through an inverter.
Use watt-hours
Watt-hours are easier to compare than amp-hours because amp-hours depend on battery voltage.
Efficiency matters
Inverters, battery management systems, temperature, and cutoff settings mean you usually cannot use every listed watt-hour.
Loads are not always steady
A fridge, pump, or heater may cycle on and off. Use an average load when you know it, and be conservative for critical equipment.
Check output limits
Runtime is separate from whether the battery can actually start or power the device. Verify max watts and surge capability too.
12V battery runtime questions this calculator answers
Battery runtime searches usually mix watt-hours, amp-hours, 12V batteries, inverter losses, and load watts. The cleanest approach is to convert everything to usable watt-hours.
12V battery run time
A 100Ah 12V battery is roughly 1,200 Wh before usable-capacity limits. Real runtime depends on how much of that energy is usable and how much power the load draws.
Read the battery runtime guideWatt-hours beat amp-hours
Amp-hours without voltage can mislead. Watt-hours make it easier to compare batteries and portable power stations.
100W load example
A 1,000 Wh battery running a 100W load looks like 10 hours before losses. With an 85% usable setting, the planning estimate is closer to 8.5 hours.
Surge is separate
Runtime does not guarantee the battery or inverter can start the device. Check continuous output and surge output before relying on the setup.
How to use this estimate
What this includes
- Battery watt-hours
- continuous load in watts
- usable-capacity/efficiency adjustment
What it does not include
- Battery age
- temperature effects
- startup surge
- inverter limits
- manufacturer-specific cutoff behavior
Common mistakes
- Using amp-hours without converting voltage
- Assuming 100% of battery capacity is usable
- Ignoring inverter losses
- Forgetting that heaters and cooking loads drain batteries very quickly
Quick questions
How do I calculate battery runtime?
Divide usable battery watt-hours by the load in watts. The calculator includes an efficiency setting so the result is not based on perfect 100% usable capacity.
Are amp-hours the same as watt-hours?
No. Amp-hours need the battery voltage to be converted into watt-hours. Comparing watt-hours is usually easier when voltage varies.
Why did my battery run out faster than estimated?
Real runtime can change because of inverter losses, temperature, battery age, surge loads, cycling loads, and manufacturer cutoff settings.
Common battery runtime examples
Battery runtime is easiest to understand when you compare usable watt-hours to the load in watts.
100 watt device
A 1,000 Wh battery running a steady 100 watt load might look like 10 hours before losses. With usable capacity and inverter loss, the real result may be closer to 8–9 hours.
12V battery
Amp-hours need voltage to be useful. A 100Ah 12V battery is roughly 1,200 Wh before usable-capacity limits and losses.
Fridge or cycling load
A fridge may not draw its rated watts continuously. Use average watts if you know them, but make sure the battery or inverter can handle startup surge.
Next steps for this project
Use these related paths when the calculator result turns into a real buying decision, budget, or quote comparison.
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