Battery Runtime Calculator for 12V, Wh, and Watt Loads
Use this when you know the battery capacity and the wattage of the device you want to run. It works best with watt-hours, but the guide below explains how to think about 12V amp-hours too.
Last updated: July 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.
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.
Related guide
Use the guide with this calculator when you need more context before buying materials or comparing options.
Related calculators
Helpful tools for nearby decisions.