Runtime is roughly the battery’s usable energy divided by the power your devices draw. The calculator below applies two adjustments that matter in practice: not all of the rated capacity is usable, and the inverter loses some energy converting DC to AC.
Runtime is an estimate: it depends on the real load, temperature and battery age.
- Estimated runtime
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The formula
Runtime (hours) = capacity (Wh) × usable share × inverter efficiency ÷ average load (W).
For example, a 1,000 Wh battery with 90% usable capacity and 85% inverter efficiency running a 100 W load gives 1,000 × 0.9 × 0.85 ÷ 100 ≈ 7.7 hours.
Why your real runtime may differ
- Average versus peak load: use the average draw over time. A refrigerator cycles on and off; a laptop draws more while charging than when full.
- DC outputs are more efficient: charging devices from USB or 12 V outputs skips the inverter, so runtime is usually longer than the AC estimate.
- Temperature: batteries deliver less in cold conditions.
- Battery age: capacity declines over the battery’s life.
- Standby draw: an inverter left on with nothing plugged in still uses some power.
To understand capacity labels, see watt-hours vs amp-hours. For refrigerators specifically, read running a refrigerator on a solar generator.
