UPS Runtime Calculator
Estimate how long a UPS or battery bank will power a load from the battery voltage, capacity, inverter efficiency and watts drawn.
How it works
Battery Wh = V × Ah × batteries · Runtime (h) = Wh × efficiency ÷ load (W)
A battery’s energy in watt-hours is its voltage times its amp-hour capacity; multiply by the number of batteries in the bank. Dividing by the load in watts gives runtime in hours, scaled down by inverter efficiency (converting DC to AC loses 5–15%). Real runtime is usually shorter still because lead-acid batteries lose capacity at high discharge rates (Peukert’s effect) and should not be drained fully.
Worked example
One 12 V, 100 Ah battery holds 12 × 100 = 1200 Wh. At 90% inverter efficiency, 1080 Wh is usable. Powering a 300 W load: 1080 ÷ 300 = 3.6 hours ≈ 3 h 36 min.
Frequently asked questions
Why does my UPS die sooner than this estimate?
Lead-acid batteries deliver less than their rated capacity under heavy loads (the Peukert effect), and you should not discharge them below ~50% to preserve lifespan. Treat the number as a best-case ceiling and halve it for a safe planning figure on lead-acid banks.
How do I convert a VA rating to watts?
Multiply the VA rating by the power factor (typically 0.6–0.9 for older UPS units, ~1.0 for modern ones). A 500 VA UPS at 0.6 power factor supports about 300 W — use that watt figure as the load here.
Does adding batteries in parallel extend runtime?
Yes. Parallel batteries at the same voltage add their amp-hours, so two 100 Ah batteries behave like 200 Ah and roughly double the runtime. Series wiring raises voltage instead, keeping amp-hours the same.
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This calculator is for educational purposes. Double-check important results before acting on them.