UPS Runtime Calculator — Calculation guide

Estimate UPS battery backup hours and minutes from battery voltage, amp-hours, count, usable capacity, efficiency and load.

Inputs

Estimate battery-backed runtime under a stable power draw. Use manufacturer data for actual battery discharge performance.

Enter battery voltage and amp-hours per battery, battery count, allowable capacity usage, conversion efficiency and average AC load in watts.

Method and assumptions

Usable Wh = battery V × Ah × battery count × usable fraction × conversion efficiency. Runtime (h) = usable Wh / AC load W.

Worked example

Two 12 V, 100 Ah batteries at 70% usable energy and 85% efficiency deliver 1,428 Wh. At 300 W the estimate is 4.76 h.

Limitations

Assumes equivalent batteries whose nominal watt-hours are additive. High discharge rates, aging, cutoff, UPS standby draw and temperature reduce runtime. Always verify DC architecture and supplier charts.

Why is actual UPS runtime shorter?

Discharge-rate losses, battery condition, inverter losses and cutoff voltages can reduce usable energy.