This calculator works out UPS runtime from its battery bank and actual load. The estimate applies usable discharge and UPS efficiency to the internal battery bank before dividing by connected watts. Replace the example values with information from your own label, bill, drawing, product sheet or land record. Use actual load and the UPS maker's discharge curve because small sealed batteries lose effective capacity at high rates. The formula and every editable assumption remain visible so the result can be checked before it is used for planning. Keep the values and result together when you compare or share this planning scenario.
Enter your details
Every input includes a simple explanation.
Private: calculations run in your browser. Saved scenarios, preferred electricity rate and land state stay on this device.
Runtime comes from battery energy and the connected watts after usable-discharge and conversion losses. These linear examples teach the chain; the UPS maker's discharge curve remains the better check for a specific model.
| Example | Inputs | Visible working | Answer |
|---|---|---|---|
| Two 12 V, 9 Ah batteries | 80% usable, 80% efficiency, 120 W load | 12 × 9 × 2 × 0.80 × 0.80 ÷ 120 | 1.15 hours |
| Two 12 V, 18 Ah batteries | 80% usable, 80% efficiency, 200 W load | 12 × 18 × 2 × 0.80 × 0.80 ÷ 200 | 1.38 hours |
| One 24 V, 100 Ah bank | 50% usable, 85% efficiency, 500 W load | 24 × 100 × 0.50 × 0.85 ÷ 500 | 2.04 hours |
Motors and compressors can need extra power when they start.
Battery age, temperature and a high discharge rate can reduce delivered energy.
Use this as a planning range. Confirm continuous rating, peak rating and battery compatibility with the manufacturer and a qualified installer.
Read the meaning first, verify the important details, then avoid the most common planning error.
The estimate applies usable discharge and UPS efficiency to the internal battery bank before dividing by connected watts.
Use actual load and the UPS maker's discharge curve because small sealed batteries lose effective capacity at high rates.
The VA printed on a UPS is not battery energy and cannot by itself predict minutes of runtime.
Updated 9 August 2026. Changes listed here describe this calculator release; they are not invented historical entries.