Approximate runtime is usable watt-hours divided by load watts. Amp-hours alone do not establish duration: battery voltage, usable capacity and conversion losses matter too.
Open the battery-runtime calculator. These are invented load scenarios, not measurements of a particular router or predictions of Iraq's electricity supply.
Work through one battery example
A nominal 12 V, 20 Ah battery represents 240 Wh. Assume, only for this example, 80% usable capacity and 90% conversion efficiency. The energy available to the load is 240 × 0.80 × 0.90 = 172.8 Wh. These percentages are not recommended settings for every battery; use the manufacturer's permitted limits.
| Assumed continuous load | Calculation | Approximate energy runtime |
|---|---|---|
| 12 W | 172.8 ÷ 12 | 14.4 hours |
| 24 W | 172.8 ÷ 24 | 7.2 hours |
| 40 W | 172.8 ÷ 40 | 4.32 hours |
7.2 hours means seven hours and 12 minutes. Actual duration can be shorter than this energy estimate.
Include everything connected
Suppose a router uses 10 W, an optical network terminal uses 8 W and a lamp uses 6 W. Together they draw 24 W, so the middle row applies. Replace those illustrative values with measurements or reliable specifications for your own equipment.
A power adapter's maximum rating may differ from the continuous load. A suitable meter or a reliable UPS reading can help establish actual consumption. Include only equipment that will remain connected throughout the backup period.
Know what the estimate leaves out
Battery age, temperature and discharge rate affect usable energy. An inverter may also consume power at light load. This calculator does not model those effects individually or reproduce a model-specific UPS discharge curve. Eaton's reference illustrates load-dependent runtimes and their limitations; it is not the specification for our invented battery scenario.
Watts measure power; watt-hours measure energy over time. A 24 W device running for three hours uses 72 Wh before supply losses. The electricity guide explains the related cost calculation.
Check voltage and connector compatibility separately, use the manufacturer's runtime curve where available, and retain a margin for real conditions. Compare an essential-device scenario with your full connected load before relying on the result.