Work out how long your inverter, UPS or solar battery will run a load, or how many batteries you need for the backup time you want. The calculator accounts for inverter losses, usable depth of discharge and the Peukert effect, the reason a lead-acid battery gives less than its label capacity at high load.
Estimate for planning. Real backup also depends on temperature, battery age and the inverter's low-voltage cut-off; the manufacturer's discharge table is the reference for final design.
| Load | Battery current | C-rate | Backup time |
|---|
The quick method divides the usable energy of the battery bank by the power drawn from it:
For a 12 V 150 Ah battery, 80% usable, 90% inverter efficiency and a 500 W load: 12 × 150 × 0.8 × 0.9 ÷ 500 = 2.6 hours. That is the figure most online calculators give, and for a lead-acid battery it is too optimistic.
A battery's Ah rating is measured at a slow discharge, usually over 20 hours for lead-acid. Discharge it faster and it delivers less. Peukert's law describes this:
The 500 W load above draws 500 ÷ 0.9 ÷ 12 = 46 A from the battery, about 0.31 C. With k = 1.2 a flooded battery reaches empty after about 2.25 hours instead of 3.2, and 80% of that is about 1.8 hours of usable backup. Lithium batteries have k close to 1.05, so their real backup stays close to the simple calculation.
| Battery type | Typical usable DoD | Peukert exponent | Notes |
|---|---|---|---|
| Lead-acid tubular / flooded | 50–80% | 1.15–1.3 | Common in home inverters; 50% DoD gives the longest life |
| Lead-acid AGM / gel | 50–80% | 1.05–1.15 | Sealed, no topping up |
| LiFePO4 | 80–95% | about 1.05 | Long cycle life, near-constant voltage |
| Li-ion NMC | 80–90% | about 1.05 | Common in packaged home batteries |
Switch the calculator to "Batteries needed", enter the backup time you want, and it works out the Ah required and the number of batteries in series (to reach the inverter voltage) and in parallel (to reach the capacity). For example, a 1,000 W load for 4 hours on a 24 V system with 12 V 150 Ah tubular batteries needs 2 in series × 2 in parallel, 4 batteries giving about 4 hours 8 minutes; the tool shows the exact count and the backup the final bank really gives.
Sizing the inverter or UPS itself? Use the UPS Sizing Calculator, and for solar systems the Solar Panel Calculator. To size the battery cables, use the Cable Size Calculator with the battery current shown here.
At 12 V with a lead-acid battery, about 1.8 hours on a 500 W load and about 4 hours on a 250 W load, using 80% of the capacity and a 90% efficient inverter with the Peukert correction. A LiFePO4 battery of the same size lasts noticeably longer at these loads.
Backup time in hours = battery voltage × Ah × usable depth of discharge × inverter efficiency ÷ load in watts. For lead-acid batteries correct the result with Peukert's law, because capacity falls at higher discharge currents.
On a 24 V system with 12 V 150 Ah lead-acid batteries, 4 batteries: 2 in series × 2 in parallel (24 V, 300 Ah), allowing for inverter losses, 80% usable capacity and the Peukert effect. With two 25.6 V 150 Ah LiFePO4 batteries in parallel you get over 6 hours. Use the Batteries needed mode for your exact figures.
The percentage of the battery's capacity you actually use before recharging. Using less of it extends battery life: lead-acid batteries last far longer at 50% depth of discharge than at 80%, while lithium batteries tolerate 80–95%.
Not by itself, the stored energy (V × Ah) is what counts. But a higher system voltage lowers the current for the same load, which reduces the Peukert loss in lead-acid batteries and the cable losses, so the real backup improves slightly.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
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