Battery Runtime Calculator
Estimate how long a battery will run your devices, directly or through an inverter — from capacity (Ah), voltage, allowed depth of discharge, inverter efficiency and load in watts, with a Peukert correction for high discharge currents. A planning estimate, not a guarantee.
Battery
Load and inverter
Before you rely on the number
- Enter the whole battery bank: two 12 V 100 Ah batteries in series are 24 V 100 Ah; in parallel they are 12 V 200 Ah. Use the capacity's rating hours from the label — most lead-acid batteries are rated at 20 hours (C20).
- The Peukert exponent is how much capacity a battery loses at higher current. Use the datasheet value if there is one; otherwise about 1.25 is a typical lead-acid figure (roughly 1.1–1.3), and lithium is close to 1. Set it to 1.00 to switch the correction off.
- Use the average power, not the nameplate peak: a fridge or a boiler pump cycles on and off. And add the inverter's own idle draw from its spec sheet — it runs the whole time the inverter is on.
- Cold and age cut capacity: at −10 °C a lead-acid battery holds about 80 % of its rated capacity, a battery counts as worn out at 80 % of its original capacity, and the Peukert exponent grows with age. Plan with a margin.
How this is calculated
First the calculator works out the current the battery has to supply: load watts divided by inverter efficiency and battery voltage. A battery's amp-hour rating is measured at a slow discharge (usually 20 hours); at a higher current it delivers less. Peukert's equation describes this: time to empty = rated hours × (capacity ÷ (current × rated hours)) raised to the Peukert exponent. Runtime is that time multiplied by the allowed depth of discharge — about 50 % for lead-acid, 80 % or more for lithium. At or below the rated current no correction is applied, and with an exponent of 1.00 the result equals the simple manufacturers' formula Ah × V × depth of discharge × efficiency ÷ watts. Peukert's equation holds for a steady load at a steady temperature; at very high currents real batteries give even less.
Common questions
How long will a 100Ah battery last?
It depends on voltage, chemistry and load. A 12 V 100 Ah battery stores 1200 Wh. Without any rate correction, a lead-acid one used to 50 % through a 90 % efficient inverter runs about 5.4 hours at 100 W. Because 100 W already draws more than the battery's 20-hour rating current, the Peukert correction brings that down somewhat; at 500 W and more the difference grows. A LiFePO4 battery used to 80–100 % runs considerably longer.
My UPS runs for minutes, not hours — why?
Home UPS units usually have small lead-acid batteries (7–9 Ah) and are loaded close to their rating, so the battery is drained in well under an hour — exactly where lead-acid loses the most capacity, and where even Peukert's equation is optimistic. Also, the UPS rating is in VA, while runtime depends on the watts your devices actually draw. For a UPS, trust the runtime table in its manual; use this calculator for a battery bank on an inverter or a UPS with large external batteries.
What should I enter for inverter efficiency?
The value from your inverter's spec sheet if you have it; otherwise 90 %. Small or cheap inverters, and inverters running at a small fraction of their rating, are less efficient, so 85 % is a safer guess for them. If your device runs directly on DC (a 12 V fridge, LED lights), enter 100 %.
Part of Home Improvement Calculators