Find out how long a battery will run a device before you buy it or head off-grid. Enter the capacity the way it is printed — mAh on a phone or power bank, Ah on a 12 V leisure battery, or Wh on a portable power station — plus the load in watts or amps. The calculator converts everything to watt-hours, applies depth of discharge and inverter losses, and gives the runtime in plain hours and minutes, with an optional Peukert correction for lead-acid batteries.
All maths runs in your browser. Sizing a mains backup instead? The UPS runtime calculator works from battery bank and VA ratings. To plan the cabling from a 12 V battery, check the voltage drop calculator, and for quick amps-watts-volts conversions use the Ohm's law calculator.
Frequently asked questions
How do I calculate how long a battery will last?
Convert the battery to energy in watt-hours (Wh = Ah × nominal voltage, or mAh ÷ 1000 × voltage), multiply by the depth of discharge you are willing to use, then divide by the power the battery actually supplies — the load in watts divided by the inverter or converter efficiency. For example, a 12 V 100 Ah battery holds 1,200 Wh; at 50 % DoD that is 600 Wh usable, and a 100 W load through a 90 % efficient inverter draws about 111 W, so it runs roughly 5 h 24 min. This calculator does exactly that and shows every intermediate value.
Why do I need the battery voltage if I already know the mAh?
Milliamp-hours measure charge, not energy. The energy a battery stores is charge times voltage, so 20,000 mAh at 3.7 V (a typical power bank) is 74 Wh, while 20,000 mAh at 12 V would be 240 Wh — more than three times as much. Power banks quote the capacity of their internal 3.7 V lithium cells, which is why a "20,000 mAh" bank delivers far fewer than 20,000 mAh at its 5 V USB output. If your battery is labelled in Wh you can skip the voltage, but you still need it to see the current draw or apply the Peukert correction.
What depth of discharge should I use?
Depth of discharge (DoD) is the share of the rated capacity you actually drain. Flooded and AGM lead-acid batteries are usually limited to about 50 % to get a reasonable cycle life; draining them flat repeatedly kills them within a few hundred cycles. LiFePO4 packs are comfortable at 80–90 % and their battery management system often cuts off near there anyway. Phone and laptop lithium-ion batteries expose close to 100 % of what the manufacturer allows, although the device shuts down a little before truly empty. Use the lower figure if you care about battery lifespan.
What is the Peukert exponent and when should I turn it on?
Peukert's law describes how lead-acid batteries deliver less capacity when discharged quickly. Capacity is rated at a slow rate, usually 20 hours (C/20); draw current faster than that and the usable amp-hours shrink. The correction is t = H × (C ÷ (I × H))^k, where H is the rated hours, C the rated Ah, I the actual current and k the Peukert exponent — typically 1.1–1.3 for lead-acid and close to 1.0–1.05 for lithium. Enable it for lead-acid banks under heavy loads; leave it off for lithium, where the linear estimate is already close.
What efficiency should I enter?
Use 100 % when a DC device runs straight off the battery at its own voltage, such as a 12 V LED strip on a 12 V battery. Use 85–95 % when a DC-DC converter sits in between, such as a power bank boosting 3.7 V cells to 5 V USB. Use 85–90 % for an inverter powering 230 V or 120 V AC appliances, and remember cheap modified-sine inverters and inverters running at a small fraction of their rating can be worse, sometimes 70 % or below. Inverters also draw an idle current of their own, which matters for small loads over long periods.
Why is my real runtime shorter than the estimate?
The estimate assumes a new battery at room temperature delivering its nameplate capacity. In practice capacity falls with age (often 20 % or more after a few years), cold temperatures cut lead-acid and lithium output noticeably, and devices such as fridges or pumps draw surge currents and cycle on and off rather than drawing a steady average. Voltage also sags under load, so a low-voltage cutoff may trigger before the calculated energy is used. Treat the result as a best case, and add a safety margin of 20–30 % when sizing a battery for something important.