Battery Runtime Calculator
How long a battery or UPS will run a load from Ah or Wh, voltage, inverter efficiency and usable depth of discharge, with Peukert correction for lead-acid batteries.
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About this toolWhat is the Battery Runtime Calculator?
How long will this battery run that load? Enter the battery capacity in amp-hours or watt-hours, its voltage, the load in watts and whether the load runs through an inverter, and this calculator estimates the runtime in hours and minutes. It works for RV and boat batteries, solar storage, portable power stations and small UPS units.
Chemistry presets fill in realistic usable depth of discharge: about half for flooded, AGM and gel lead-acid if you want a long service life, and 80 to 100% for lithium iron phosphate (LiFePO4). Lead-acid presets also switch on Peukert correction, which accounts for the capacity a lead-acid battery loses when it is drained faster than its rated time.
Turn the question around with the target runtime field: it tells you the amp-hours you need to run the same load for, say, eight hours. A table shows the runtime at a quarter, half, double and four times the load.
- Ah or Wh input with any battery voltage
- Chemistry presets for depth of discharge and Peukert exponent
- Inverter efficiency or direct DC loads
- Battery size needed for a target runtime
- Runtime table at other loads, copy and share
How to use the Battery Runtime Calculator
- Choose the battery chemistry; this sets typical depth of discharge and Peukert exponent, which you can edit.
- Enter capacity in Ah or Wh and the nominal voltage of the battery or bank.
- Enter the load in watts and choose AC through an inverter (with its efficiency) or DC direct.
- Leave Peukert on for lead-acid and enter the hour rating of the capacity, usually 20.
- Read the runtime, battery current and C-rate, and enter a target runtime to see the battery you would need.
Worked examples
100 Ah AGM battery, 100 W AC load
Input
12 V, 100 Ah, 100 W through a 90% inverter, 50% DoD, Peukert k 1.15, C/20
Output
4 h 55 min (5 h 24 min without Peukert), 9.26 A
Peukert trims about 9% at this modest current. For 8 hours you would need about 153 Ah.
LiFePO4 battery and a 300 W load
Input
12.8 V, 100 Ah, 300 W through a 90% inverter, 90% DoD, Peukert off
Output
3 h 27 min, 26 A (0.26C)
Lithium gives most of its capacity at higher currents, so no Peukert correction is applied.
Flooded bank at a heavy load
Input
12 V, 225 Ah, 500 W through a 90% inverter, 50% DoD, k 1.25
Output
1 h 42 min, versus 2 h 26 min ideal
At 46 A the bank only delivers about 158 Ah of its 225 Ah rating, a 30% loss.
Background and insight
The ideal runtime is simple energy accounting: hours = (capacity in Wh × usable depth of discharge) ÷ (load W ÷ inverter efficiency). A 12 V 100 Ah battery holds 1,200 Wh. If you only use half to protect a lead-acid battery, and feed a 100 W AC load through a 90% efficient inverter that draws 111 W from the battery, the runtime is 600 ÷ 111 = 5.4 hours.
Lead-acid capacity depends on discharge rate. Batteries are rated at a fixed time, usually 20 hours (C/20), and deliver less when discharged faster. Wilhelm Peukert described this in 1897 with t = H × (C ÷ (I × H))^k, where C is the rated capacity in Ah over H hours, I the actual current and k the Peukert exponent. k is 1 for an ideal battery, roughly 1.1 to 1.3 for lead-acid (lower for AGM, higher for flooded), and close to 1.05 for LiFePO4. A 100 Ah C/20 battery at 10 A with k = 1.2 lasts 20 × 0.5^1.2 = 8.7 hours, not 10.
Peukert's law is an approximation fitted to constant-current discharges at room temperature. Cold reduces capacity further (a lead-acid battery near freezing may give 20% less), aging reduces it steadily, and inverters draw idle power of several to tens of watts. Use manufacturer discharge tables when you have them; they are more accurate than any single exponent.
Common use cases
RV and van builds
Check how long a fridge, fan and lights will run overnight before you need to start the engine or solar.
Home backup
Estimate how many hours a power station or battery bank keeps a router, CPAP or sump pump going.
UPS planning
Work out runtime for a network rack from its watt draw and the UPS battery Wh.
Comparing chemistries
See how much usable runtime LiFePO4 adds over lead-acid of the same rated Ah.
Tips and limitations
- Battery voltage matters for Ah but not for Wh. Two 12 V 100 Ah batteries in series are 24 V 100 Ah, in parallel 12 V 200 Ah; both store 2,400 Wh.
- Measure the actual load with a plug-in watt meter. Fridges and pumps cycle, so use the average, not the peak.
- Draining lead-acid below 50% repeatedly shortens its life a lot. LiFePO4 tolerates 80 to 90% routinely.
- Add inverter idle draw (often 10 to 30 W on larger units) to the load if it stays on.
- Peukert matters most at high currents. At the rated 20-hour current it changes nothing.
Frequently asked questions
How long will a 100Ah battery last?
Divide usable watt-hours by the load. A 12 V 100 Ah battery is 1,200 Wh; at 50% depth of discharge and a 100 W load through a 90% inverter it lasts about 5.4 hours, or about 4.9 hours once Peukert loss is included for AGM.
How do I calculate battery runtime?
Runtime (h) = battery Wh × usable fraction ÷ (load W ÷ efficiency). For lead-acid, correct for Peukert effect at currents above the rated 20-hour current.
What is Peukert's law?
An empirical rule that a lead-acid battery delivers less capacity at higher currents: t = H × (C ÷ (I × H))^k. The exponent k is about 1.1 to 1.3 for lead-acid and near 1.05 for lithium.
How deep can I discharge a lead-acid battery?
Plan on 50% for a long life. Deeper cycles are possible but cut the number of cycles sharply. LiFePO4 batteries are commonly used to 80 to 90% depth of discharge.
How do I convert amp-hours to watt-hours?
Multiply by the battery voltage. 100 Ah at 12.8 V is 1,280 Wh.
Why does my UPS run for less time than calculated?
Batteries lose capacity with age and in the cold, UPS inverters have losses and idle draw, and many UPS units shut down well before the battery is empty. Use the manufacturer runtime chart when available.
Is there an in-depth guide to the Battery Runtime Calculator?
Yes. “Battery Runtime and Peukert's Law: How Long Will It Last?” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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