Estimate how long a UPS or inverter battery will keep your equipment running. Enter the batteries, the inverter efficiency and the load, and optionally apply Peukert's law, which shortens lead-acid runtime at high discharge currents.
Last updated:
Batteries
V
Ah
Rated at the 20-hour rate for most lead-acid batteries
%
100% to the UPS cut-off; use 80% for aged batteries
%
Load
Load given as
W
VA
Many consumer UPS ratings assume 0.6; computers with PFC are about 0.9
AGM about 1.1–1.25, flooded 1.2–1.3, lithium 1.02–1.05
h
The C20 rate (20 h) is standard for lead-acid
Enter your values to see the result.
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How to use the UPS Runtime Calculator
Enter the battery voltage and Ah of one battery, how many there are and whether they are in series or parallel.
Set the inverter efficiency and how much of the capacity is usable.
Enter the load in watts, or in VA with a power factor.
Keep Peukert's law on for lead-acid batteries and set the exponent.
Read the runtime and compare other loads in the table and chart.
UPS runtime formulas
Quantity
Formula
Battery energy (Wh)
V × Ah × number of batteries
Linear runtime (h)
Wh × usable fraction × efficiency ÷ load W
Battery current (A)
Load W ÷ (bank voltage × efficiency)
Peukert runtime (h)
H × (C ÷ (I × H))^k × usable fraction
Real power (W)
VA × power factor
Peukert's law matters most for small lead-acid batteries running a heavy load. A 9 Ah battery is rated for 0.45 A over 20 hours; a UPS may pull 10–15 A from it, so the capacity actually delivered can be well under half the label value.
Typical Peukert exponents
Battery chemistry
Typical k
Lithium iron phosphate (LiFePO4)
1.02–1.05
AGM sealed lead-acid
1.10–1.25
Gel lead-acid
1.10–1.25
Flooded lead-acid
1.20–1.30
The manufacturer's runtime chart for your UPS model is the best reference; this calculator gives an estimate for planning.
Frequently asked questions
How do I calculate UPS runtime?
Runtime (hours) = battery voltage × amp-hours × number of batteries × inverter efficiency ÷ load in watts. Two 12 V 9 Ah batteries (216 Wh) at 90% efficiency running 300 W give about 216 × 0.9 ÷ 300 = 0.65 h, or 39 minutes, before any Peukert correction.
What is Peukert's law?
Peukert's law describes how a battery's usable capacity drops as the discharge current rises: t = H × (C ÷ (I × H))^k, where C is the rated Ah over H hours, I the current and k the Peukert exponent. With k = 1 there is no loss; lead-acid batteries are typically 1.1 to 1.3.
Why is real UPS runtime shorter than the simple estimate?
Small UPS batteries are discharged far faster than their 20-hour rating, so lead-acid loses capacity (Peukert effect). Battery age, low temperature, and the UPS cutting off before the battery is fully empty also reduce runtime.
How do I convert VA to watts?
Watts = VA × power factor. A UPS rated 1,500 VA at 0.6 power factor can deliver 900 W. For the runtime what matters is the real power in watts drawn by your equipment.
Does wiring batteries in series or parallel change runtime?
No. Two 12 V 9 Ah batteries store 216 Wh either way: 24 V 9 Ah in series or 12 V 18 Ah in parallel. The UPS design determines which wiring it needs.
What efficiency should I use?
Most UPS and inverters convert battery power to AC at about 85–95% efficiency at moderate load, lower at very light loads. 90% is a reasonable default if the data sheet does not say.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.