Bar shows the load as a share of the UPS rated watts. Estimates assume a full charge at about room temperature. The manufacturer’s runtime chart for your model is the best check.
Formula and breakdown
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This UPS runtime calculator estimates how long an uninterruptible power supply will keep your computer, router, NAS, alarm panel or small server running during a power cut, for home and small office users in the UK, Ireland, Europe and elsewhere. It works from the battery blocks inside the UPS, your load in watts or VA, inverter efficiency and battery age, and it applies Peukert’s law so that lead-acid batteries are not credited with energy they cannot deliver at high current.
Quick answer: A UPS runtime calculator divides usable battery energy by the load. Runtime in hours = battery Wh x inverter efficiency x battery health / load in W. Two 12 V 9 Ah blocks (216 Wh) at 85% efficiency give about 37 minutes at 300 W in theory, but about 22 minutes once lead-acid high-current losses are included.
What Is a UPS Runtime Calculator?
A UPS runtime calculator is a tool that predicts how many minutes a UPS can supply a given load from its battery after mains power fails. In plain words: the battery stores a fixed amount of energy in watt-hours, the inverter loses some of it turning DC into AC, an older battery holds less, and the load in watts decides how fast the rest is used.
The simple formula is: runtime (h) = V x Ah x efficiency x health / W. This ups runtime calculator also uses Peukert’s law, t = H x (C / (I x H))^k, where H is the hour rating of the battery’s Ah figure, C the capacity, I the battery current and k the Peukert exponent. With k = 1 the two methods give the same answer, which is the right setting for lithium batteries.
How Do You Use the UPS Runtime Calculator?
- Pick the battery chemistry. Lead-acid sets a Peukert exponent of 1.15; lithium sets it to 1.
- Enter the voltage and Ah of one battery block, as printed on it, and how many blocks are in series and in parallel. Extra battery packs add parallel strings.
- Enter the load in watts, or in VA with the load’s power factor.
- Enter the UPS rated output in watts and the inverter efficiency on battery from the datasheet.
- Lower the battery health figure if the batteries are a few years old.
- Read the runtime, the simple estimate without Peukert, and runtimes at 25%, 50% and 100% of the UPS rating.
How Long Will a UPS Last at 300 W?
A typical small desktop UPS with two 12 V 9 Ah lead-acid blocks lasts roughly 20 to 25 minutes at 300 W when the batteries are new, and less as they age. The table shows why the simple energy sum overstates runtime at higher loads: the battery current rises, and a lead-acid battery delivers less of its rated capacity when discharged quickly.
| Load (W) | Battery current (A) | Simple estimate (min) | With Peukert k = 1.15 (min) | With Peukert at 80% health (min) |
|---|---|---|---|---|
| 50 | 2.45 | 220 | 171 | 137 |
| 100 | 4.90 | 110 | 77 | 62 |
| 200 | 9.80 | 55 | 35 | 28 |
| 300 | 14.7 | 37 | 22 | 17 |
| 450 | 22.1 | 25 | 14 | 11 |
| 600 | 29.4 | 18 | 10 | 8 |
| 900 | 44.1 | 12 | 6 | 5 |
Example values: 24 V, 9 Ah (216 Wh) pack rated at the 20-hour rate, 85% inverter efficiency, Peukert exponent 1.15. Halving the load more than doubles the runtime, which is why shutting down non-essential devices during a cut buys so much time. The Wikipedia article on uninterruptible power supplies puts typical UPS runtime at 5 to 15 minutes on battery, which matches a desktop unit working near its rated load.
Why Is the Real Runtime Shorter Than Watt-Hours Divided by Watts?
Real UPS runtime is shorter than the simple sum because of three losses: the inverter wastes part of the energy, lead-acid batteries lose capacity at high current, and batteries hold less charge as they age. Inverter losses are covered by the efficiency input; use your model’s datasheet figure.
The high-current loss is the biggest one for a UPS. Battery blocks are usually rated at a 20-hour discharge, but a UPS empties them in minutes. The Wikipedia article on Peukert’s law gives lead-acid exponents typically between 1.1 and 1.3, about 1.05 to 1.15 for AGM and 1.1 to 1.25 for gel. It also notes that the law ignores temperature and that the exponent tends to rise as a battery ages. Applying a 20-hour rating to a discharge of a few minutes is a long extrapolation, so treat the result as an estimate and check the manufacturer’s runtime chart.
Ageing is handled by the health input. If a UPS that once ran for 30 minutes now manages 24, its battery is at about 80% health, and the same ratio applies at any load. For general battery runtime outside a UPS, our battery runtime calculator uses the same principles.
Should I Use VA or Watts to Work Out UPS Runtime?
Use watts: runtime depends on real power drawn from the battery, so a VA figure must be multiplied by the load’s power factor first. A UPS sold as 1,500 VA might be rated at 900 W, and both limits apply. Exceeding either can make the UPS shut down or refuse to switch to battery.
Real power is VA x power factor. A device drawing 500 VA at a power factor of 0.9 uses 450 W. The kVA to kW calculator does the same conversion for larger loads. The ups runtime calculator warns when your load is above the UPS watt rating, and the bar shows how much of the rating you are using.
How Can I Get Longer Runtime From a UPS?
The quickest way to double runtime is to roughly halve the load: put only the router, modem and essential devices on the battery-backed outlets and let the rest drop off. Other options are:
- Add an extended battery pack if your model supports one; enter it as an extra parallel string.
- Replace ageing batteries with the type the manufacturer specifies.
- Keep the UPS within the operating temperature and ventilation conditions in its manual.
- Use the UPS software to shut computers down cleanly before the battery runs out.
For longer outages, a portable power station or home battery holds far more energy; the power station runtime calculator estimates those. Hardwired UPS units, and any change to fixed wiring, must be installed by a qualified electrician; in the UK the work must meet BS 7671. Lead-acid batteries should be replaced and recycled by following the manufacturer’s instructions.
Worked Example: Keeping Broadband and a Laptop Dock Online
Example: a home office in Galway runs a router, an ONT, a small network switch and a laptop dock, measured together at 60 W with a plug-in power meter. The UPS is rated 900 W and has two 12 V 7 Ah lead-acid blocks, rated at 20 hours, which are three years old and estimated at 80% health. Inverter efficiency is taken as 85% as an example value.
- Pack: 12 V x 2 = 24 V, 7 Ah, 168 Wh.
- Battery current: 60 / 0.85 / 24 = 2.94 A.
- Simple estimate: 7 / 2.94 x 0.80 = 1.90 h, about 1 h 54 min.
- Peukert, k = 1.15: 20 x (7 / (2.94 x 20))^1.15 x 0.80 = 1.38 h, about 1 h 23 min.
Removing a 150 W desktop from the same UPS is what makes this possible; at 210 W the same batteries would last about 19 minutes.
Frequently Asked Questions
How do I calculate UPS battery backup time?
Multiply battery voltage by Ah to get watt-hours, multiply by inverter efficiency and battery health, then divide by the load in watts. Two 12 V 9 Ah blocks give 216 Wh; at 85% efficiency and 300 W that is 0.61 h in theory. For lead-acid, apply Peukert’s law for a more realistic figure.
How long does a 1500 VA UPS last?
It depends on the batteries inside and the load, not on the VA rating. A 1,500 VA unit rated at 900 W with two 12 V 9 Ah blocks lasts roughly 6 minutes at full load and about 22 minutes at 300 W, based on this ups runtime calculator’s Peukert estimate with new batteries.
Does a UPS last longer with a smaller load?
Yes, and more than in proportion. Halving the load roughly halves the battery current, and a lead-acid battery delivers more of its capacity at lower current. In the table above, going from 300 W to 100 W raises runtime from about 22 to about 77 minutes.
Why is my UPS runtime shorter than when it was new?
Batteries lose capacity as they age, so the same load drains them sooner. If runtime has dropped from 30 to 20 minutes, the battery is at about two thirds of its original capacity. Enter that as battery health, and consider replacing the batteries with the type the manufacturer specifies.
Can I add a bigger battery to a UPS for longer runtime?
Only in the way the manufacturer allows. Some models accept extended battery packs, which you can enter as extra parallel strings. Connecting larger or different batteries puts the charger and batteries outside what the unit was designed for, so follow the model’s manual or ask the manufacturer.
What Peukert exponent should I use for a lithium UPS?
Use 1 unless the manufacturer gives another figure. Peukert’s law and its published exponents, typically 1.1 to 1.3, describe lead-acid batteries. With an exponent of 1 the calculator gives the simple energy estimate, which you can then compare with the runtime chart the manufacturer publishes for your model.
Checked October 2026 by the Solaxyra Editorial Team. Sources: Wikipedia: Uninterruptible power supply, Wikipedia: Peukert’s law, CyberPower: manufacturer runtime charts.