Most UPS calculators divide the battery's watt-hours by the load and stop there. For a lead-acid battery drained in minutes, that answer is often twice the real one, because the battery's capacity is measured over twenty hours and it gives much less when it is emptied fast.
This UPS calculator does the sum the battery makers' own numbers support: the load, the inverter's losses, the current that leaves the battery, and Peukert's law for what a lead-acid battery gives at that current. The simple sum is printed beside it so you can see the gap, every figure it supplies names its source, and the list of what it cannot tell you is below the answer.
In this guide
The UPS calculator
Add up what is plugged into the battery outlets: the PC, the monitor, the router. A plug-in power meter gives the real figure; the PSU calculator's total is a PC's peak, which is the cautious one.
The W figure, not the VA one: a box that says 1500VA/900W is 900 here.
Most desk-size UPSes run one or two 12 V batteries. The spec sheet or the replacement battery's label says how many.
The Ah figure on each battery, such as 12V 9Ah. Batteries in series share one figure; do not add them up.
100 for a new battery. IEEE 1188 counts a sealed lead-acid battery as worn out at 80.
How that is worked out, line by line
- Energy on the battery's label216 Wh
- Power taken from the battery235 W
- Current from the battery9.8 A
- Capacity left at that battery health9.0 Ah
- Load as a share of the UPS's watt rating22 %
- What label energy divided by the load would say55 min
On those numbers you are in the 15 minutes to an hour band, which is the section below.
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Where every number here comes from
No part of this was measured on a bench here. This site works from published specifications and from what owners report, and every number below is either one you typed in or a published figure with its source named next to it.
The figures this page supplies
- Share of the battery's power that reaches the outlets: 85 %. An allowance, not a measurement: consumer UPS makers do not publish their inverter's efficiency on battery. It is set so that, with Victron's exponent below, this page's answers for CyberPower's CP1500PFCLCD land either side of CyberPower's own published runtimes (the last two lines): short at half load, long at full load.
- Peukert exponent for a lead-acid battery: 1.2500. Victron Energy's battery monitor manual: 1.25 is its default and 'an acceptable average value for most lead acid batteries'. The higher it is, the more capacity a battery loses when drained fast.
- Hours a battery's Ah rating is measured over: 20 h. Sealed lead-acid batteries of the kind UPSes use are rated at the 20-hour rate: Yuasa's NP7-12 is 7 Ah when drained over 20 hours, and gives less when drained faster.
- CyberPower's published runtime for the CP1500PFCLCD at half load: 10 min. CyberPower's own figure for its 1000 W unit with two 12 V 8.5 Ah batteries carrying 500 W. Not used in the arithmetic; it is the check on it.
- CyberPower's published runtime for the CP1500PFCLCD at full load: 2.5000 min. CyberPower's own figure for the same unit carrying its full 1000 W. Not used in the arithmetic.
Why each step is the step it is
- Energy on the battery's label:
batt_v * batt_ah. Volts times amp-hours. This is the figure most runtime calculators start and finish with. - Power taken from the battery:
load_w / (eff_pct / 100). The inverter turns battery power into mains power and loses some on the way, so the battery supplies more than the outlets deliver. - Current from the battery:
draw_w / batt_v. Power divided by voltage. This current, not the wattage, is what decides how much of its rated capacity a lead-acid battery gives. - Capacity left at that battery health:
batt_ah * health_pct / 100. The label's Ah figure scaled by the health you gave it. A battery at 80% holds four fifths of what it held new. - Load as a share of the UPS's watt rating:
load_w / ups_w * 100. Over 100% the UPS cannot carry it on battery at all. Close to 100% the battery is drained fastest, which is where this arithmetic is least reliable (see the limits below). - What label energy divided by the load would say:
cap_ah * batt_v * eff_pct / 100 / load_w * 60. The simple sum: watt-hours times efficiency over watts. It assumes the battery gives its full 20-hour capacity however fast it is drained, which a lead-acid battery does not, so it runs long. - Minutes on battery:
rate_h * Math.pow(rate_ratio, peukert_k) * 60 + valid. Peukert's law: a battery rated C amp-hours over H hours lasts H times (C over current times H) to the power k when drained at that current. With k above 1, draining it faster takes more than its share of the capacity away.
What this cannot tell you
Every calculator has an edge, and a page that hides its edge is worse than no page at all. Here is this one's.
- It is arithmetic on published figures. No UPS or battery was run down for it, and this site has never had any of these units in its hands.
- Near the UPS's full rating the answer runs long: Victron's manual warns that a battery drained very fast gives even less than Peukert's law predicts, and CyberPower publishes a shorter full-load runtime than this page works out. Under about a quarter of an hour, plan on less than it says.
- At very light loads the UPS's own electronics take a share of the battery that this does not count, so a router-only runtime of many hours will come out shorter in practice.
- Battery health is a guess unless the UPS reports it. Lead-acid batteries lose capacity with age and with heat, and a UPS that has sat in a warm cupboard for years may hold well under its label.
- A PC's draw moves from second to second. Gaming pulls far more than an idle desktop, so put in the figure for what it will be doing when the power goes, or its peak to be safe.
- It covers lead-acid batteries, which nearly every desk-size UPS uses. A lithium battery loses much less to fast draining, and this exponent would make its runtime look shorter than it is.
What that runtime is good for
These follow from the answer above and nothing else. Each heading below covers one span of results, so read the one the calculator pointed you at and ignore the rest.
Under 5 minutes
Enough to save and shut down, if you are at the desk when the power goes. Set the UPS's software to shut the PC down by itself, because nothing else will happen in time. This site has not covered a UPS yet, so it is not going to name one.
5 to 15 minutes
Enough to finish what you are doing and shut down cleanly, and to ride out the flickers and short cuts that make up most outages. Near the bottom of this range, plan on less than the number says: a battery drained fast gives even less than the arithmetic predicts.
15 minutes to an hour
Enough to keep working through a short outage, or to keep the router up while you decide what to do. A battery that has aged will move you down a band, so check the health box against what the UPS reports.
More than an hour
A light load on a large battery, such as a router and a modem on their own. At loads this light the UPS's own electronics take a share this page does not count, so the real figure will be shorter than it says.
Best Gaming Router Under $200, 5 Honest Picks for 2026 covers this range in full.
Past what this calculator covers
Either the load is more than the UPS's watt rating, which it cannot carry on battery at all, or it is so light that the battery would drain slower than its 20-hour rating, where this arithmetic stops. A bigger UPS fixes the first; for the second, go by the maker's runtime chart.
If you do not know what your PC draws, the PSU calculator adds up its peak from the parts. That is the cautious figure to put in here, since a PC at peak empties a battery fastest. A UPS calculator can only be as good as the load you give it.
Questions about the UPS calculator
How long will a UPS run my PC?
Work out the current the load draws from the battery, then apply Peukert's law to the battery's rated capacity. The example this UPS calculator opens with is a 200 W load on a 900 W UPS with 24 V of 9 Ah batteries. Allowing 85% for the inverter, the battery supplies 235 W, which is 9.8 A, and that gives about 25 minutes. Halve the load to 100 W and it gives 61 minutes, more than twice as long, because a battery drained more slowly gives more of its capacity.
Why does a UPS run for less time than its battery's watt-hours suggest?
Because a lead-acid battery's Ah figure is measured over 20 hours, and a UPS drains it in minutes, which gives far less. The simple sum, label energy times 85% over the load, says 55 minutes for the example on this page; Peukert's law, with Victron's published exponent of 1.25 for lead-acid, says 25. The gap grows with the load: at 600 W the simple sum says 18 minutes and Peukert's law 6.
What size UPS do I need?
One whose watt rating is at least your load, with batteries big enough for the minutes you want. The watt figure is the one that counts: when a box lists a VA figure and a smaller W figure, the W figure is what it carries, and Schneider Electric's white paper on watts and volt-amps explains why. The example's 200 W is 22% of a 900 W UPS. Put in more than the watt rating and this UPS calculator refuses, because the UPS would not carry it on battery at all.
How much runtime does an old UPS battery lose?
More than its lost capacity alone suggests. IEEE 1188 counts a sealed lead-acid battery as worn out at 80% of its rated capacity, and on the example this page opens with that cuts the runtime from 25 minutes to 19. Peukert's law scales the runtime by the capacity to the power 1.25, so a fifth less capacity costs about a quarter of the minutes. If the UPS reports its battery health, put that figure in the box.
How close is a UPS runtime calculator to the maker's own figures?
Close enough to plan with, not to the minute. CyberPower publishes 10 minutes for its CP1500PFCLCD carrying 500 W, half its rating, on 24 V of 8.5 Ah batteries; this page works out 8, on the short side. At the full 1000 W CyberPower publishes 2.5 minutes and this page 3, on the long side, because a battery drained that fast gives even less than Peukert's law predicts. Near a UPS's full rating, go by the maker's runtime chart.
Sources
- Victron Energy: battery capacity and the Peukert exponent (BMV-712 Smart manual): Sets out Peukert's law, the 20-hour (C20) rating it works from, and 1.25 as the default exponent for lead-acid batteries. It also warns that at very high currents a battery gives even less than the law predicts.
- Yuasa NP7-12 sealed lead-acid battery specification sheet: A typical UPS battery, rated in Ah at the 20-hour rate, which is the rating this page's Ah box takes.
- CyberPower CP1500PFCLCD product page: The maker's published runtimes at half and full load for a 1000 W unit with two 12 V 8.5 Ah batteries, which this page's answers are set against.
- IEEE 1188: maintenance, testing and replacement of VRLA batteries: Recommends replacing a sealed lead-acid battery once it holds less than 80% of its rated capacity, which is the worn-out end of the health box.
- Schneider Electric white paper 15: Watts and Volt-Amps, Powerful Confusion: Explains why a UPS's watt rating, not its larger VA figure, is the one a computer's load has to fit under.
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