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Battery Runtime

UPS Runtime Calculator

A UPS calculator for battery backup time: estimate how long your uninterruptible power supply keeps a computer, router or NAS running when the power goes out. Enter the UPS battery size and what is plugged into it.

Updated · Free, no sign-up · Runs in your browser

Quick answer

A 1,000 VA UPS with one 12 V 9 Ah battery runs a 100 W load for about 28 min and a 20 W router and modem for about 2 h 57 min. Runtime falls faster than the load rises, because small lead-acid batteries lose capacity at high currents.

Calculate UPS runtime

Ah
V
Wired in
W

Add up the watts of everything running at the same time.

Advanced settings
%
%
%

Battery type fills in typical values. Peukert's exponent models how lead-acid loses capacity at high loads (1.0 = no loss). Health is the share of its original capacity an older battery still holds. How these work.

Estimated runtime

28 min

≈ 0.46 hours on 108 Wh at 100 W

Usable energy
86 Wh
Battery current
9.8 A
Power from battery
118 W
Discharge rate
1.09C
  • This load pulls 1.1C from the battery. Lead-acid batteries deliver noticeably less than their rating at this rate, and the voltage may sag below the inverter cut-off early.
  • Draining lead-acid past 50% shortens its life. 80% is fine for occasional outages but not for daily cycling.

How this was worked out

  1. Battery energy: 108 Wh rated
  2. Power drawn from the battery: 100 W ÷ 85% efficiency = 118 W
  3. Rate effect (Peukert 1.15): the battery delivers 63% of its rating at this load
  4. Runtime: 108 Wh × 63% × 80% usable ÷ 118 W = 0.46 h

Runtime at other loads

LoadRuntime
25 W2 h 17 min
50 W1 h 2 min
100 W28 min
150 W17 min
200 W13 min

UPS runtime chart

Estimated runtime for common UPS battery setups, using sealed lead-acid (AGM) batteries in good condition, 85% inverter efficiency, and the UPS shutting down at about 80% of the battery’s capacity.

UPS battery50 W100 W200 W400 W
1 × 12 V 7 Ah (small 600–850 VA UPS)46 min21 min9 min4 min
1 × 12 V 9 Ah (1000 VA)1 h 2 min28 min13 min6 min
2 × 12 V 9 Ah (1500 VA)2 h 17 min1 h 2 min28 min13 min
2 × 12 V 18 Ah (2000–3000 VA)5 h 3 min2 h 17 min1 h 2 min28 min

Doubling the load more than halves the runtime. A 7 Ah battery at 400 W is discharging at several times its rated current, and lead-acid delivers far less of its rated capacity that fast.

How to find your UPS load in watts

  • Read the UPS display or software. Most units show the load in watts or as a percentage of capacity. Multiply the percentage by the UPS watt rating (not the VA).
  • Use a plug-in power meter on each device for a few minutes of normal use.
  • Estimate from typical values: a cable modem and Wi-Fi router draw 10–25 W together, a laptop 30–90 W, an office desktop with monitor 100–250 W, and a gaming PC 300–600 W under load.

How UPS runtime is calculated

The calculator turns the battery into watt-hours (Ah × V × number of batteries), takes off the inverter loss inside the UPS, and applies Peukert’s law for sealed lead-acid batteries. That last step matters more for a UPS than for almost any other battery: UPS batteries are small and the loads are large compared to them, so they routinely discharge in 10 to 30 minutes, where a lead-acid battery gives well under its 20-hour rating. The full method is on how battery runtime is calculated.

Planning a bigger backup for a home office or network rack? Use the battery size calculator to work out the capacity for the hours you need, or compare with a portable power station.

Frequently asked questions

How long will a UPS last during a power outage?

It depends on the battery and the load. A typical 1,000 VA UPS with one 12 V 9 Ah battery runs a 100 W load (a desktop PC at idle, or a router plus a laptop) for about 28 min, and a 20 W router alone for 2 h 57 min. Desktop UPS units are built to give you time to save work and shut down, not to ride out long outages.

How do I find my UPS battery capacity?

The replacement battery label shows voltage and amp-hours, for example 12 V 9 Ah. Check the UPS manual or the battery compartment, and count the batteries: units of 1,500 VA and up usually have two 12 V batteries in series (24 V).

Is the VA rating the same as runtime?

No. VA and watts are the maximum load the UPS can power, not how long it runs. Two UPS units with the same VA rating can have very different batteries and runtimes. Runtime comes from the battery capacity and your actual load.

Why does my UPS run for less time than the calculator shows?

UPS batteries are small sealed lead-acid batteries that wear out in 3 to 5 years, and capacity drops long before they fail completely. Set battery health under Advanced settings to 70–80% for a battery that is a few years old. High loads also cut lead-acid capacity sharply, which the calculator models with Peukert’s law.

How can I get more runtime from a UPS?

Plug only the essentials into the battery-backed outlets (the router, modem and the computer itself, not printers or speakers), replace old batteries, or choose a UPS that accepts an external battery pack. For hours of runtime, a LiFePO4 power station or a home inverter suits better than a desktop UPS.