UPS Runtime Calculator
This UPS runtime calculator estimates how long an uninterruptible power supply can run your equipment on battery. Enter the battery voltage and amp hours, how many batteries are in series, the load in watts, and the inverter efficiency, and the UPS runtime calculator returns the backup time in minutes, the battery current, and the bank energy, with a correction for the capacity lead-acid batteries lose at high discharge rates.
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The UPS Runtime Calculator is part of our Electronics collection. Browse more electronics to size, convert, and check your work alongside this tool.
What the UPS runtime calculator does
A UPS draws current from its batteries through an inverter, and the current is high because the load runs for minutes, not hours. Batteries deliver less than their label capacity at high currents, so a simple watt hour division overstates runtime. The UPS runtime calculator finds the battery current, then applies Peukert's law to the 20 hour rated capacity to give a realistic backup time.
UPS runtime formula
C is the 20 hour rated capacity in Ah and k is the Peukert exponent: about 1.2 for sealed lead-acid and 1.05 for lithium. The simple estimate, bank Wh × efficiency ÷ load, is shown for comparison.
UPS runtime calculator example
A 1,500 VA UPS with two 12 V 9 Ah batteries feeding a 300 W load at 90% efficiency:
The simple watt hour estimate says 39 minutes, about twice the real figure, because a 9 Ah battery cannot deliver its full rating at almost 14 A.
How to use the UPS battery runtime calculator
Find the battery voltage and amp hours on the battery label, usually 12 V and 7 to 9 Ah in small units. Enter how many batteries are wired in series; a 24 V UPS has two. Enter the total load in watts, the inverter efficiency, and the battery type. Keep usable capacity at 100% for a UPS that runs to its cutoff, or lower it for an aged battery. Press Calculate.
Reading your UPS backup time calculator result
Treat the result as a new-battery figure at room temperature. Batteries lose capacity with age, often 20 to 40% after three years, and cold cuts it further. If you need a set runtime for a safe shutdown, size for at least 1.5 times that time. Halving the load more than doubles the runtime on lead-acid, so moving non-essential gear off the UPS pays off.
When to use a UPS backup time calculator
- Choosing a UPS for a computer or server
- Checking backup time for a router and modem
- Sizing an external battery pack
- Planning a safe shutdown window
- Comparing lead-acid and lithium UPS units
Common UPS runtime calculator mistakes
- Using VA as the load. Enter real watts.
- Dividing Wh by watts. That overstates lead-acid runtime.
- Ignoring battery age. Old batteries hold far less.
- Counting parallel batteries as series. Enter batteries in series; add parallel strings as more Ah.
Tips for using a UPS battery backup calculator
- Plug only critical gear into the battery outlets.
- Run a self-test every few months.
- Replace sealed lead-acid batteries every 3 to 5 years.
Peukert's law and UPS battery calculation
Battery capacity is rated over 20 hours, a gentle current of C ÷ 20. A UPS pulls many times that, and lead-acid chemistry cannot keep up, so usable capacity falls. Peukert's law models this with the exponent k: the higher k, the bigger the loss at high current. For loads that run for hours rather than minutes, the battery life calculator gives a simpler estimate. To convert the battery rating between amp hours and watt hours, use the amp hour calculator.
UPS battery size calculator: longer backup
To stretch runtime, add an external battery pack or move to a UPS with a higher bank voltage. Doubling the batteries more than doubles runtime on lead-acid, because each one runs at half the current. For backup measured in hours, such as a home office through a long outage, size a dedicated bank with an inverter using the battery bank calculator.
UPS runtime chart (sealed lead-acid, 90% efficiency)
| Load | 2 × 12 V 9 Ah | 2 × 12 V 18 Ah | 4 × 12 V 9 Ah |
|---|---|---|---|
| 100 W | 73 min | 168 min | 168 min |
| 200 W | 32 min | 73 min | 73 min |
| 300 W | 20 min | 45 min | 45 min |
| 450 W | 12 min | 28 min | 28 min |
| 600 W | 9 min | 20 min | 20 min |
| 900 W | 5 min | 12 min | 12 min |
UPS Runtime Calculator FAQ
How long will a UPS last?
It depends on the battery and load. Two 12 V 9 Ah batteries run a 300 W load for about 20 minutes.
How do you calculate UPS backup time?
Find the battery current from the load and bank voltage, then apply Peukert's law to the 20 hour capacity, as this UPS runtime calculator does.
Why is my UPS runtime shorter than the battery Wh suggests?
Lead-acid batteries deliver less capacity at high current, and the inverter loses about 10%. Old batteries lose more.
Does a bigger VA rating mean longer runtime?
No. VA sets the maximum load; runtime comes from battery capacity.