UPS Runtime Calculator

Estimate how long a UPS will keep a load running from its battery voltage, amp-hours and inverter efficiency. Calculates in your browser.

Estimated runtime

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Total minutes-
Battery energy-
Usable energy-

Show the math

Battery energy is voltage × amp-hours × battery count; runtime is the usable share divided by the load:

runtime = (V × Ah × n × DoD × efficiency) ÷ load (W)

This is an estimate, real runtime is shorter under heavy load (Peukert's effect) and falls as batteries age. Lead-acid banks are often sized for about 50% depth of discharge to protect their life. Calculated entirely in your browser.

What this does

A UPS runtime calculator estimates how long a UPS keeps a load running from its battery voltage, amp-hours and inverter efficiency.

How to use it

  1. Enter battery voltage, amp-hours and count.
  2. Enter the load in watts.
  3. Set depth of discharge and efficiency.
  4. Read the runtime.

How it works

Real runtime is shorter under heavy load (the Peukert effect) and as batteries age, so plan for less than the best case.

runtime = (V × Ah × DoD × efficiency) ÷ load watts

Understanding your result

This is a best-case runtime; real backup time is shorter under heavy load (the Peukert effect) and shrinks as batteries age, so plan for noticeably less than the figure shown. A UPS is meant to ride out brief outages or allow a clean shutdown, not to run for hours, so size it to your actual graceful-shutdown time plus a safety margin.

Example

A 12 V 100 Ah battery at 90% efficiency runs a 300 W load for about 3.6 hours.

Sources & methodology

Last updated .

Frequently asked questions

How is UPS runtime calculated?

Battery energy is voltage × amp-hours × number of batteries. Multiply by the usable depth of discharge and inverter efficiency, then divide by the load in watts. A 12 V 100 Ah battery at 90% efficiency runs a 300 W load for about 3.6 hours.

Why is real runtime shorter?

Batteries deliver less than their rated capacity under heavy load (the Peukert effect), lose capacity as they age, and should not be fully discharged, so plan for less than the calculated best case.

What depth of discharge should I use?

Lead-acid banks are often sized for about 50% depth of discharge to protect battery life, while lithium can go deeper. Set the figure to match your battery chemistry.