What this calculator works out
This calculator divides a UPS battery's usable energy by the load it supports, applying an efficiency factor, and returns the runtime in minutes.
UPS runtime is not linear with load. Doubling the load more than halves the runtime, because batteries deliver less total energy when discharged faster.
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Before you rely on a figure
- Battery capacity in watt-hours is sometimes published and sometimes has to be derived from the battery voltage and amp-hour rating.
- Load should be the actual measured draw, not the equipment's nameplate rating.
- 85% efficiency covers inverter losses. It does not cover the non-linear discharge effect, which makes real runtime at high loads shorter than this suggests.
- Batteries lose capacity with age. A three-year-old UPS delivers noticeably less than its specification.
The purpose of most small UPS installations is a clean shutdown rather than continued operation. Five minutes is enough to close files and power down safely; running through a long outage needs a generator. Sizing for graceful shutdown is cheaper and more achievable than sizing for endurance, and it prevents the failure a UPS actually exists to prevent — filesystem and database corruption from an abrupt loss of power.
How this calculator works
Energy divided by power:
Hours = battery watt-hours × (efficiency ÷ 100) ÷ load in watts
Minutes = hours × 60The calculation assumes constant discharge efficiency, which overstates runtime at high loads. Peukert's effect means a battery discharged quickly delivers less total energy than the same battery discharged slowly — so a UPS rated for 30 minutes at half load will give considerably less than 15 at full load.
Manufacturers publish runtime curves rather than a single figure for exactly this reason, and those curves are more reliable than any calculation.
Worked example: a 500 Wh UPS
Using the default figures — 500 Wh of battery, a 120 W load, 85% efficiency:
- Estimated runtime: 213 minutes
Three and a half hours, which is far longer than most small UPS units achieve — and that is the calculation's limitation showing. At a 120 W load a 500 Wh battery is being discharged gently, so the linear figure is closer to reality here than it would be at 400 W. Treat the number as an upper bound and check the manufacturer's runtime curve, which accounts for the discharge behaviour this arithmetic ignores.
Common mistakes
- Treating the figure as reliable at high loads. Real runtime falls faster than linearly.
- Using nameplate power for the load. Measure it.
- Confusing VA with watts. UPS ratings in VA exceed the watts they can actually supply.
- Never testing. A UPS with a failed battery reports healthy until the power goes.
- Sizing for endurance rather than shutdown. Running through an outage needs a generator.
Frequently asked questions
What is the difference between VA and watts?
Volt-amperes measure apparent power and watts measure real power. A UPS rated 1,500 VA might supply only 900 W, the ratio being its power factor. Sizing from the VA figure overstates what the unit can actually support, and it is the most common UPS sizing error.
How long do UPS batteries last?
Typically three to five years for sealed lead-acid, less in a warm environment — heat is the main determinant. Capacity declines throughout that period rather than failing suddenly, so a five-year-old UPS may deliver a fraction of its original runtime while reporting itself healthy.
Should I test the UPS?
Yes, regularly. A self-test confirms the unit switches over; a full runtime test under real load confirms the battery still holds charge. Many organisations discover a failed battery only during an actual outage, which is the one occasion it matters. Scheduling a test is the difference between a UPS and an assumption.
Do I need a UPS at home?
For a desktop computer or a NAS, it prevents corruption from abrupt power loss, which is a real risk for filesystems and databases. For a router and a few small devices it keeps connectivity through brief cuts. It is a modest cost against the alternative of a corrupted array, and the smallest units are inexpensive.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
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References
- Health and Safety Executive — electrical safety and safe handling of batteries
- National Cyber Security Centre — resilience and availability guidance for systems
Sources are checked at publication and can change — how I choose and check references.