Ask “how long will my UPS run?” and the honest answer is “it depends” — on the load, the battery, its age and its temperature. The one thing that is certain is that a battery delivers far less than its rated capacity at high discharge rates. This guide explains why, so the runtime number in our UPS calculator makes sense — and why we confirm the exact figure per model.
What determines runtime
Runtime is energy ÷ power: how much usable energy the battery holds, divided by how fast your load draws it, adjusted for inverter efficiency. Double the load and you roughly halve the runtime — but only roughly, because batteries do not discharge linearly.
That is why a UPS quoted at “up to X minutes” can disappoint: the headline figure is usually at a light load. At your real load it will be shorter.
The Peukert effect — the non-linear bit
Peukert’s law describes how a battery’s usable capacity falls as the discharge current rises. For valve-regulated lead-acid (VRLA) batteries the exponent is around 1.25, so a battery rated for a 20-hour discharge can deliver under half that capacity when emptied in an hour or two — exactly the regime a UPS runs in.
The practical consequence: doubling the load cuts runtime by more than half. Our calculator applies a Peukert correction so its estimate is realistic rather than the optimistic straight-line figure, and it plots the curve so you can see the effect.
Lithium-ion batteries have a much smaller Peukert effect (exponent near 1.05), holding their rated capacity better at high load — one of several reasons they are taking over. See VRLA vs lithium-ion.
Internal batteries vs extended battery modules
A standalone UPS gives short runtime — typically 5–15 minutes at half load — because the internal battery is small. To run longer you add extended battery modules (EBMs): each adds a fixed amount of energy, so two EBMs roughly triple the runtime of the base unit.
This is why one “exact runtime” per model name is misleading: the same UPS can run 6 minutes or 60 depending on how many EBMs are fitted. Tell us your target and we size the battery string to hit it.
Age and temperature steal capacity
VRLA batteries lose capacity as they age and are usually replaced every 3–5 years; a 4-year-old battery may hold only 80% of its original capacity, so a UPS that gave 15 minutes when new may give 10–12.
Heat is the biggest enemy: battery life roughly halves for every 10°C above 20–25°C. A hot comms cupboard quietly destroys runtime — another reason to size cooling properly with the cooling calculator.
How to get the runtime you need
Decide the target first (bridge-to-generator vs full graceful shutdown — see what size UPS do I need), then choose a UPS plus enough battery to hit it with margin for ageing.
Use the UPS calculator to estimate runtime for your load, then let us confirm the exact minutes for the specific Eaton or APC model and battery configuration — including EBMs and replacement schedule.