How it's calculated
Battery current = Load W ÷ Efficiency ÷ Battery bank voltage
Runtime = 20 × (Ah ÷ (Current × 20)) ^ k × Usable fraction
UPS batteries are discharged quickly, often in 5–30 minutes, so Peukert's law is used to allow for the lower capacity lead-acid batteries give at high current. The batteries in most UPS units are wired in series, so their voltages add up.
Worked example
A home office UPS with two 12 V 9 Ah batteries (24 V) running a 300 W load at 85% efficiency draws about 14.7 A. That gives roughly 22 minutes, well short of the 37 minutes a simple Ah ÷ A calculation would suggest.
Tips
- UPS batteries typically last 3–5 years. Runtime falls steadily as they age.
- Only plug essential equipment into the battery-backed sockets.
- For long outages, a larger external battery bank or a power station with a pass-through/UPS mode gives far more runtime.
Frequently asked questions
How long will a 1500VA UPS last?
It depends on the battery, not the VA rating. A typical 1500 VA UPS with two 12 V 9 Ah batteries runs a 300 W load for about 15–20 minutes.
What's the difference between VA and watts?
VA is apparent power. Watts are real power. For computer equipment, watts are roughly 0.6–0.9 × VA. UPS units list both.