UPS Runtime Calculator
Estimate how long a UPS will keep your equipment running during a power cut.
How to use this tool
Enter battery voltage, capacity in Ah, number of batteries, your load in watts, and inverter efficiency. The result applies a depth-of-discharge limit and the efficiency loss to estimate runtime. Real runtime is lower when batteries are old or cold, so treat this as an upper bound.
Worked example
2 × 12V 9Ah batteries in series (24V, 9Ah) ≈ 216 Wh; at 80% usable and 90% efficiency, a 150 W load runs ≈ 1.04 h.
Frequently asked questions
Why is my real runtime shorter?
Battery age, temperature, a non-linear discharge curve (Peukert effect) and load spikes all reduce it.
VA or watts?
Size by watts of real load and keep it under about 70–80% of the UPS watt rating; VA is apparent power.
How long do UPS batteries last?
Typically 3–5 years for sealed lead-acid; replace on schedule.