How UPS runtime is estimated
Divide the load by the battery voltage and inverter efficiency to find the battery current. Small sealed lead-acid batteries deliver far less than their rated amp-hours at high current (the Peukert effect), so we scale the 20-hour rating accordingly.
Battery current I = Load W ÷ (Battery V × Efficiency)
Runtime (h) = 20 × (Ah ÷ (I × 20))^1.2
Example: 300 W on a 1500 VA UPS with two 12 V 9 Ah batteries
I = 300 ÷ (24 × 0.9) = 13.9 A. Runtime = 20 × (9 ÷ 278)^1.2 = 0.33 h, about 20 minutes.
Sizing a UPS
- Keep the load under 80% of the UPS's watt rating, not just its VA rating.
- VA ≈ W ÷ power factor. Modern PC power supplies have a power factor of about 0.9–0.99.
- For longer runtime, add an external battery pack rather than a bigger UPS.
- Lead-acid UPS batteries typically last 3–5 years.
Frequently asked questions
How long will a 1500VA UPS last?
With two 9 Ah batteries, about 20 minutes at 300 W and 5–8 minutes at 700 W.
What is the difference between VA and watts on a UPS?
VA is apparent power; watts is real power. The UPS must meet both ratings for your load.
Why is runtime not linear with load?
Lead-acid capacity drops at higher discharge rates, so doubling the load more than halves the runtime.