How to use it
- Enter the voltage and Ah of one battery and the number of batteries (UPS packs are wired in series: two 12 V batteries make 24 V).
- Enter the load in watts, or choose VA and give the power factor.
- Keep or change the inverter efficiency (90 % is a typical starting value) and the usable depth of discharge.
- Read the runtime in minutes and hours, the stored and usable energy and the current the batteries supply.
Formula
- Stored energy E = V × Ah × n (Wh)
- Usable energy = E × DoD × η
- Runtime t = usable energy / P (hours), × 60 for minutes
- Load in watts P = VA × PF when given in VA
- Battery current I = P / (η × V × n)
Worked example
A small UPS has two 12 V 9 Ah batteries and powers a 200 W load. The stored energy is 12 × 9 × 2 = 216 Wh. Using 80 % of it with a 90 % efficient inverter leaves 216 × 0.8 × 0.9 = 155.52 Wh for the load, so the runtime is 155.52 / 200 = 0.78 h ≈ 46.7 minutes. The batteries supply 200 / (0.9 × 24) = 9.26 A.
A 500 VA load at power factor 0.6 is 300 W; on four 12 V 7 Ah batteries (336 Wh, 85 % efficiency, 80 % DoD) it runs about 45.7 minutes.
Tips
- Halving the load more than doubles the runtime in practice, because batteries deliver more of their capacity at lower current.
- Shut down gracefully. Plan the runtime you need to save work and power off, plus a margin, not the theoretical maximum.
- Replace old batteries. Sealed lead-acid UPS batteries lose capacity over a few years; a runtime test tells you when.