UPS Battery Sizing Calculator
Battery amp-hours for a load and runtime target
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UPS Battery Sizing Calculator is built for battery amp-hours for a load and runtime target — fast, free, and private. Just enter Load (W), Backup Time (minutes) and Battery Bank Voltage (V); UPS Battery Sizing updates the result as you type. UPS Battery Sizing ties each output to its formula. Anyone comparing options in Engineering can use UPS Battery Sizing to compare scenarios. Nothing typed into UPS Battery Sizing leaves your device or the tab. UPS Battery Sizing sits in the Engineering section next to free online ups battery sizing calculator and ups battery sizing calculator ah runtime backup load inverter. UPS Battery Sizing goes from question to answer fast. UPS Battery Sizing is free, instant and local.
What the defaults need
A 600 W load held for 30 minutes, from a 12 V battery bank through an 85% efficient inverter, needs about 29.4 Ah of battery capacity.
That is the theoretical figure at the 20-hour discharge rate. A battery asked for its whole capacity in half an hour delivers far less than its label claims, so the real requirement is larger.
The formula
Ah = (load in W × hours) ÷ (battery volts × inverter efficiency).
600 W × 0.5 h = 300 Wh of energy. 300 ÷ (12 × 0.85) = 300 ÷ 10.2 = 29.4 Ah.
Worked example
Load 600 W
Runtime 30 min = 0.5 h
Energy = 300 Wh
Battery at 12 V, 85% efficient → 10.2 Wh per Ah
300 ÷ 10.2 = 29.4 Ah
Why the real battery is bigger
- Peukert effect. A lead-acid battery rated 100 Ah at the 20-hour rate might deliver only 55–65 Ah at a one-hour rate. Drawing 29.4 Ah in half an hour is a high rate.
- Depth of discharge. Flooded lead-acid should not go below 50%, gel to about 60%, lithium to 80–90%. Doubling the figure is the practical rule for lead-acid.
- Age and temperature. Capacity falls with cold and with age; a three-year-old battery may hold 70% of its rating.
- Inverter idle draw. 10–20 W of standby consumption over the runtime is small here but not zero.
Sizing for this example
A 29.4 Ah theoretical draw, run through 50% depth of discharge and a Peukert derate of about 1.3 at this rate, points to roughly 75–80 Ah of lead-acid, or about 35 Ah of lithium where the discharge curve is flatter. Always check the manufacturer’s discharge table at your target runtime rather than scaling the 20-hour rating.
What this does not cover
Recharge time, charger sizing, cable and fuse ratings, and the surge current of a motor load are all outside this calculation. A 600 W load with a 1,800 W startup surge needs an inverter and wiring sized for the surge.
Frequently Asked Questions
Why 29.4 Ah and not 50 Ah?
Because the energy needed is only 300 Wh — 600 W for half an hour — and at 12 V with 85% efficiency that is 29.4 Ah. Longer runtimes scale linearly; an hour at 600 W needs 58.8 Ah.
Why does the real battery need to be bigger?
Peukert losses at high discharge rates and the depth-of-discharge limit both cut usable capacity. For lead-acid, plan on roughly double the calculated figure.
Does battery voltage change the amp-hours?
Yes, inversely. The same 300 Wh at 24 V needs 14.7 Ah rather than 29.4, because each amp-hour carries twice the energy.
What efficiency should I use?
85% is reasonable for a decent pure-sine inverter at moderate load. Cheap modified-sine units can be 70–75%, and efficiency drops sharply at very low load.
How long will this run a fridge?
A fridge drawing 600 W while running but cycling about a third of the time averages 200 W, so the same battery would last roughly 1.5 hours of real operation — but compressor startup surges are the limiting factor.
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