A battery label in amp-hours does not by itself tell you how long an appliance will run. You also need the battery-bank voltage, the amount of charge you plan to use, the power consumed by the load, and losses between the battery and the device. Treat the result as a planning estimate and check equipment ratings before connecting anything.
Convert the battery label into energy
Nominal energy in watt-hours is approximately voltage × amp-hours. A 12 V, 100 Ah bank represents about 1,200 Wh nominally. A 24 V, 100 Ah bank represents about 2,400 Wh. Comparing the two by “100 Ah” alone hides a twofold difference in energy. Enter the voltage of the full bank, not the voltage printed on one battery if multiple batteries are wired in series. Victron Energy’s battery bank guide explains how series and parallel connections affect voltage and capacity; its guidance applies to system wiring and does not turn a runtime estimate into a safety assessment.
If you need to compare possible series and parallel arrangements, use the Battery Series Parallel Calculator. Its energy result helps compare arrangements; it does not establish wiring safety or compatibility.
Find usable energy and account for losses
Suppose the 12 V, 100 Ah bank can supply 80% of nominal energy within your selected operating range, and the inverter delivers 85% of battery-side energy to the AC load. Modeled usable AC energy is 1,200 × 0.80 × 0.85 = 816 Wh. A steady 200 W load gives 816 ÷ 200 = 4.08 hours, about 4 hours 5 minutes. That is a modeled result, not a guarantee: battery condition, temperature, discharge rate, inverter standby draw and changes in the appliance load can shorten actual time.
Run the complete case in the Inverter Battery Backup Time Calculator. For a DC load without an inverter, use the 12V Battery Runtime Calculator and enter losses appropriate to that delivery path. Avoid applying inverter losses twice.
Check watts and VA separately from hours
Enough stored energy does not prove an inverter or UPS can power the connected equipment. A device may draw a much higher startup surge than its normal running watts; an inverter also has a continuous power rating, and a UPS often displays a VA rating alongside its watt limit. Check the manufacturer specifications for the actual model. The UPS Battery Backup Time Calculator estimates runtime, while hardware ratings still determine whether a load is supported.
Size from the other direction
If you know the appliance load and required operating time, start with the energy demand. A 200 W load for four hours uses 800 Wh at the appliance. With 85% delivery efficiency and an 80% usable battery fraction, the nominal battery requirement is 800 ÷ (0.85 × 0.80) ≈ 1,176.5 Wh, before an additional planning reserve. At 12 V that is about 98.1 Ah nominal. The Inverter Battery Capacity Calculator handles the sizing direction and reserve explicitly.
Write down the voltage, battery capacity, charge range, continuous watts, startup demand and conversion assumptions used. A traceable estimate is more useful than a precise-looking number with hidden inputs. For critical or safety-related installations, follow the equipment manuals and a qualified installer’s guidance.
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