Estimate how long a battery can power an appliance using wattage, battery voltage, amp-hours, and inverter efficiency.
Check the appliance label, power supply, or a plug-in power meter. If you are not sure, start with a preset below.
12 V is common for cars and RVs, 24 V for larger systems, and 48 V for solar battery banks.
Use the rated amp-hour capacity. For batteries in parallel, add the Ah values.
Use 100% for direct DC loads. For AC inverters, typical values are 85% to 95%.
Use 50% for lead-acid if you want to preserve the battery, 80-90% for many lithium batteries, or the limit recommended by the manufacturer.
With 100 W on a 12 V / 100 Ah battery, using 80% of capacity, estimated usable runtime is 8.64 h.
Estimated current draw: 9.26 A. Make sure your battery and cables can safely handle this continuous load.
Formula: runtime (h) = V × Ah × depth of discharge × efficiency ÷ W.
Battery used
Battery remaining
Estimated runtime
Energy delivered
10%
90%
52 min
0.09 kWh
20%
80%
1.73 h
0.17 kWh
30%
70%
2.59 h
0.26 kWh
40%
60%
3.46 h
0.35 kWh
50%
50%
4.32 h
0.43 kWh
60%
40%
5.18 h
0.52 kWh
70%
30%
6.05 h
0.60 kWh
80%
20%
6.91 h
0.69 kWh
90%
10%
7.78 h
0.78 kWh
100%
0%
8.64 h
0.86 kWh
Quick presets
Use this free battery duration calculator to estimate how long a battery will run an appliance, inverter, router, laptop, fridge, light, CPAP machine, or small off-grid setup.
Enter the appliance wattage, battery voltage, battery capacity in Ah, and the inverter efficiency. The calculator shows estimated runtime, battery remaining, current draw, and energy delivered in kWh.
This is useful for solar battery banks, RVs, boats, emergency backup power, camping power stations, and simple DIY electronics planning.
Battery runtime formula
The calculator uses this formula:
runtime hours = battery voltage × battery Ah × depth of discharge × efficiency ÷ appliance watts
Example: a 12 V 100 Ah battery with 90% inverter efficiency stores about 1.08 kWh of usable energy. A 100 W appliance would run for about 10.8 hours before full discharge.
What values should I enter?
Watts (W): the power used by the device. Check the label, power supply, manual, or use a plug-in power meter.
Battery voltage (V): common systems are 12 V, 24 V and 48 V.
Battery capacity (Ah): the amp-hour rating printed on the battery.
Efficiency (%): use 100% for direct DC loads, or 85-95% for most AC inverters.
Important battery notes
Lead-acid batteries should usually not be discharged to 0%. Lithium batteries often allow deeper discharge, but you should still follow the battery manufacturer's limits. The result is an estimate; real runtime changes with battery age, temperature, inverter idle draw, startup surge, and how stable the appliance load is.
