How battery runtime is calculated
A battery's stored energy in watt-hours (Wh) is its voltage multiplied by its capacity in amp-hours (Ah). Not all of it is usable: lead-acid and AGM batteries are usually kept above about 50% charge to protect their life, while lithium iron phosphate (LiFePO4) batteries can typically use 80–100%. If the load runs on AC through an inverter, some energy is also lost in the inverter, commonly around 10%.
Battery energy (Wh) = volts × amp-hours × number of batteries
Usable energy = Wh × depth of discharge × inverter efficiency
Runtime (hours) = usable Wh ÷ load watts
Battery needed = load × hours ÷ depth of discharge ÷ inverter efficiency
This assumes batteries are connected so their energy adds up (two 12 V 100 Ah batteries hold 2,400 Wh whether wired in series as 24 V 100 Ah or in parallel as 12 V 200 Ah).
Worked example: lithium
Two 12 V, 100 Ah lithium (LiFePO4) batteries power a 300 W load through an inverter that is 90% efficient, using 90% of their capacity.
- Stored energy: 2 × 12 × 100 = 2,400 Wh.
- Usable: 2,400 × 0.9 × 0.9 = 1,944 Wh.
- Runtime: 1,944 ÷ 300 = 6 hours 29 minutes.
- To run 300 W for 8 hours you need about 2.96 kWh of battery, about 247 Ah at 12 V.
The same bank with lead-acid batteries
With lead-acid or AGM batteries limited to 50% discharge, the same 2,400 Wh bank gives only 1,080 Wh of usable energy and about 3 hours 36 minutes at 300 W. Eight hours would need about 5.33 kWh (444 Ah at 12 V). Lead-acid capacity also falls at high discharge rates and in the cold, so real runtime can be shorter still.
Measuring your load
Add up the watts of everything that will run at the same time: a router and modem, a few LED lights, a laptop, a CPAP machine, a TV or a refrigerator. Refrigerators and freezers cycle on and off, so their average draw over a day is much lower than their running watts; use the energy label's kWh per year ÷ 8,760 for an average. A plug-in power meter gives the most accurate figure.
UPS units and portable power stations
Power stations and solar generators list capacity in Wh. Enter the battery voltage and Ah from the specifications, or enter 1 V with the Wh figure as the Ah. Use 100% depth of discharge if the stated Wh is already the usable capacity, and add the inverter efficiency for AC loads. For a computer UPS, the stated VA rating is the maximum load, not the runtime; runtime comes from the battery's Wh.
Sizing for solar and off-grid
For an off-grid or backup solar system, size the battery for the energy you need overnight or through a cloudy day, then size the panels to recharge it; the solar panel calculator helps with the second step. Keep the current in mind too: a 1,200 W inverter on a 12 V battery draws over 100 A, which needs heavy cables and fuses (see the watts to amps calculator).
Charging the battery back up
Runtime is only half of a backup plan; the battery must also be recharged. Charging time is roughly the energy to replace divided by the charger's power, plus losses: refilling 1,944 Wh with a 500 W charger takes about four hours or a little more. Solar panels, a generator or a vehicle alternator can all recharge a bank, and lithium batteries usually accept a faster charge than lead-acid. Check that the charger matches the battery chemistry and voltage.
Common mistakes
- Using the full Ah rating of a lead-acid battery. Regularly draining it below about 50% shortens its life sharply.
- Adding volts and amp-hours incorrectly. Series adds voltage, parallel adds Ah; watt-hours add either way.
- Forgetting inverter losses and idle draw. Many inverters draw some power even with no load.
Estimates only. Follow the battery and inverter manufacturer's limits.
Frequently asked questions
How long will a 100Ah battery run a 300W load?
A 12 V 100 Ah battery holds 1,200 Wh. With 90% usable and a 90% efficient inverter, about 972 Wh, or about 3 hours 14 minutes at 300 W.
How do I calculate battery backup time?
Volts × Ah × depth of discharge × inverter efficiency ÷ load watts. Two 12 V 100 Ah lithium batteries run 300 W for about 6.5 hours.
What size battery do I need to run 300 W for 8 hours?
About 2.96 kWh of lithium at 90% usable and 90% inverter efficiency, or about 5.33 kWh of lead-acid at 50% usable.
How much of a lead-acid battery can I use?
About 50% is a common limit for long life; lithium iron phosphate batteries typically allow 80–100%.
Sources
- US Department of Energy, Energy Saver. Batteries for solar and backup power.
- Battery and inverter manufacturers' datasheets: usable capacity, depth of discharge and efficiency.
- National Renewable Energy Laboratory (nrel.gov). Behind-the-meter battery storage basics.
Last reviewed 27 October 2026. Results are estimates; see our disclaimer.