Quick answer
Runtime (hours) = capacity (Ah) × voltage × usable % × inverter efficiency ÷ load (W). A 100 Ah 12.8 V lithium battery runs a 60 W load for about 17 hours through an inverter.
How to use the Battery Runtime Calculator
- Choose your battery type. This fills in a sensible usable capacity (depth of discharge), which you can change.
- Enter the battery voltage: 12.8 V for a 12 V LiFePO4 battery, 12 V for lead-acid, or the nominal voltage of your power station.
- If you power 230 V/120 V devices through an inverter, keep the inverter option on.
- Enter the battery capacity and the load, then press Calculate.
- Switch to What battery do I need? to size a battery for a target runtime.
Example
A 100 Ah 12.8 V LiFePO4 battery powers a 60 W laptop and router through an inverter (90% efficient):
100 Ah × 12.8 V = 1,280 Wh → usable 1,280 × 90% = 1,152 Wh → after the inverter 1,152 × 90% = 1,037 Wh
Runtime: 1,037 Wh ÷ 60 W ≈ 17.3 hours. The same capacity in lead-acid, used only to 50%, would give about 9.6 hours.
How it works
The calculator converts everything to watt-hours (Wh), the actual energy stored:
- Energy = capacity (Ah) × voltage (V)
- Usable energy = energy × depth of discharge
- Delivered energy = usable energy × inverter efficiency (if used)
- Runtime = delivered energy ÷ load (W)
Depth of discharge (DoD): lead-acid batteries last much longer if you only use about half their capacity. Lithium iron phosphate (LiFePO4) batteries can safely use 80-100%.
Real runtimes are often a little shorter: lead-acid capacity drops at high currents (the Peukert effect), cold weather reduces capacity, and many devices don't draw constant power. Plan with a 10-20% safety margin.
Real-world runtime examples
- Phone from a 20,000 mAh power bank: the cells hold about 74 Wh (at 3.7 V). After conversion losses, expect about 3-4 full charges of a typical phone.
- CPAP overnight: a typical CPAP without the humidifier uses 30-60 W. A 500 Wh power station covers about one to two nights.
- Router during a power cut: a home router uses 10-20 W. A 300 Wh power station keeps it online for over 12 hours.
- Camping fridge: a 12 V compressor fridge averages 15-30 W over a day. A 100 Ah lithium battery runs it for roughly 2-3 days without charging.
- Cold weather: at 0 °C, lead-acid batteries lose about 20% of their capacity, and lithium batteries shouldn't be charged below freezing.
Frequently asked questions
How long will a 100Ah battery last?
A 12 V 100 Ah battery stores about 1,200 Wh. At a 100 W load, a lithium battery (90% usable) lasts about 10.8 hours directly, or about 9.7 hours through a 90% efficient inverter. A lead-acid battery used to 50% lasts about half as long.
How do I convert Ah to Wh?
Multiply amp-hours by the battery voltage: Wh = Ah × V. For example, 50 Ah × 12 V = 600 Wh. Power stations are usually rated in Wh already.
How long will a power station run a fridge?
Fridges cycle on and off, so use their average consumption. A typical fridge uses about 1-1.5 kWh per day, or roughly 40-60 W on average, so a 1,000 Wh power station would run it for around 15-20 hours.
Why use only 50% of a lead-acid battery?
Deep discharges dramatically shorten a lead-acid battery's life. Using only 50% of its capacity can give several times more charge cycles than discharging it fully.
What inverter efficiency should I use?
Good pure sine wave inverters reach 90-95% at medium loads, but efficiency is lower at very light loads. 85-90% is a realistic planning value.