"How long will it run?" is the first question everyone asks about an inverter battery. The answer comes from one simple formula - no guesswork needed.
The formula
Backup hours = battery energy (kWh) x usable fraction x inverter efficiency / load (kW)
- Battery energy = voltage x amp-hours / 1000. A 51.2 V 100 Ah battery holds 51.2 x 100 / 1000 = 5.12 kWh.
- Usable fraction - about 0.8 for LiFePO4 lithium (rated at 80% depth of discharge), about 0.5 for lead-acid.
- Inverter efficiency - use about 0.9 to allow for conversion losses.
- Load - the total watts running, divided by 1000.
Example 1: a PowerRun 51.2 V 100 Ah lithium battery
5.12 kWh x 0.8 x 0.9 = about 3.7 kWh delivered.
- At 500 W (lights, fans, TV, router): 3.7 / 0.5 = about 7 hours
- At 800 W (add a fridge): 3.7 / 0.8 = about 4.5 hours
- At 2 kW (add a 1.5 ton AC): 3.7 / 2 = under 2 hours
The 25.6 V 200 Ah battery stores the same 5.12 kWh, so it gives the same backup on a 24 V inverter.
Example 2: a 12 V 150 Ah lead-acid battery
12 x 150 / 1000 = 1.8 kWh. Usable: 1.8 x 0.5 x 0.9 = about 0.8 kWh. At 500 W that is roughly 1.6 hours - which is why lead-acid systems need several batteries for long cuts.
Working backwards: what capacity do I need?
Flip the formula: battery kWh needed = load (kW) x hours / (0.8 x 0.9) for lithium.
For 800 W over 6 hours: 0.8 x 6 / 0.72 = 6.7 kWh. The nearest PowerRun options are the 25.6 V 280 Ah (7.17 kWh) for a 24 V inverter or the 51.2 V 200 Ah (10.24 kWh) for a 48 V inverter.
Three tips for longer backup
- Switch to BLDC fans and LED lights. A BLDC fan uses less than half the power of a regular fan.
- Keep heavy loads off backup. Geysers, irons and induction cooktops drain a battery in minutes.
- Add solar. With a hybrid inverter, the sun recharges the battery during the day, so it starts every evening full.
Let the calculator do it
Our battery calculator runs these numbers for you, or message us on WhatsApp with your load and the hours you need. Browse the full range of PowerRun lithium batteries.