The formula
- Capacity is the battery size in watt-hours, printed on the station.
- Efficiency is the share you actually get out. Converting battery DC to household AC loses energy as heat, and the electronics use some too: 0.85 is a realistic planning figure for AC outlets, 0.9 for DC and USB.
- Average load is the total watts of everything plugged in, averaged over time (see below).
Worked examples
One appliance
A 55" TV (90 W) on a 500 Wh station: 500 × 0.85 ÷ 90 = 4.7 hours.
Several appliances
Add the watts first. A modem and router (20 W), a laptop (45 W) and two LED bulbs (18 W) total 83 W. On a 1,000 Wh station: 1,000 × 0.85 ÷ 83 = 10.2 hours.
Appliances that cycle
A fridge runs at about 150 W, but only about 40% of the time, so its average is about 60 W. On a 1,000 Wh station: 1,000 × 0.85 ÷ 60 = 14.2 hours. Using 150 W would wrongly give 5.7 hours.
Converting amp-hours (Ah) to watt-hours
Batteries are often rated in amp-hours. Multiply by the battery's nominal voltage to get watt-hours:
A 12 V 100 Ah lithium battery (nominal 12.8 V) holds about 1,280 Wh. Note that power banks often quote mAh at 3.7 V, the cell voltage, so their Wh figure is what to compare.
Why real runtime is shorter than the math
- Inverter idle draw: the AC inverter uses 10–30 W just by being on. That hits small loads hardest.
- Cold: lithium batteries deliver less below about 50°F (10°C). Keep the station indoors.
- Battery age: capacity slowly drops with use. LiFePO4 stations typically keep about 80% after 3,000+ cycles; older NMC units fade faster.
- High loads: efficiency falls a little near the station's maximum output.
- Label watts: appliance labels show maximum draw, which often overstates average use (or understates it for older appliances).
- Starting below 100%: if the station sat for months, it may not be full when the power goes out. Top it up every few months.
The runtime calculator does the math for any combination of appliances, and the appliance wattage chart lists typical averages.