What an inverter does
An inverter converts the direct current (DC) of a battery bank into household alternating current (120 V AC in the United States). RVs, boats, vans, cabins and backup systems use one to run appliances from batteries. Undersizing it trips the inverter; oversizing it costs money and wastes power at idle.
Sizing in three steps
- Running load: add the wattage of everything that runs at the same time.
- Surge: motors, compressors and pumps draw two to seven times their running wattage for a moment when they start. The inverter’s surge rating must cover the largest start, plus whatever is already running.
- Margin: choose a continuous rating about 20% to 25% above the running load, so the inverter does not run at its limit.
Battery current
The inverter draws a large DC current from the battery:
DC current = load watts ÷ (battery voltage × efficiency)
A 1,500 W load on a 12 V bank at 90% efficiency draws about 139 A. Cables and fuses must be sized for that current. Above about 150 A, a 24 V or 48 V battery bank makes the wiring much simpler.
Runtime
Runtime (hours) = battery Ah × voltage × usable depth of discharge × efficiency ÷ load watts
A 200 Ah 12 V battery used to 50% (lead-acid) runs 1,500 W for about 0.72 hours, or 43 minutes. A lithium battery of the same size, used to 80% or 90%, runs about 70% longer.
Typical appliance loads
| Appliance | Running | Starting surge |
|---|---|---|
| Fridge | 100 to 200 W | 600 to 1,200 W |
| Microwave (1,000 W cooking) | 1,400 to 1,600 W | about the same |
| Coffee maker | 800 to 1,200 W | about the same |
| Laptop | 50 to 100 W | none |
| Air conditioner (RV) | 1,300 to 1,800 W | 2,500 to 3,500 W |
| Power tools | 500 to 1,500 W | up to 3 times |
Pure sine or modified sine?
- Pure sine wave inverters reproduce grid power. Use them for motors, compressors, chargers, medical devices and electronics.
- Modified sine wave inverters are cheaper but can make motors run hot and some chargers fail.
Safety
- Install a fuse or breaker within a short distance of the battery positive terminal, rated about 125% of the maximum DC current.
- Keep DC cables short and use the cross-section recommended by the inverter maker.
- Connect to a building’s wiring only through a transfer switch installed by a licensed electrician.