Do not add appliance nameplates and buy the next size up. Build a load schedule showing what runs together, what cycles and what has a high starting current.
Load worksheet
| Load | Running watts | Starting or surge | Expected overlap | Daily energy |
|---|---|---|---|---|
| Battery charger or inverter-charger | ||||
| Air conditioning | ||||
| Watermaker | ||||
| Hot-water element | ||||
| Galley appliance | ||||
| Washing machine | ||||
| Tools or pumps |
Motor and compressor loads can draw several times running current during startup. Inverter air conditioners and soft starters can change the profile but must be assessed from actual specifications. Power factor means apparent power in kVA and real power in kW are not interchangeable for every load.
Oversizing has costs: weight, purchase price, installation space and long periods at unfavourably light load. Undersizing causes voltage and frequency instability, breaker trips, overheating and poor motor starting. Load management may permit a smaller set by preventing the water heater, charger and air conditioner from demanding peak power together.
For battery charging, include charger efficiency and taper. A 3 kW charger input does not deliver 3 kW into the battery, and a nearly full battery may accept far less. Compare generator run time with alternator, solar and inverter capability as one energy system.