how do you calculate inverter surge needs for real appliances?

[SeiferAlmasy]

I’m still confused about why an inverter that looks big enough on paper can still fail when a motor load starts. I see continuous and surge ratings, I see appliance watt labels, and I see people mention starting current and power factor, and my brain just stops. I’m trying to keep it simple and buy something that works without spending more than I need.

This weekend I watched a friend’s backup setup handle their lights and internet smoothly and it was honestly satisfying how normal everything felt during an outage. Then they said the key was choosing an inverter that could handle motor surges, not just the steady load, and that is the part I do not understand. What is your simple rule of thumb for sizing, and when does oversizing actually help versus just wasting money?

 

[Alan]

A practical rule is size for your steady loads, then make sure the inverter surge rating comfortably covers your biggest motor start. If you have a fridge, freezer, well pump, or furnace blower, those are usually the deciding loads. What’s the largest motor appliance you want to run.

 

[YukariTakeba]

If the appliance label only shows amps, you can estimate watts as volts times amps, then assume startup is several times higher for motors. Some appliances list LRA or starting amps, which is basically the surge hint. If you can find that number, it makes sizing way easier. I also try to run only one motor starting at a time during an outage. Staggering loads can save you money.

 

[solarM4n]

Beyond just the size, make sure you're getting a Pure Sine Wave inverter. Cheaper modified sine inverters make motors run hot, loud, and inefficiently.

Sometimes a motor won't even start on a cheap inverter, even if the wattage is high enough, simply because the shape of the power is too messy for the motor's internals to handle