Sizing an inverter

Inverters get sized wrong in one of two ways. Someone adds up a day’s watt-hours, or they forget how hard the air conditioner pulls when it starts. Here’s how to avoid both.

Size for what runs at the same time

An inverter’s continuous rating has to cover every load that’s on at once. Daily watt-hours don’t matter here. The Megawatt battery calculator estimates that peak from the appliances you check. A luxury coach with the AV package on and the air conditioner off comes to 2,300 W. Check multi-zone air conditioning and the peak jumps by another 4,500 W.

Add a margin

The solar planner multiplies your peak by 1.25 and picks the next size up, somewhere from 1,000 to 6,000 W. That margin is ours. It’s not from the National Electrical Code, so change it if your installer uses a different one.

Don’t forget surge

A roof air conditioner can pull several times its running watts for a moment when the compressor starts. The calculator doesn’t track surge, so check two numbers yourself: the inverter’s surge rating and the air conditioner’s start-up draw from its own manual.

Bigger loads, higher voltage

The same peak picks your battery voltage. Past 3,000 continuous watts, the calculator leaves 12 volts behind. Past 5,000, it leaves 24. That’s why a residential fridge plus air conditioning tends to push you toward a higher-voltage bank and a bigger inverter.

What we link, and what we don’t

Megawatt’s Amazon list has the batteries and the solar packs. It doesn’t have an inverter yet, so you won’t see one recommended here. And as the original solar calculator put it: talk to a certified electrician before you change the electrical system.