Sizing an inverter, starting with the coffee maker

Here’s the morning that sells a lot of inverters. You’re dry camping, the coffee maker kicks on, someone starts the microwave, and the whole coach goes quiet. The batteries were fine. The inverter just couldn’t carry both at once.

Watts right now, not watt-hours all day

Batteries and solar are sized in watt-hours, the total over a day. An inverter is sized in watts, the load at one moment. So add up everything you’d ever run together. The rating plate on the bottom or back of each appliance lists its watts.

The battery bank calculator estimates that peak from the boxes you check. A luxury coach with the AV package on and the air conditioner off comes to 2,300 watts in its table. Tick the multi-zone air conditioning box and the peak jumps by another 4,500.

Add a margin, then round up

The solar planner multiplies your peak by 1.25 and picks the next size up from 1,000, 2,000, 3,000, 4,000, 5,000, or 6,000 watts. The 1.25 is a planning margin we chose. It isn’t a code requirement, so if your installer uses a different one, go with theirs.

Run that luxury coach through it and 2,300 watts becomes 2,875. The next size up is a 3,000 W inverter.

The surge problem

Motors and compressors pull a big burst when they start. A roof air conditioner can draw several times its running watts for a moment. The original calculator doesn’t publish a surge figure, so we won’t make one up. Compare the inverter’s surge rating with the startup numbers in the appliance manual.

Bigger inverter, higher battery voltage

The same peak watts set 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 pull a plan toward a 24 or 48 volt bank.

James doesn’t link an inverter on Amazon right now, so you won’t find an inverter card here. These are the batteries the calculator pairs with bigger loads.