How to size a solar generator

A "solar generator" is a battery with an optional solar input. It does not generate electricity, and without panels it is limited to whatever you charge into it. The search term is useful because that is what people type, but the device is a battery, and sizing it is about two separate numbers that are easy to confuse.

Watts and watt-hours are not the same

Watts measure how fast energy flows. Watt-hours measure how much you have stored. A station rated at 1,000 W of output can still hold only 500 Wh of energy, which means it can run a 1,000 W load for about half an hour, not for a whole night.

When you look at a spec sheet, the output rating tells you what it can start. The capacity tells you how long it lasts. You need both to clear your load, and they are checked independently.

Panel wattage is not charge rate

A 400 W panel does not deliver 400 W into the battery. Panel ratings assume full sun at a specific temperature, and real conditions rarely match. Angle, cloud, season, and heat all reduce output, and the charge controller converts at a loss.

A practical planning figure is that you will see somewhere between half and three quarters of the panel's nameplate rating on a good day. Treat solar as a way to extend a stay rather than a way to run a load indefinitely.

Working out your number

1. List the devices you need and the hours per day for each. Multiply watts by hours to get watt-hours.
2. Apply a duty cycle to anything with a compressor or thermostat. A fridge does not draw its nameplate wattage continuously.
3. Divide by inverter efficiency, roughly 0.85, to account for conversion losses.
4. Add a reserve margin. Fifteen percent is tight, twenty is standard, more than thirty if you expect cold weather.

The calculator does all four steps and shows you which of capacity or output is the binding constraint.

What panels will and will not do

Panels are best understood as a top-up. If your daily use is 1,500 Wh and your panel array realistically delivers 300 W for five good hours, you can recover most of a day's use. If your use is a space heater at 1,500 W continuous, no practical array on a portable station will keep up, and the honest answer is that this is the wrong device for that job.

Next:size the load in the calculatoror readhow to estimate CPAP backup power.