Off-Grid Solar Calculator

Tap the appliances you run from the list above and the calculator adds up their average watt-hours for you. The wattage on each is a typical average, so your exact model may pull a little more or less. For a fridge or freezer it shows the average draw across a full day (the compressor only runs part of the time), which is why it looks lower than the sticker on the back. If you already metered your own gear, edit any row, or switch to the manual total and type your own daily watt-hour figure.

The annual figure is compiled from published state-level summaries of NREL solar data (fixed south-facing panels); we have not re-run PVWatts for every state ourselves. It is also an average: a sunny ridge does better than a shaded valley a few miles away, and winter runs much lower than the yearly number. For an exact figure at your address, look it up free on NREL PVWatts, and turn on the winter toggle if you run off-grid year-round.

Divide your total panel wattage by your battery voltage to get the amps, then add about 25 percent for cold, sunny days when panels can briefly spike above their rating. A 600 watt array on a 12 volt battery is about 50 amps before margin, or roughly 62 amps once you add the 25 percent, so you would pick an 80 amp MPPT controller. Always round up to the next standard size, never down. Amperage is not the whole story: also confirm your panel string fits the controller's voltage window and maximum PV input.

Multiply your daily watt-hours by how many cloudy days you want to ride out, then divide by your usable depth of discharge, which is about 80 percent for LiFePO4 and 50 percent for lead-acid. The calculator also adds about 10 percent for inverter and round-trip losses, since AC appliances draw through the inverter, then turns the result into watt-hours and amp-hours at your system voltage. More days of autonomy means a bigger, pricier bank, so most off-grid setups land at two or three days.

The calculator picks this for you from the size of your system, so you do not have to. As a rule, small van and weekend setups run 12 volts, larger rigs run 24 volts, and cabins or homes run 48 volts. Higher voltage moves the same power with less current, which means thinner, cheaper, safer wiring once a system gets large. You can override the pick under Advanced if you already know what you want.

MPPT controllers pull 15 to 30 percent more energy from the same panels, especially in cold weather and when the panel voltage runs higher than the battery voltage. PWM is cheaper but wastes that extra harvest, so it only makes sense on small, matched 12 volt systems. For anything you rely on, an MPPT controller pays for itself.