How to Size an Off-Grid or Backup Solar System (2026)

Sizing an off-grid or backup system comes down to four parts that depend on each other: how much energy you use, how long you want to run without sun, how much sun you get, and how much power you need at once. Get those four numbers and the rest is arithmetic.

Use the free sizing calculator to do the math instantly, or follow the steps below to understand what it is doing.

Step 1: your daily energy use

Add up the watt-hours of everything you run in a day. A fridge, lights, and electronics land around 3 to 5 kWh. A small home is often 8 to 15 kWh. A full home with heating or cooling can be 20 to 40 kWh.

Step 2: your battery bank

Multiply daily use by the days of backup you want, then divide by the usable depth of discharge. LiFePO4 batteries safely use about 90% of their capacity, so a system that needs 16 kWh of usable storage wants a bank closer to 18 kWh total.

Step 3: your solar array

Divide daily watt-hours by your peak sun hours, then add roughly 25% for losses. Sunnier regions need fewer panels for the same energy.

Step 4: charge controller and inverter

The charge controller is sized to the array and system voltage. The inverter is sized to the largest load you might run at once, with headroom on top.

Plug your numbers into the calculator to see a full recommendation.