How to Size an Off-Grid Solar System (Panels, Battery & Inverter)

"How many solar panels do I need?" is the most common off-grid question — and the answer always starts in the same place: your daily energy use. Here's a clear method for sizing every part of the system.

Step 1 — Find your daily watt-hours

List every appliance, multiply watts by hours per day, and total it. That number — your daily watt-hours (Wh) — drives everything else. Be realistic about run-times; this is where most people over- or under-build.

Step 2 — Size the battery bank

Decide how many days of backup (autonomy) you want — 1 day for a sunny climate with a generator on standby, 2–3 for cloudy regions. Multiply your daily Wh by that number. Then account for depth of discharge: lithium (LiFePO4) can use ~80–100%, lead-acid only ~50%, so a lead bank needs to be roughly twice as big. Convert to amp-hours by dividing by your system voltage.

Step 3 — Size the solar array

Divide your daily Wh by your local peak sun hours (typically 3–5), then add about 25% for wiring, heat, and charging losses. That's the array wattage you need to replace what you use each day. If you run year-round, size for your worst winter month.

Step 4 — Size the charge controller

Take your array watts and divide by your battery voltage to get charging amps, then add margin — that's your MPPT controller's minimum current rating. Also confirm your panel string's cold-weather voltage stays under the controller's maximum PV input.

Step 5 — Size the inverter

Add up the watts of everything that could run at the same time — that's your continuous rating. Then check the biggest motor's surge; the inverter's peak rating must cover it. A 3,000W inverter that surges to 6,000W, for instance, can start a pump that a same-rated but lower-surge unit can't.

A quick worked example

Say you use 3,000Wh/day, want 2 days of backup, and get 4 peak sun hours:

• Battery: 3,000 × 2 = 6,000Wh usable → about a 24V / 300Ah lithium bank.
• Array: 3,000 ÷ 4 × 1.25 ≈ 940W of panels.
• Controller: ~940W ÷ 24V ≈ 40A, so a 40–50A MPPT.
• Inverter: a 3,000W unit if your peak simultaneous load stays under it, with surge headroom for motors.

Let the tool run the numbers

Our System Builder does all of this from a simple appliance list — including the cold-voltage and NEC ampacity checks that are easy to miss by hand — and shows you exactly which panels, batteries, and inverters fit together.