Guide

How to size an off-grid solar system

Sizing an off-grid system is five small calculations done in order. Here's each one, the exact formula our calculator uses, and a full worked example for a van.

Step 1 — Your daily energy use (watt-hours)

For every appliance, multiply watts × hours per day × quantity. Add them up. That's your daily watt-hours (Wh) — the single most important number. Anything powered through an inverter (AC appliances) should be divided by about 0.9 to account for inverter losses.

Step 2 — Battery bank

You can't use 100% of a battery. Usable depth of discharge (DoD) is about 80% for LiFePO4/lithium and 50% for lead-acid (AGM or flooded). Decide how many days of backup you want for cloudy stretches.

Battery Wh = daily Wh × days of backup ÷ DoD

Battery Ah = Battery Wh ÷ system voltage

Step 3 — Solar array

Your panels have to replace everything you used, plus charging and wiring losses (~25%). "Peak sun hours" is how many hours of full-strength sun your location effectively gets — roughly 3 in a cloudy northern winter, 5–6 in a sunny southern summer.

Solar watts = daily Wh ÷ (peak sun hours × 0.75)

Step 4 — Charge controller

The controller sits between panels and battery. Size it for the current the array produces, with a 25% safety margin. Use MPPT (not PWM) for almost all modern builds — it harvests 20–30% more.

Controller amps = solar watts ÷ system voltage × 1.25

Step 5 — Inverter (only if you run AC appliances)

Size a pure sine wave inverter to cover the largest set of AC appliances you'd run at once, with surge headroom (motors and compressors spike on startup). If everything you own is 12V DC, you don't need one.

Worked example: a full-time van

Loads: 12V fridge (50W, 8h), LED lights (20W, 5h), Starlink (50W AC, 10h), laptop (60W AC, 4h).

• Daily energy ≈ 1,400 Wh (AC loads grossed up for inverter loss)
• Battery (LiFePO4, 1 day, 80% DoD): 1,400 ÷ 0.8 ÷ 12V ≈ 150 Ah
• Solar (4 sun hours): 1,400 ÷ (4 × 0.75) ≈ 470W → a 500W array
• Controller: 500 ÷ 12 × 1.25 ≈ 52A → a 60A MPPT
• Inverter: covers the ~110W of AC loads → a small 300–600W pure sine unit

These are planning estimates. Off-grid wiring must follow local electrical code; have complex builds reviewed by a qualified installer.