Off-Grid Solar Calculator

How to size an off-grid solar system

Every off-grid system — a campervan, an overland rig, a remote cabin — is sized in the same four steps. Get the first one right and the rest follow.

Step 1 — Your daily energy use (the foundation)

Add up the watt-hours every appliance uses in a day: watts × hours run per day. A 45 W fridge that effectively draws power 10 hours a day (its compressor cycles) uses 450 Wh; ten hours of a 12 W light is 120 Wh. The total is the single most important number in your whole build, because the array and battery are both sized from it. The builder above does this sum for you.

Step 2 — The solar array

Your panels have to replace a full day's energy during the limited hours the sun is strong, and they never perform at their rated wattage in the field.

Array watts = Daily Wh ÷ (Peak sun hours × System efficiency)

The peak sun hours are the equivalent hours of full 1000 W/m² sun your location gets — roughly 3 in the cloudy Pacific Northwest, 4–5 across most of the US, and 6–8 in the desert Southwest. Crucially, use your worst month (usually December), not the annual average, or the system will fall short exactly when you need it. The system efficiency factor (~0.75) bundles together temperature derating, wiring and charge-controller losses, dirt, and battery charging inefficiency.

Step 3 — The battery bank

The battery carries you through the night and through cloudy days. Two things set its size: how many days of autonomy you want, and how deeply you can discharge the chemistry.

Battery Wh = Daily Wh × Days of autonomy ÷ Depth of discharge

Lead-acid is limited to about 50% depth of discharge, so you buy double what you'll use; LiFePO4 lithium goes to 80–90%, which is why it has taken over van and RV builds despite the higher sticker price. Divide the result by your system voltage to get the amp-hour rating you'll shop for (e.g. 2,400 Wh ÷ 12 V = 200 Ah).

Step 4 — Inverter and charge controller

The inverter must be big enough to run whatever AC appliances you'll use at the same time — size it to your realistic simultaneous peak, not your daily total. The charge controller sits between panels and battery; size its current rating to the array watts divided by battery voltage, with a 25% safety margin, and choose MPPT over PWM for anything beyond a trickle.

A worked example — a campervan

Suppose your appliance list totals 800 Wh/day, you're in a location with 4 worst-month peak sun hours, you want 2 days of autonomy, and you run a 12 V LiFePO4 bank at 80% DoD with 0.75 system efficiency:

• Array: 800 ÷ (4 × 0.75) = 267 W → round up to 300 W (3 × 100 W panels)
• Battery: 800 × 2 ÷ 0.80 = 2,000 Wh ÷ 12 V = 167 Ah → a 200 Ah battery
• Charge controller: 300 W ÷ 12 V × 1.25 ≈ 31 A → a 30–40 A MPPT

That's a textbook small-van setup, and it lines up with what most builders actually install. Push the same appliances into a December in Seattle (≈2 sun hours) and the array requirement nearly doubles — which is exactly why worst-month sun hours, not averages, decide whether your system works in January.

Common appliance loads