How to Size a Solar System (Watts & Amp-Hours)

Sizing a solar system boils down to answering one question: how many watt-hours (Wh) do you need per day? Once you know that number, every other component — panels, batteries, controller, inverter — sizes itself through straightforward math.

This is the condensed, practical version of sizing. For the comprehensive step-by-step with regional sun-hour data and real-world examples, see our complete sizing guide.

The Quick Sizing Formula

Step 1: Daily Watt-Hours

List every device. Multiply watts × hours of daily use = Wh per device. Sum them all. Example: LED lights (40W × 5h = 200 Wh) + fridge (50W × 12h = 600 Wh) + laptop (60W × 4h = 240 Wh) = 1,040 Wh/day.

Step 2: Panel Wattage

Apply a 1.25× efficiency buffer, then divide by your peak sun hours:

(Daily Wh × 1.25) ÷ Peak Sun Hours = Panel Watts

(1,040 × 1.25) ÷ 5 = 260W. Oversize by 25%: target ~325W of panels.

Step 3: Battery Capacity (Amp-Hours)

Decide on days of autonomy (1.5–2 for off-grid), then factor in depth of discharge:

Daily Wh × Autonomy Days ÷ Battery Voltage ÷ Usable DoD = Ah

1,040 × 2 ÷ 12 ÷ 0.9 (LiFePO4) = 193Ah. A 200Ah LiFePO4 bank covers this.

With AGM at 50% DoD: 1,040 × 2 ÷ 12 ÷ 0.5 = 347Ah. Nearly double the capacity needed.

Step 4: Charge Controller

Panel watts ÷ battery voltage = minimum controller amps. Add 25% headroom.

325W ÷ 12V = 27A → 30A MPPT minimum, 40A recommended for expansion room.

Step 5: Inverter

Sum the wattage of everything that could run simultaneously. Add 20% for startup surges. Most off-grid builds need a 1,000W–3,000W pure sine wave inverter.

Common Sizing Mistakes

Using label wattage instead of actual draw. A fridge label might say 150W but it cycles on and off — actual daily consumption might be 50W average. Use a Kill-A-Watt meter for accurate numbers.

Forgetting inverter idle draw. Inverters consume 15–40W just to stay on, 24/7. That's 360–960 Wh/day of overhead on large inverters. Factor this into your load calculation.

Not accounting for winter. If you're off-grid year-round, size panels for your worst-case month (typically December/January in the Northern Hemisphere), not your annual average. Winter sun hours can be 30–50% less than summer.