Sizing an off-grid solar system incorrectly is the single most common and costly DIY mistake. Too small and you'll constantly run out of power. Too big and you've wasted thousands of dollars. This guide walks you through the exact professional methodology — step by step — in plain English.
List every device you plan to power. For each one, multiply its wattage by the hours per day you use it. This gives you watt-hours (Wh) per day.
| Appliance | Watts | Hours/Day | Wh/Day |
|---|---|---|---|
| Refrigerator | 150 | 8 (compressor cycles) | 1,200 |
| LED Lighting (8 bulbs) | 80 | 5 | 400 |
| Laptop | 65 | 6 | 390 |
| Phone Charging (2) | 30 | 3 | 90 |
| Internet Router | 20 | 24 | 480 |
| TV (LED 55") | 120 | 3 | 360 |
| Total | 2,920 Wh/day |
Add 20% for system inefficiencies (wiring losses, inverter losses, battery charge/discharge losses):
Peak sun hours (PSH) measure the daily equivalent hours of full-strength sunlight (1,000 W/m²). This is not total daylight — it's a standardized measure of your solar resource.
For conservative sizing, use your winter worst-case PSH — typically 20–30% lower than the annual average.
Example using 3,504 Wh/day at 5.0 PSH:
3,504 ÷ (5.0 × 1,000 × 0.80) = 3,504 ÷ 4,000 = 0.876 kW → round up to 1.2 kW
With 400W panels: 1,200W ÷ 400W = 3 panels minimum (install 4 for headroom).
Your battery bank must store enough energy to power your home through nights and cloudy days. Design for your "autonomy days" — how many days without sun your system must handle.
Example: 3,504 Wh/day × 2 days ÷ 0.85 ÷ 1,000 = 8.2 kWh battery bank
In amp-hours at 48V system voltage: 8,200 Wh ÷ 48V = 171 Ah @ 48V
MPPT charge controllers are sized by amperage output to the battery bank.
Example: (1,600W ÷ 48V) × 1.25 = 41.7A → use a 40A or 60A MPPT controller
Popular 40A controllers: Renogy Rover, EPEver Tracer. 60A: Victron SmartSolar 60A, EPEver 60A.
Your inverter must handle your peak surge load — the largest simultaneous draw including motor start-up surges.
Example: Fridge (400W surge) + laptop + lighting + router = 800W continuous → choose a 1,500W or 2,000W inverter
| Parameter | Result |
|---|---|
| Daily Load | 3,504 Wh/day (after losses) |
| Location | North Carolina (4.7 PSH) |
| Solar Array | 4 × 400W panels = 1,600W |
| Battery Bank | 10 kWh LiFePO4 (200Ah @ 48V) |
| Charge Controller | 60A MPPT |
| Inverter | 2,000W pure sine wave |
| Est. DIY Cost | $4,800–6,500 |
| Autonomy | 2.5 days without sun |
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