Solar sizing looks like a guess until you realize it's usage divided by sun, corrected for losses. Get the three numbers right and the panel count falls out cleanly.
Usage drives everything
Your electricity bill is the starting point — the monthly figure, divided into a daily average. Sizing is matching generation to that number, which is why two identical houses can need very different systems if one heats electrically and the other doesn't.
Sun hours and losses
Panels produce at rated power only during peak sun hours. Divide by that figure, then divide again by (1 − losses) to account for the gap between lab ratings and real-world output. Together these two corrections turn an optimistic estimate into a workable one.
From kW to panels to roof
Kilowatts convert to a panel count through wattage, and panels to roof area at about 1.7 m² each. The final step — orientation, tilt, shading and structural checks — is exactly what a professional survey adds before anyone signs.
Frequently asked questions
Divide your system size (daily usage ÷ sun hours, adjusted for losses) by the panel wattage. A 900 kWh/month home at 5 sun hours with 400 W panels needs about 20 panels.
The hours per day when sunlight is intense enough for panels to produce at their rated power. It averages about 4–5.5 depending on location — the key variable in any solar estimate.
Around 20% — from inverter efficiency, heat, soiling, shading and wiring. Most sizing calculators bake this in as a derating factor.