Solar Panel Sizing Calculator

Enter your average monthly electricity usage in kWh (check your utility bill; most bills report this directly), your location's peak sun hours per day, your target panel wattage (400W is a common residential panel size in 2026), a system derate factor (80% is a reasonable default covering inverter losses, wiring losses, soiling, and temperature effects), and what percentage of your usage you want the system to offset.

Worked example: 864 kWh/month usage (the 2026 US household average), 4.98 peak sun hours/day (the 2026 US average), 400W panels, 80% derate, 100% offset target. Daily usage = 864 × 12 ÷ 365 = 28.41 kWh/day. Target daily production (100% offset) = 28.41 kWh. Required system size = 28.41 ÷ (4.98 × 0.80) = 7.13 kW. Panel count = 7.13 kW × 1000 ÷ 400W = 17.8, rounded up to 18 panels (you can't install a fraction of a panel). Actual system size with 18 panels = 18 × 400W = 7.2 kW, producing about 10,470 kWh/year, slightly above your 10,368 kWh/year usage, which is expected since panel counts round up.

A second example, sunnier location: 1,200 kWh/month usage, 6.5 peak sun hours/day (Arizona-level sun), same 400W panels and 80% derate, 100% offset. Daily usage = 39.45 kWh/day. Required system size = 39.45 ÷ (6.5 × 0.80) = 7.59 kW. Panel count = 7.59 × 1000 ÷ 400 = 18.97, rounded up to 19 panels, a 7.6 kW system producing about 14,425 kWh/year. Despite using 39% more electricity than the first example, this home needs only 1 additional panel, because its peak sun hours are 30% higher.

To see whether that system size makes financial sense given installation cost, incentives, and utility rate escalation, run the resulting system cost through the solar payback calculator.

Daily usage (kWh) = Monthly usage × 12 ÷ 365.

Target daily production = Daily usage × Offset % ÷ 100 (set offset above 100% to intentionally oversize for future EV charging or electrification).

Required system size (kW) = Target daily production ÷ (Peak sun hours × Derate factor).

Panel count = ROUND UP(Required system size × 1000 ÷ Panel wattage), always rounded up, since partial panels aren't installable.

Actual system size = Panel count × Panel wattage ÷ 1000.

Annual production estimate = Actual system size × Peak sun hours × 365 × Derate factor.

Estimated roof area = Panel count × ~19.6 sq ft, based on a typical 400W residential panel footprint of roughly 17.5-21 sq ft. Treat this as a rough planning number, not a substitute for a site survey.

A solar panel's nameplate wattage is measured under lab-standard test conditions that real rooftops rarely match. The derate factor bundles together every real-world loss: inverter conversion loss (typically 2-4%), wiring and connection losses (1-3%), soiling from dust and debris (2-5% depending on climate and cleaning frequency), and temperature, the biggest single factor. Panels lose efficiency as they heat up, and a hot summer rooftop can run panels 25-40°C above the standard test temperature, cutting output by 10-15% during peak sun hours.

Stacked together, these losses commonly total 15-25%, which is why this calculator defaults to an 80% derate factor (a 20% total loss) rather than assuming panels perform at their full nameplate rating. Using 100% (no derate) instead of 80% would understate your required system size by roughly 20%, exactly the kind of gap that leaves a homeowner short of their target offset after installation.

If you're also sizing battery backup rather than just grid-tied panels, the solar battery sizing calculator and the solar inverter sizing calculator handle those adjacent calculations using the same system-size output from this page.

Wind has an equivalent gap between the nameplate rating and the real output, and it is even larger. The Wind Turbine Savings Calculator checks a quoted capacity factor against what the rotor swept area can physically extract at your site's mean wind speed.

Peak sun hours is not the same as daylight hours. It's a measure of total daily solar energy compressed into an equivalent number of hours at maximum intensity (1,000 W/m²). The continental US ranges from about 3.0-4.2 peak sun hours in the Pacific Northwest and northern Great Lakes up to 6.5-7.5 in the desert Southwest, with a national average of 4.98.

Because peak sun hours sits in the denominator of the system-sizing formula, a location with low sun needs a proportionally larger system to produce the same energy. The 2.2x panel-count difference between a 900 kWh/month home in Arizona (13 panels) versus an identical home in Seattle (29 panels) illustrates just how much this single input moves the result. Always use your actual location's figure rather than a national average if precision matters for your budget.

This is a planning-stage sizing tool, not a substitute for a professional site survey. It doesn't account for roof orientation and tilt (a south-facing roof at your latitude's optimal tilt angle outperforms a north-facing or flat roof by 20% or more), shading from trees or nearby structures, roof structural capacity, or local permitting and setback requirements that can reduce usable roof area.

It also assumes a flat, year-round average usage pattern. Homes with strong seasonal swings (heavy summer AC load or winter electric heating) may want to size against their peak month rather than the annual average used here, to avoid a system that undershoots during the highest-usage season.

Finally, panel wattage and derate factors continue to improve year over year; check your specific panel model's real-world specifications rather than relying solely on the 400W/80% defaults if you're finalizing a purchase decision. Sizing answers how many panels you need, not whether they pay for themselves; the solar panel payback guide covers ROI, incentives and bill savings once you know the system size.