“How many solar panels do I need?” is the first question every homeowner asks — and the answer is not a guess. It is your electricity usage divided by how much sunlight your roof actually gets, adjusted for real-world system losses. This calculator does that math from the one number you already know: your monthly bill.
Solar Panel Sizing Calculator
How many panels do you actually need? Size a system from your real electricity use, your region’s sun, and a target offset.
Sizing estimate using regional peak-sun-hours and a standard 0.80 system derate (inverter, wiring, soiling, temperature losses). Actual production varies with roof orientation, shade, and weather. The 30% federal Residential Clean Energy Credit applies to owned systems through 2032. Not tax or financial advice.
How to use this calculator
Enter your monthly electricity use — either as a dollar bill (the calculator converts it using your rate) or directly in kWh if you know it. Pick your region so the math uses realistic peak sun hours, choose a panel wattage, and set how much of your usage you want solar to cover. The result is the panel count, system size in kilowatts, roof area required, expected annual production, and the cost before and after the 30% federal tax credit.
The sizing math, in plain terms
Four numbers decide everything:
- Annual usage — your monthly kWh × 12, scaled by your target offset.
- Peak sun hours — not daylight hours. It is the equivalent hours per day of full-strength (1,000 W/m²) sun your location averages. The desert Southwest gets ~6.5; the Pacific Northwest ~3.8.
- System derate — real systems lose ~20% to inverter conversion, wiring, heat, dust and panel tolerance. The industry-standard 0.80 factor is built in.
- Panel wattage — modern residential panels are ~400 W. Higher-wattage panels mean fewer panels for the same system size.
System size (kW) = daily kWh needed ÷ (peak sun hours × 0.80). Panel count = system watts ÷ panel watts, rounded up.
A worked example
A home using 900 kWh/month in California (5.8 sun hours), targeting 100% offset with 400 W panels:
- Annual need: 900 × 12 = 10,800 kWh
- Daily: ~29.6 kWh
- System: 29.6 ÷ (5.8 × 0.80) ≈ 6.4 kW
- Panels: 6,400 ÷ 400 ≈ 16 panels
- Roof: ~280 sq ft
- Cost at $2.75/W: ~$17,600 → ~$12,300 after the 30% credit
The same house in the Pacific Northwest (3.8 sun hours) needs roughly 24–25 panels for the identical usage — sun hours, not house size, is the biggest driver.
Why “100% offset” is not always the goal
- Net metering rules matter. Where you get full retail credit for exports, slight oversizing is fine. Where buyback is low (NEM 3.0-style), oversizing wastes money — size closer to daytime use or add a battery.
- Future load. Planning an EV or heat pump in the next few years? Size to 120–150% now; adding panels later costs more per watt than including them upfront.
- Roof limits. The calculator shows required roof area (~17.5 sq ft per panel). Usable, unshaded, correctly-oriented roof is often the real constraint, not budget.
Frequently asked questions
Is the panel count exact? It is a sound planning estimate. A site survey adjusts for your exact roof pitch, azimuth, shading and local weather. Expect the final design within roughly 10–15% of this number.
What are peak sun hours? The number of hours per day your location would receive if all sunlight arrived at full 1,000 W/m² intensity. It already averages in cloudy days and winter — it is not the same as hours of daylight.
Why the 0.80 derate? Panels are rated in lab conditions. Real output is lower due to inverter losses, heat, wiring resistance, dust and age. 0.80 is the widely used industry default; high-quality installs may reach 0.82–0.85.
Should I size for 100% of my bill? Often yes for full-retail net metering. Under low-export-credit policies, sizing to your daytime usage plus a battery usually beats a big grid-export system. Check your utility’s current net metering rules.
Does roof direction change the count? Yes. South-facing at the optimal tilt is the benchmark. East/west roofs produce ~10–20% less, so you would need proportionally more panels. A site assessment refines this.
How much roof do I need? About 17.5 sq ft per standard panel including spacing. A 16-panel system needs roughly 280 sq ft of clear, well-oriented roof.
Does the 30% tax credit apply to me? The federal Residential Clean Energy Credit is 30% of the system cost for owned systems (cash or loan) through 2032, with no income limit. Leases and PPAs do not qualify for the homeowner — the third party claims it.
Will more panels always save more? Only up to your usage plus any export value. Beyond what you use and what your utility credits, extra panels generate power you are barely paid for. The offset input lets you find that sweet spot.
Methodology
Annual consumption is monthly kWh (entered directly or derived from bill ÷ rate) times twelve times the target offset. Required DC system size is daily consumption divided by regional peak sun hours and a 0.80 derate. Panel count rounds up to whole panels; production is the resulting system size times sun hours times 365 times the derate. Cost uses your installed dollar-per-watt; the after-credit figure applies the 30% federal Residential Clean Energy Credit. Estimates for planning and education only — not an engineering design, quote, or tax advice.
Written by the CalcCottage team. We show the real number, not the marketing number.
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