ganak.appSolar Panel Calculator
Free solar panel calculator. Estimate system size, number of panels, installation cost, payback period, and 25-year savings with monthly production chart.
AZ/NV: 6-7h · CA/TX: 5-6h · NY/OH: 3.5-4.5h · WA/OR: 3-4h
Inverter, wiring, shading, temperature · 14% is typical
100% Electricity Coverage • 400W panels
Recommended system size
8.00kW
Financial Summary
Cost, savings, and return on investment
25-Year Net Savings: $44,943
272% ROI · $61,463 total value · 3%/yr escalation
System Details
Panel count, area, and daily production
Monthly Energy Production
Estimated kWh output by month (seasonal variation)
Environmental Impact
Annual carbon reduction from your solar system
How Solar Panel Sizing Works
Understanding the key factors that determine your system
Solar panel sizing depends on three core factors: your electricity consumption, your location's solar resource (peak sun hours), and system efficiency losses. The formula is:
Core Formula:
System Size (kW) = Annual kWh ÷ (Peak Sun Hours × 365 × Efficiency)
Peak sun hours measure the equivalent hours of full sunlight your location receives daily. This is not the same as daylight hours — a location with 12 hours of daylight might only have 4-5 peak sun hours because morning and evening sunlight is weaker. The southwestern US averages 6+ hours, while the Pacific Northwest averages 3-4.
System losses of 14% account for inverter conversion (3-4%), wiring (2%), temperature effects (5-8%), shading, soiling, and panel mismatch. NREL's PVWatts calculator uses 14% as the standard default.
Solar Panel Cost & Financial Returns
Understanding the economics of going solar
The average cost of solar in the US is $2.50–$3.50 per watt installed (2026), meaning a typical 8 kW system costs around $23,600 before incentives at $2.95/W. The federal Investment Tax Credit (ITC, US only) reduces this by 30% through 2032, dropping to 26% in 2033 and 22% in 2034.
Payback Period Calculation
The payback period accounts for annual electricity rate increases (historically ~3%/year) and panel degradation (~0.5%/year). Most systems pay for themselves in 6-10 years, then generate 15-19 years of pure savings. A typical system returns 200-400% on investment over its 25-year warranty life.
In the US, state incentives, net metering policies, and local rebates can further reduce costs. Some states like California, New York, and Massachusetts offer additional credits worth $1,000-$5,000. Other countries have their own solar incentive programs — adjust the tax credit slider to match your region.
Common Mistakes When Sizing Solar Panels
Avoid these pitfalls for an accurate estimate
Using daylight hours instead of peak sun hours
A day with 14 hours of sunlight doesn’t mean 14 peak sun hours. Peak sun hours account for angle, cloud cover, and intensity. Using daylight hours will dramatically undersize your system.
Ignoring system losses
A 10 kW system doesn’t produce 10 kW continuously. Inverter losses, wiring, temperature, and shading reduce output by 10-25%. Always factor in at least 14% system losses.
Not accounting for seasonal variation
Solar production varies 40-60% between summer and winter in most US locations. A system sized to produce 100% of annual usage will overproduce in summer and underproduce in winter.
Forgetting roof constraints
Not all roof area is usable — vents, chimneys, shading from trees, and orientation reduce the effective area. South-facing roof sections produce the most energy in the northern hemisphere.
Choosing the Right Panel Wattage
Panel efficiency and when it matters
Modern solar panels range from 350W to 550W. Higher-wattage panels produce more power per panel, meaning you need fewer of them. This matters most when roof space is limited.
| Wattage | Best For | Cost/Panel |
|---|---|---|
| 350W | Budget installs, large roofs | $175-$250 |
| 400W | Most residential installs | $200-$320 |
| 450W | Limited roof space | $280-$400 |
| 500W+ | Commercial or tight spaces | $350-$500 |
For most homes, 400W panels offer the best balance of cost and efficiency. Choose 450W+ only if your roof space is tight or you want to maximize production from a small area.
Frequently Asked Questions
Common questions about solar panel sizing, cost, and savings
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Last updated Mar 22, 2026