Solar Panel Calculator
System size, output and payback period for rooftop solar.
Last updated
Your details
Roughly 4 in northern Europe, 5–6 in India and the southern US.
Result
- System size needed
- 3.42 kW
- Estimated annual generation
- 4,867 kWh
- Annual bill saving
- $38,933.33
- Estimated system cost
- $170,940.17
- Simple payback period
- 4.4 years
- Roof area needed
- 18 m²
- 25-year savings
- $802,393.16
About 7 panels at 550 W each
Roughly 2.6 m² per panel
Net of the system cost, ignoring degradation and rate rises
Why the derate factor matters
A 1 kW panel array does not deliver 1 kW. Inverter losses, high panel temperatures, wiring resistance, dust and imperfect angles combine to cost roughly 20–25% of nameplate capacity. This calculator applies a 0.78 factor, which is the standard industry assumption.
Peak sun hours is not daylight hours. It is the equivalent number of hours at full 1,000 W/m² intensity — a twelve-hour day might deliver only five peak sun hours.
Payback depends on policy as much as physics
The simple payback above assumes every generated unit displaces a purchased one. In practice that depends on your net-metering rules: full net metering makes the assumption roughly true, while low export tariffs mean daytime surplus is worth far less than what you pay at night.
Subsidies, tax credits and rising electricity prices all shorten payback; battery storage lengthens it substantially while improving resilience.
Formula & methodology
What this calculator assumes
- A derate factor of 0.78 accounts for inverter loss, temperature, wiring, soiling and mismatch. It is the long-standing rule of thumb; a well-sited modern system with a good inverter may reach 0.80–0.85.
- Peak sun hours are a location property, roughly 4 in northern Europe and 5–6 in India or the southern United States. Using a national average for a specific roof can be badly wrong.
- Assumes an unshaded array at a reasonable pitch and orientation. Shading is not linear — a small shadow can cost far more than its area suggests.
- Ignores panel degradation, typically 0.5% a year, and says nothing about whether generation coincides with your consumption.
Sources
Frequently asked questions
Should I size for my full usage?
Not always. If your export tariff is poor, sizing to your daytime consumption rather than your total often pays back faster than a larger system.
What sun-hours figure should I enter?
Peak sun hours for your location, which is the daily solar energy expressed as hours at full rated intensity — typically three to four in northern Europe, five to six across much of India, the southern United States and Australia. It is not the same as hours of daylight, and using daylight hours will roughly double the estimate.
Why is my actual output lower than the panel rating?
Panels are rated under standard test conditions that real roofs never meet. Temperature, dust, inverter losses, cable losses, shading and panel orientation together typically cost 15 to 25% of the nameplate figure, which is why the estimate applies a derating factor rather than using the rating directly.
Does the payback period account for rising electricity prices?
No, and that makes the figure conservative. It uses the tariff you enter held constant, so if prices rise the real payback arrives sooner. It also excludes maintenance, inverter replacement around year ten, and any subsidy or feed-in tariff available where you are.