Solar Panel Efficiency Calculator
Calculate actual vs rated solar panel efficiency, temperature derating losses, and 30-year degradation projection.
Efficiency Results
Temperature Derating Analysis
30-Year Degradation Projection
Based on 0.5% annual degradation rate (industry standard).
| Year | Est. Output (W) | Efficiency (%) | Total Loss (%) |
|---|---|---|---|
| 1 | 398W | 21.42% | -0.5% |
| 2 | 396W | 21.31% | -1% |
| 3 | 394W | 21.21% | -1.5% |
| 4 | 392.1W | 21.1% | -2% |
| 5 | 390.1W | 21% | -2.5% |
| 6 | 388.1W | 20.89% | -3% |
| 7 | 386.2W | 20.79% | -3.4% |
| 8 | 384.3W | 20.68% | -3.9% |
| 9 | 382.4W | 20.58% | -4.4% |
| 10 | 380.4W | 20.48% | -4.9% |
| 11 | 378.5W | 20.37% | -5.4% |
| 12 | 376.6W | 20.27% | -5.8% |
| 13 | 374.8W | 20.17% | -6.3% |
| 14 | 372.9W | 20.07% | -6.8% |
| 15 | 371W | 19.97% | -7.2% |
| 16 | 369.2W | 19.87% | -7.7% |
| 17 | 367.3W | 19.77% | -8.2% |
| 18 | 365.5W | 19.67% | -8.6% |
| 19 | 363.7W | 19.57% | -9.1% |
| 20 | 361.8W | 19.47% | -9.5% |
| 21 | 360W | 19.38% | -10% |
| 22 | 358.2W | 19.28% | -10.4% |
| 23 | 356.4W | 19.18% | -10.9% |
| 24 | 354.7W | 19.09% | -11.3% |
| 25(warranty) | 352.9W | 18.99% | -11.8% |
| 26 | 351.1W | 18.9% | -12.2% |
| 27 | 349.4W | 18.8% | -12.7% |
| 28 | 347.6W | 18.71% | -13.1% |
| 29 | 345.9W | 18.62% | -13.5% |
| 30 | 344.2W | 18.52% | -14% |
How It Works
This calculator analyzes your solar panel performance across three dimensions: efficiency, temperature impact, and long-term degradation.
- Rated efficiency — calculated as rated wattage divided by panel area (converted to m²) times standard irradiance (1,000 W/m²). This is the manufacturer specification under Standard Test Conditions (STC).
- Actual efficiency — uses your measured real-world output instead of rated wattage to show true performance.
- Performance ratio — the ratio of actual output to rated output, expressed as a percentage. Values below 75% may indicate issues like shading, soiling, or equipment problems.
- Temperature derating — converts ambient temperature to Celsius, calculates how many degrees above STC (25°C), and applies the temperature coefficient to estimate heat-related losses.
- Degradation projection — models output decline over 30 years using the industry-standard 0.5% annual degradation rate, so you can forecast long-term performance.
Frequently Asked Questions
What is a good solar panel efficiency rating?
Most residential solar panels have an efficiency between 18% and 22%. Premium panels from manufacturers like SunPower and REC can reach 22-23%. Anything above 20% is considered high efficiency. The average panel sold today is around 20-21% efficient.
How does temperature affect solar panel efficiency?
Solar panels lose efficiency as temperature rises above 25°C (77°F). The typical temperature coefficient is -0.3% to -0.5% per degree Celsius above STC. On a 95°F day, panels can lose 7-12% of their rated output. This is why panels in hot climates often produce less than expected despite abundant sunshine.
How much do solar panels degrade over time?
Solar panels degrade at roughly 0.5% per year on average. After 25 years (the standard warranty period), panels typically still produce about 87-88% of their original rated output. High-quality panels may degrade as little as 0.3% per year. Most manufacturers guarantee at least 80% output at year 25.
What is the performance ratio of a solar panel?
The performance ratio compares actual energy output to the rated (nameplate) capacity. A ratio of 85% means the panel produces 85% of its rated wattage under real-world conditions. Losses come from temperature, shading, wiring, inverter conversion, and soiling. A performance ratio of 75-85% is typical for well-installed residential systems.