What Is Solar Performance Ratio and What's a Good Score?
Last updated: 2026-04-06 · Solar Benchmark
What Is Solar Performance Ratio and What's a Good Score?
Performance ratio (the percentage of expected energy your system actually produces) is the most reliable metric for comparing solar systems across locations and climates. A US residential system with a performance ratio of 0.85 is producing 85% of what physics says it should, given the sunlight available at its location. Scores above 0.85 are normal; below 0.75 warrants investigation.
Performance Ratio Benchmarks for US Residential Systems
| PR Score | Status | What It Means |
|---|---|---|
| Above 0.95 | Excellent | System is performing near theoretical maximum |
| 0.85 to 0.95 | Normal | Expected range for a healthy residential system |
| 0.75 to 0.85 | Watch | Below average; check for soiling, shading, or aging equipment |
| 0.60 to 0.75 | Alert | Action recommended; likely a hardware or configuration issue |
| Below 0.60 | Critical | Probable hardware fault; contact installer immediately |
(Source: pvlib physics modeling thresholds, calibrated against Open-Meteo ERA5 weather data)
How Performance Ratio Is Calculated
Performance ratio = actual energy produced / expected energy given available irradiance.
The expected energy calculation requires knowing the solar irradiance at the system's location for the time period measured. This is where most homeowner tools fall short: monitoring apps like Enphase Enlighten and mySolarEdge show actual production but don't calculate expected production from weather data. Without both numbers, PR cannot be computed.
A proper PR calculation uses hourly irradiance data (global horizontal irradiance, direct normal irradiance, and diffuse horizontal irradiance), applies the Perez transposition model to account for roof tilt and orientation, applies the Faiman temperature correction model, and then divides actual metered output by the model result.
PVWatts uses Typical Meteorological Year averages. In any specific month or year, actual weather diverges from the TMY average, which is why PVWatts estimates frequently miss actual production by 20–40%. Physics engines using ERA5 actual hourly weather data reduce that deviation to 5–7%. Learn more at /resources/methodology.
Why Performance Ratio Matters More Than Raw kWh
Two 8kW systems in different cities will produce different kWh totals. The Phoenix system produces about 14,600 kWh/year; the Boston system produces about 9,280 kWh. If both have PR scores of 0.82, both are performing normally for their location. If the Boston system has a PR of 0.71, something is wrong, and 2,000+ kWh/year is being lost.
This is the core value of PR: it accounts for weather. A cold, cloudy year reduces production but also reduces expected production. A system with PR 0.88 in a bad weather year is still healthy.
What Affects Performance Ratio
- Soiling: Dust on panels in dry climates reduces PR by 0.03–0.07 without regular cleaning.
- Shading: Partial shading in a string-wired system degrades PR by 0.05–0.15 depending on shade fraction and inverter type.
- Panel degradation: Each year of panel aging reduces PR by roughly 0.005 (0.5% of total).
- Inverter efficiency: String inverters operate at 96–98% efficiency at optimal load. Efficiency drops at low irradiance levels, particularly in winter mornings.
- Wiring losses: Resistance in undersized or degraded wiring shows up as sustained PR reduction across all weather conditions.
- Module mismatch: Panels from different production batches or manufacturers with different power ratings wired together will underperform the rated capacity.
Frequently Asked Questions
Q: What performance ratio should I expect from a new solar system?
A: A new system in good condition should have a PR between 0.78 and 0.90. Values above 0.90 are possible with premium equipment and no shading. Values below 0.78 on a new installation suggest a configuration issue or hardware problem from the start.
Q: My installer says my system is performing fine but I think production is low. How do I check?
A: Ask for the expected production in kWh for the last 12 months, based on actual weather data at your address, not a long-run TMY average. Divide your actual production by that expected figure. If the result is below 0.80 and your installer can't explain why, request a site inspection.
Q: Does PR change over time?
A: Yes. Performance ratio declines slowly as panels degrade. PERC panels lose about 0.5%/year, so PR typically drops from 0.83 to 0.78 over 10 years under normal conditions. A sharp PR drop in a single year, rather than a gradual decline, points to a specific event: a damaged panel, a failed microinverter, or new shading from a growing tree.
Q: What's the difference between performance ratio and capacity factor?
A: Capacity factor compares actual output to theoretical maximum assuming 24-hour sunshine. Performance ratio compares actual output to what the system should produce given the sunlight that actually arrived. PR is the better diagnostic because it isolates system issues from weather variation.
Data: pvlib physics modeling + Open-Meteo ERA5 weather data | Last updated: 2026-04-06 | Solar Benchmark