Before running a full simulation, solar installers need a fast, credible production number to put in front of a customer. This estimator multiplies the system size by bundled monthly irradiance for the chosen climate zone and applies a performance ratio to give monthly and annual energy yield, plus an optional bill-offset figure.
How it works
The peak-sun-hours method is the industry-standard quick estimate. The daily irradiance in kilowatt-hours per square metre is numerically equal to the equivalent peak sun hours, because the module rating is defined at one kilowatt per square metre. So each month:
E_month = kWp x H_daily x days_in_month x PRwhere H_daily is the average daily irradiance for that month and PR is the performance ratio that derates for real-world losses. Summing the twelve months gives annual yield, and dividing by the system size gives specific yield in kilowatt-hours per kilowatt peak per year.
Reading the monthly profile
Monthly energy production is not evenly distributed — the profile follows the irradiance pattern of your climate zone. In a temperate zone, June and July often produce two to three times the energy of December and January. This matters practically for bill-offset calculations: a system designed to cover 100% of an annual bill will overproduce heavily in summer and underproduce in winter, making net-metering credit carry-forward policies important to the customer’s economics.
The monthly breakdown also helps identify whether sizing up for winter production is economically sensible. In many temperate climates the answer is no — the additional panels serve only the worst few months and the extra capital cost is rarely recovered.
Climate zone specific yield expectations
The specific yield (kWh per kWp per year) varies significantly by zone:
| Zone | Typical specific yield range |
|---|---|
| Desert (e.g., Sonoran, Saharan) | Above 1,800 kWh/kWp |
| Mediterranean | Roughly 1,400–1,800 |
| Subtropical | Roughly 1,400–1,700 |
| Tropical | Roughly 1,200–1,600 (clouds reduce it) |
| Temperate (e.g., northern Europe) | Roughly 800–1,100 |
| Continental (cold winters, hot summers) | Roughly 1,000–1,400 |
These are illustrative ranges for reasonably well-sited systems. A shaded or poorly oriented roof can fall well below the lower end of the range.
Performance ratio: the biggest variable
The performance ratio collapses many real-world loss factors into one number. Typical contributions:
- Inverter efficiency: 2–5% loss
- Module temperature derating: 3–8% loss (higher in hot climates)
- Soiling and dust: 1–4% loss
- DC wiring losses: 1–2%
- Module mismatch: 1–2%
A clean, well-cooled, recently installed system in a mild climate might reach 0.83–0.85. A hot, dusty site with older equipment might run 0.70–0.75. Choosing the performance ratio honestly is the most important judgement call in this tool.
Accuracy and next steps
The bundled irradiance values are representative climatology for each broad zone at roughly latitude tilt. They are ideal for the first proposal conversation and a rough payback estimate, but they are not site-specific. For a binding quote, move to NREL PVWatts or SAM with the site’s typical meteorological year data, the actual tilt and azimuth, and a real shading analysis.