☀️ Solar
Solar panel output: watts, area and sun hours
A solar panel is rated in watts, but the energy it delivers depends on where it is, how it is mounted and how hot it gets. Here is how to turn a nameplate rating into a realistic daily and annual figure.
Nameplate watts are a laboratory number
Panel wattage is measured at standard test conditions (STC): 1,000 W/m² irradiance, 25 °C cell temperature, and an air mass of 1.5. A 400 W panel produces 400 W only in that laboratory flash test. In the field, irradiance varies through the day, cells run hotter than the air, and soiling, wiring and inverter losses take a share.
Use the nameplate as a comparison tool, not a promise. Two 400 W panels from different manufacturers can have different efficiency and different real-world output.
Efficiency is watts per area
Efficiency = rated power ÷ (1,000 W/m² × panel area) × 100. A 400 W panel with an area of 1.7 m² is 400 ÷ 1,700 = 23.5% efficient. That is the share of the sunlight falling on the panel that becomes DC electricity at STC.
Efficiency matters most when roof area is limited. A 22% panel costs more per watt but needs less roof for the same array. If you have plenty of area, a lower-efficiency panel can be the cheaper choice.
| Panel | Area | Efficiency | Watts/m² |
|---|---|---|---|
| 250 W | 1.6 m² | 15.6% | 156 |
| 300 W | 1.7 m² | 17.6% | 176 |
| 350 W | 1.8 m² | 19.4% | 194 |
| 400 W | 1.9 m² | 21.1% | 211 |
| 450 W | 2.0 m² | 22.5% | 225 |
The same wattage can come from different areas. A high-efficiency panel is not automatically a better investment; it depends on the cost per watt and the value of the roof space.
Peak sun hours
Peak sun hours (PSH) are the number of hours at 1,000 W/m² that would deliver the same daily irradiance. A location with 4.5 PSH receives about 4.5 kWh/m² per day. PSH fold latitude, season, clouds and haze into one number.
Daily energy (kWh) = array size (kW) × PSH × derate factor. A 4 kW array at 4.5 PSH and 0.8 derate produces 4 × 4.5 × 0.8 = 14.4 kWh/day on an average day. Annual energy is daily × 365, about 5,256 kWh.
That is an average. Winter days produce less and summer days more. Use monthly PSH data for a serious estimate, not a single annual figure.
| Location | PSH/day |
|---|---|
| London | 2.7 |
| Berlin | 2.9 |
| Tokyo | 3.4 |
| Sydney | 4.3 |
| Madrid | 4.8 |
| São Paulo | 4.3 |
| Mumbai | 5.0 |
| Cape Town | 5.0 |
| Dubai | 5.5 |
| Phoenix | 5.8 |
These are rounded annual averages for a well-oriented, unshaded site. Local microclimate, tilt, shading and the data source can move them by 10–20%.
Derating: where the watts go
| Loss | Typical range | Notes |
|---|---|---|
| Temperature | 5–15% | Cells run hotter than ambient |
| Inverter | 3–8% | Conversion and standby losses |
| Wiring | 1–3% | DC and AC resistance |
| Soiling | 2–5% | Dust, pollen, bird droppings |
| Mismatch | 1–3% | Panels not perfectly matched |
| Shading | 0–30%+ | Can be much worse than it looks |
| Availability | 1–2% | Downtime and maintenance |
A combined derate of 0.75–0.85 is a sensible quick estimate. Use 0.8 for a first pass, then replace it with a local design tool or PVWatts for a real installation.
Temperature
Cell temperature is usually 20–30 °C above ambient in full sun. A panel with a −0.35%/°C coefficient loses about 7–10% at 45–55 °C cell temperature versus 25 °C. That loss is already inside the derate factor if you use 0.8, so do not subtract it twice.
Cool, windy, open-rack mounting runs cooler than a flush roof mount. High-altitude sites run cooler but may have more irradiance.
Orientation, tilt and shading
North-facing panels in the northern hemisphere lose a lot of annual yield; east/west splits production across the day. Tilt near latitude is a common starting point for year-round output, but a steeper tilt favours winter and a shallower tilt favours summer.
Shading is not linear: a small shadow on one cell can disproportionately reduce a series string. Bypass diodes limit the damage but do not eliminate it. Keep panels clear of chimneys, trees and future obstructions.
What the numbers mean for a roof
To estimate array size, divide the available roof area by the panel area. A 20 m² roof with 2 m² panels fits 10 panels, about 4 kW at 400 W each, assuming no gaps for access or shading. Real layouts lose 10–20% to edges, walkways and setbacks.
Check the inverter limit and local grid rules. A 4 kW array with a 3.6 kW inverter is common; clipping losses are small in most climates but larger in very sunny, cool locations.
Quick checks
- Use monthly peak sun hours, not a single summer figure.
- Apply a derate of 0.75–0.85.
- Do not double-count temperature loss.
- Check shading at the worst time of year.
- Use a local design tool for a real installation.
Frequently asked questions
Sources
- NREL PVWatts Calculator and documentation.
- NREL National Solar Radiation Database (NSRDB).
- IEC 61215 — design qualification and type approval of PV modules.
- IEC 61724 — photovoltaic system performance monitoring.
- Manufacturer datasheets for module dimensions, temperature coefficients and power tolerances.