Hong Life CalculatorsUnderstanding the solar panel system calculator — concept and principle
This calculator sizes a rooftop solar PV system to cover a given yearly electricity use. From your consumption, local peak sun hours, panel rating and a performance ratio, it returns the system size in kWp, the number of panels, the expected yearly output and a simple payback period. It is a quick first check before asking installers for quotes.
How it is calculated
- System size kWp = yearly kWh ÷ (peak sun hours × 365 × PR).
- Panels = kWp × 1,000 ÷ panel watts, rounded up.
- Yearly output = panels × panel watts ÷ 1,000 × peak sun hours × 365 × PR.
- Installed size = panels × panel watts ÷ 1,000, the size of the whole-panel system.
- Simple payback = installed size × cost per kW ÷ (self-used kWh × price per kWh), where self-used kWh is the yearly output capped at your yearly use; exports are not valued.
The principle
Panel ratings (Wp) are measured under standard test conditions of 1,000 W/m² of sunlight at 25 °C cell temperature. "Peak sun hours" restates a location's daily solar energy as the number of hours at that 1,000 W/m² level: 3.8 kWh/m² per day equals 3.8 peak sun hours. Multiplying kWp by peak sun hours therefore gives the ideal daily output. The performance ratio (PR) then accounts for real losses: heat, inverter conversion, wiring, dirt, mismatch and minor shading. Well designed systems usually reach a PR of about 0.75–0.85.
Worked example
- Inputs: 4,000 kWh per year, 3.8 peak sun hours, 430 W panels, PR 0.8, cost 1,200 per kW, electricity 0.25 per kWh.
- Size: 4,000 ÷ (3.8 × 365 × 0.8) = 4,000 ÷ 1,109.6 ≈ 3.60 kWp.
- Panels: 3,604.9 ÷ 430 = 8.38, rounded up to 9 panels (3.87 kWp).
- Output: 3.87 × 3.8 × 365 × 0.8 ≈ 4,294 kWh per year.
- Payback: 3.87 × 1,200 = 4,644 in cost ÷ (the 4,000 kWh you use × 0.25 = 1,000 per year) ≈ 4.64 years; the extra 294 kWh exported is not counted.
| Peak sun hours per day (local) | System size (kWp) |
|---|---|
| 0.95 | 14.42 |
| 1.9 | 7.21 |
| 2.85 | 4.81 |
| 3.8 | 3.6 |
| 4.75 | 2.88 |
| 5.7 | 2.4 |
| 7.6 | 1.8 |
How to read the result
Output above your use means a surplus that is exported or stored. Payback depends heavily on how much solar power you use yourself versus export, since export tariffs are often lower than the retail price. If you can sell the surplus, payback gets a little shorter than shown; a low self-use share makes it longer.
Common mistakes
- Using daily sunshine hours from weather reports; peak sun hours are an energy figure, not hours of daylight.
- Ignoring roof orientation, tilt and shading, which can change output considerably.
- Forgetting panel degradation and inverter replacement over a 20–25 year life.
Frequently asked questions
Where can I find peak sun hours for my location?
Tools such as PVGIS from the European Commission's Joint Research Centre or NREL's PVWatts give irradiation and expected output for a specific site.
Does a panel produce its rated wattage?
Rarely. The rating applies at standard test conditions; on hot days output is lower because efficiency drops with cell temperature.
Sources: European Commission JRC, PVGIS (Photovoltaic Geographical Information System) · NREL, PVWatts Calculator documentation · IEC 61724-1, Photovoltaic system performance — Part 1: Monitoring