Last reviewed 1 Oct 2026 · Facts as of 1 Oct 2026 · 8 min read
Solar radiation basics
- Solar constant : the energy per unit time on a unit area perpendicular to the sun's rays at the mean Earth–Sun distance, outside the atmosphere — about 1361–1367 W/m² (older books give 1353 or 1367).
- On passing through the atmosphere radiation is absorbed and scattered (by ozone, water vapour, CO₂, dust and air molecules).
| Component | Meaning |
|---|---|
| Direct (beam) radiation | Arrives in a straight line from the sun's disc |
| Diffuse radiation | Scattered by the atmosphere/clouds, arrives from all of the sky |
| Global (total) radiation | Direct + diffuse on a surface |
| Reflected radiation (albedo) | From the ground; matters for tilted and bifacial panels |
- Air mass (AM) is the relative path length of the beam through the atmosphere: ( = zenith angle). AM 0 = outside the atmosphere; AM 1 = sun overhead; AM 1.5 (zenith angle 48.2°) is the standard for testing PV.
- Insolation is the energy received per unit area over a time, in kWh/m²/day. India averages about 4–7 kWh/m²/day with about 250–300 clear sunny days a year — among the best in the world; Rajasthan, Gujarat and Ladakh are particularly rich.
- Peak-sun-hours = daily insolation in kWh/m² (equal to the hours of 1 kW/m² sun) — used for sizing: 5 kWh/m²/day = 5 peak-sun-hours.
Instruments
| Instrument | Measures |
|---|---|
| Pyranometer | Global (or diffuse, with a shade ring) radiation on a horizontal surface (thermopile or silicon sensor) |
| Pyrheliometer | Direct beam radiation (tracks the sun through a narrow tube) |
| Sunshine recorder (Campbell–Stokes) | Hours of bright sunshine (burns a card) |
| Albedometer | Reflected radiation |
Sun angles
Declination (°), hour angle . . For maximum yearly energy a fixed panel is tilted at about the latitude, facing south (northern hemisphere).
Photovoltaic effect and cells
Photovoltaic effect: when photons with energy greater than the band gap strike a p–n junction, they create electron–hole pairs; the built-in field separates them, driving a direct current through the external load.
- Silicon band gap ≈ 1.1 eV; a single silicon cell gives about 0.5–0.6 V (open-circuit about 0.6–0.7 V) regardless of area; current is proportional to area and irradiance.
| Cell type | Efficiency (commercial) | Remarks |
|---|---|---|
| Monocrystalline Si | about 18–23 % | Highest efficiency, uniform dark colour; PERC, TOPCon, heterojunction variants |
| Polycrystalline Si | about 15–18 % | Cheaper, bluish speckled look |
| Thin film (a-Si, CdTe, CIGS) | about 9–19 % | Flexible, better in diffuse and hot conditions |
| Perovskite, tandem | Lab values above 25 % | Under development |
| Concentrator PV | high | Needs tracking and direct sun |
I–V characteristics
FormulaKey PV parameters
- Short-circuit current (voltage = 0) — proportional to irradiance.
- Open-circuit voltage (current = 0) — falls with temperature.
- Maximum power point .
- Fill factor (typically 0.7–0.85).
- Efficiency , with = irradiance (W/m²) and = area (m²).
- Standard Test Conditions (STC): W/m², cell temperature 25 °C, AM 1.5; the rating in Wp (watt-peak) is the output at STC.
Worked ExampleExample — efficiency and fill factor
A module has V, A and W, area 1.7 m².
. .
Temperature and shading effects
- Power falls about 0.3–0.5 % per °C above 25 °C (the temperature coefficient of power); drops about 0.3 %/°C, rises very slightly. Cells run 20–30 °C above ambient, so summer output is lower than the rating.
- Partial shading of one cell can reduce the string output a lot (it acts as a load); bypass diodes across groups of cells limit the loss.
- Soiling (dust) can cut output by several percent; regular cleaning matters in India.