Question 1

How is the optimal tilt angle for a solar panel determined?

Accepted Answer

The optimal tilt angle maximizes annual irradiance on the panel surface. For Japan, this is roughly equal to the local latitude, around 30–35° facing true south. More precisely, the tilted-surface irradiance GT = Gb×Rb + Gd×(1+cosβ)/2 + G×ρ×(1−cosβ)/2 is summed over all months, and the angle β that yields the highest annual total is found by bisection.

Question 2

What are peak sun hours (PSH)?

Accepted Answer

Peak Sun Hours (PSH) represent the daily irradiance equivalent to 1 kW/m² (standard test conditions, STC). For example, an annual GHI of 1400 kWh/m² gives 1400÷365 ≈ 3.8 h/day. A 1 kWp PV system generates approximately PSH × (1−losses) kWh per day.

Question 3

What is the Hottel clear-sky model?

Accepted Answer

Hottel (1976) estimates beam and diffuse irradiance under clear-sky conditions using standard atmospheric transmittance. The beam transmittance is τb = a0 + a1×exp(−k/cosθz), where coefficients are corrected for altitude and climate zone. Actual monthly irradiance is further adjusted for cloudiness using the Ångström–Page equation H̄ = H0×(a + b×S/S0).

Question 4

What are solar declination and hour angle?

Accepted Answer

Solar declination δ is the angle between the sun and the equatorial plane due to Earth's axial tilt (23.45°), calculated as δ = 23.45°×sin(360°(284+N)/365) where N is the day of year. At summer solstice δ ≈ +23.45°; at winter solstice δ ≈ −23.45°. The hour angle ω is the angular displacement of the sun from solar noon, changing at 15° per hour (ω = 0 at noon). Together with latitude φ, these two angles determine solar altitude and azimuth.

Solar Radiation & Optimal Tilt Angle Calculator

Solar Position & Tilted-Surface Irradiance