---
title: "Solar constant, insolation, irradiance, peak sun"
subject: "Energy Environment and Society"
chapter: 3
key: "semester_7/energy_environment_and_society:solar-constant-insolation-definitions"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/solar-constant-insolation-definitions
site: BCT Engineering
---

# Solar constant, insolation, irradiance, peak sun

Energy Environment and Society · Chapter 3: Renewable Energy Sources

- Asked in: 2072 Chaitra; 2069 Chaitra, 3 marks; 2071 Shrawan, 2 marks; year not given (asked 3 times)
- Video: [Solar Radiation ll part-1](https://www.youtube.com/watch?v=--GfdbavEk8) — ELECTRICAL ENGINEER
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/solar-constant-insolation-definitions
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-3

## Question

Define solar constant, extraterrestrial radiation, insolation, irradiance, global irradiation, peak sun and sunshine hour.

## Short answer

Solar constant is about 1367 W/m2, the solar energy per unit time on a perpendicular surface at mean earth-sun distance outside the atmosphere. Insolation is solar energy received per area over time, irradiance is instantaneous W/m2, and global irradiation is beam plus diffuse on a horizontal surface. Peak sun hours equals daily insolation in kWh/m2 divided by 1 kW/m2; sunshine hours are bright direct sunshine hours.

## Answer

| Term | Definition |
|---|---|
| **Solar constant** (Gsc) | Energy from the sun per unit time received on a unit area of surface perpendicular to the rays, at the mean earth–sun distance, just outside the atmosphere. Value: about **1367 W/m²** (the modern value is about 1361 W/m²). |
| **Extraterrestrial radiation** | Solar radiation received on a surface outside the earth's atmosphere, before any absorption or scattering. It varies slightly through the year (±3.3%) with earth–sun distance. |
| **Insolation** | Solar radiation energy received on a given surface area over a given time (INcoming SOLar radiATION). Usually in kWh/m²/day. |
| **Irradiance** (G) | Rate at which radiant energy falls on a unit area at an instant, in W/m². (Power per unit area.) |
| **Global irradiation / radiation** | Total solar radiation on a horizontal surface at the earth's surface, i.e. beam plus diffuse radiation. |
| **Irradiation** (energy) | Irradiance integrated over time (hour or day), in kWh/m² or MJ/m². (Called *hourly* or *daily* irradiation.) |
| **Peak sun hours** | Number of hours per day for which the solar irradiance would be 1000 W/m² to give the same daily energy. Example: 5 kWh/m²/day = 5 peak sun hours. Kathmandu typically has about 4.5–5.5 peak sun hours. |
| **Sunshine hour** | Number of hours in a day when the direct sunshine is bright enough to burn a mark on a recorder (Campbell–Stokes), i.e. direct radiation above about 120 W/m². |

Relation: *peak sun hours (h) = daily insolation (kWh/m²/day) ÷ 1 kW/m²*.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which description fits the solar constant?
   A. About 1367 W/m2 on a surface perpendicular to the rays, outside the atmosphere at mean earth-sun distance
   B. About 1000 W/m2 on the ground at noon
   C. Daily energy in kWh/m2 on a horizontal surface
   D. Radiation measured by a Campbell-Stokes recorder
   Answer: A. The solar constant is the flux just outside the atmosphere on a perpendicular surface, at mean distance, about 1367 W/m2. (Key point: Solar constant)

2. What is the difference between irradiance and irradiation?
   A. Irradiance is energy; irradiation is power
   B. They are the same quantity
   C. Irradiance is only beam; irradiation is only diffuse
   D. Irradiance is power per unit area (W/m2); irradiation is its integral over time (kWh/m2)
   Answer: D. Irradiance is an instantaneous rate; integrating over time gives irradiation (energy per area). (Key point: Irradiance vs irradiation)

3. A site receives 5 kWh/m2/day. How many peak sun hours is this?
   A. 0.5 hours
   B. 5000 hours
   C. 5 hours
   D. 1000 hours
   Answer: C. Peak sun hours = insolation / 1 kW/m2 = 5 / 1 = 5 hours. (Key point: Peak sun hours)

## Related in Energy Environment and Society

- [Beam, diffuse and global radiation](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Abeam-diffuse-global-radiation) — Energy Environment and Society, chapter 3
- [Solar radiation, solar cell and plant](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-radiation-cell-plant) — Energy Environment and Society, chapter 3

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