Energy from sunlight

The idea

The solar energy received by a surface is E = intensity × area × time, with intensity (insolation) in J s⁻¹ m⁻², area in m² and time in s.

A sunny patch

A flat roof lies in strong sunshine. We want the energy it receives.

If the roof area doubles and the sunshine stays the same for the same time, what happens to the energy received?

• It doubles
• It halves
• It stays the same

Change area and time

Change the area and the time, and watch the energy.

The recipe

More squares of sunlight and more seconds both give more energy. The intensity tells how much energy each square metre gets in each second.

The solar energy received by a surface is E = intensity × area × time, with intensity (insolation) in J s⁻¹ m⁻², area in m² and time in s.

Notes

E = intensity × area × time, with intensity in J s⁻¹ m⁻², area in m² and time in s.

Check yourself

Insolation is 1 kW m⁻², which is 1000 J s⁻¹ m⁻². How much energy does 2 m² receive in 3600 s? Give your answer as a number of 10⁶ J.

Answer: 7.2 × 10⁶ J

With the same intensity, how much energy does 3 m² receive in 1800 s? Give your answer as a number of 10⁶ J.

Answer: 5.4 × 10⁶ J

In E = intensity × area × time, which unit goes with the intensity?

Time must be put in the formula in seconds. How many seconds are in 1 hour?

• J s⁻¹ m⁻² — correct. Yes!
• J. The joule is the unit of the energy E, not of the intensity.
• m². The square metre is the unit of the area.
• 60 s. 60 s is one minute. One hour is 60 minutes, so 3600 s.
• 3600 s — correct. Yes!
• 100 s. An hour is 60 minutes of 60 seconds each, which is 3600 s, not 100 s.