Solar irradiance is a ratio of power received from the sun divided by the collecting surface area:

This calculator is useful for measuring solar irradiance on Earth using a jar of water. A jar full of water with a black absorbing material covering the rear interior of the jar is placed in the shade and comes to thermal equilibrium with the air. At measurement time (usually near noontime), the temperature of the water in the jar is measured and the jar is placed directly facing the sun for a short time interval (usually 20 minutes). At the end of the time interval, the temperature of the water in the jar is measured again.

The value measured for solar irradiance on the surface of the Earth is smaller than the solar irradiance above the Earth's atmosphere. On a very clear day at noontime, the atmosphere will absorb/reflect 20% of the incident solar energy.