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How to Measure Solar Intensity and Factors that Affect Intensity
The sun delivers large amounts of energy to the surface of the earth.
Figuring out how to
measure solar intensity, even if it's just a general idea, is
useful to help determine the best position to place solar panels in
order to generate the most solar power.
High above the clouds, satellites have measured the solar intensity
to be 1.366 kW per square meter. As the sunlight filters through the
atmosphere, some of the energy is reflected back into space while
some is absorbed by the air. The remaining amount of solar energy
to strike the surface of the earth averages about 1 kW per square meter.
This immense amount of energy powers the wind, weather and water
cycle that influence the entire world. Solar panels can tap into this
energy but their efficiency is greatly affected by the solar intensity
(the strength of the sunlight). Here are some factors that reduce
the solar intensity:
- At high noon, the angle at which the sun's ray strikes the earth's
surface is most direct and the sunlight feels hot. Solar panels produce
the peak amount of electricity at this time of day. At sunrise and
sunset, the angle is less direct and the sunlight feels weak. Less
power is generated when the angle of incidence between the sunlight
and the earth is low.
- As the earth orbits the sun, the distance between the two celestial
bodies change. Solar panels produce more power in summer when the sun
is high above the horizon and less power in winter when the sun is low.
- In regions near the equator, the sunlight is most direct.
The further away from the equator, the more seasonal variations you
will see in solar power production.
- Atmospheric conditions also reduce the intensity of the sunlight
before it reaches the surface of the earth. Air pollution may absorb
up to 5% of the solar energy while clouds may absorb up to 30%.
- Shadows from trees and other tall obstacles can reduce the
efficiency of solar panels up to 70%, even if only a portion of
the solar array is blocked. Debris such as leaves and dust can
also prevent the solar cells from producing peak amounts of
electricity by 10%.
You don't need complex equipment to measure solar intensity.
Before installing a solar array and storage system, a small,
inexpensive solar panel and data collector can be used to give
you a general idea of how much solar energy is available in
that location. It will record voltage,
current and the time of day and will show how the electricity
production changes from day to day and season to season. Place
it in an open, unobstructed location, in the same area where
you plan to install the larger solar panels.
Solar intensity is not the same as air temperature. Even
on a cold winter day, if the solar panels face directly
towards the sun, the solar cells will still produce electricity
despite the cold air temperature. An automatic mechanical system
& sensors can be used to align the panels in the best possible
angle, but the extra components may be cost prohibitive for most
household applications. A fixed-mount position for solar panels
is the easiest and most affordable option for residential home