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Solar Radiation Resource Site EvaluationJust how much sunshine reaches your site?The performance of solar heating systems depends upon the amount of solar radiation reaching your solar heating panels. Every place on Earth receives solar energy at least a part of the year. Even Alaska has adequate solar resources, at least a part of the year. The amount of solar energy that actually reaches where you live depends upon these factors:
A common problem in placing a solar heating panel is shading. The solar heating panels in the picture below performance are diminished because of shading. Evaluate your site to determine if shading will impact the amount of solar radiation reaching your solar panels. Courtesy DOE 
Seasonal changes in Sun’s angle along with shortened daylight, and weather also affect annual sun hours or solar isolation. Solar insolation refers to the amount of solar energy, direct or diffuse, reaching the earth per a unit of time. Begin by evaluating your sites potential to capture the Sun’s free energy. Technically, solar radiation is the visible and near visible electromagnetic radiation emitted by the Sun. Woo - let’s just call it “solar energy” for short. The Suns solar energy transforms into heat for solar hot water. Regardless of where you live, the solar energy potential is adequate! In northern climates, you will need higher efficiency solar heating panels or more panels. Solar heating is used from Alaska to Antarctica. You may also be interested in
Freeze Protection for Solar Water Heating Systems. Evacuated tube solar heating panel installation being installed in a cold climate. Courtesy DOE. 
Below is a map showing the best areas for solar hot water in the United States. However, where you live affects the size of solar heating panels needed. In areas designated as good or moderate, the solar energy potential is adequate – you will need a larger solar collector. Your solar professional will help you determine the right size collector for your site. Generally, the capacity of your solar heating system will not provide 100% of your hot water needs year round. In the northern hemisphere, January is usually the month with the lowest solar insolation level. System can meet about 50% to 75% of your hot water requirements. How Much Solar Energy - Solar IsolationThe amount of solar energy or solar insolation level determines the right size solar collector for your site. The word “solar insolation” is a contraction of "incident solar radiation;" not to be confused with insulation. Scientists measure the incoming solar energy or insolation as the amount of solar radiation reaching the Earth’s surface. The amount of insolation received is controlled by the Sun’s angle and location in the sky. Average Daily Solar Radiation Per Month in January in the US. Courtesy DOE
The closer the Sun is to the horizon the lower the solar isolation striking a solar collector. The values of solar insolation are commonly expressed in kWh/m 2/day. This amounts to how much solar energy that strikes a square meter of the earth’s surface in a single day. The Sun strikes the surface at different angles ranging from nearly directly overhead on June 21 and just above the horizon on December 21 – the shortest day of the year in the northern hemisphere. Therefore – when the Sun is nearly vertical, during the summer, the Earth receives the maximum amount of solar energy. If you live in the southern hemisphere the maximum amount of solar energy is received on December 21 and the minimum on June 21. The time it takes to evaluate your site’s solar resource makes for a successful solar hot water heating system that meets your needs for many years. Pay particular attention to shading from trees and buildings. An experienced solar hot water dealer can help.
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