The Amount of solar energy available is determined by the number of photons that reach your photo voltaic cell, and whether or not these photons energize an electron into movement therefore creating a current. The number of photons, or amount of sunshine, which hits your solar cell needs to be determined to see the number of cells or solar panels you will require for your solar system. This energy will vary substantially according to your latitude, the time of year, and cloud cover your region can expect.

What Techniques are Available to Determine Solar Energy?

The two factors that are considered are the angle of the sun to your solar panets and the amount of hours of sunshine available at your residences location. This combination is expressed as insolation and is computed as an average irradiance measured in kilowatt hours per square meter per day. A solar irradiance of one thousand watts per square meter is the standard for the energy obtained from the sun at noon in the middle of summer. Usually solar irradiance is measured in the quantity of kilowatts per square meter per day so if it was possible for the sun to be up for 8 hours and always at the highest power, solar irradiance would be 8..

Solar irradiance will vary considerably during the year especially in northern latitudes. For instance, Chicago will have an irradiance of just 1.6 in January, yet it will be 6.1 in June, and the yearly average will be 4.0. This means that solar energy in December is 70% less than in June. Nearer to the equator, we find that the similar numbers for Phoenix are 3.0 in January and 7.8 in June for an average of 5.5 for the year. If you are looking for an off grid application, your solar system will have to be 2-3 times higher than required by June figures. Values for your place are easily accessible on the web because NASA's weather satellites have been obtaining this data around the globe for many years.

You can Plan your System Sizing Using Irradiance for your Locale.

If we know how many kilowatts we require in order to meet the energy requirements of our house, we are now in a position to size our system. If we are planning a grid-tie system, you can use the average annual irradiance for your calculations because the goal is to make your payment to the power company zero for a full year. If however, you need an off-grid system, you will need to use the December value for irradiance, and the system will have excess capacity for the majority of of the year.

If you divide your requirements for the day by the irradiance, you will obtain the amount of watts your system needs to generate. A grid-tied house in Phoenix using 20kw per day divided by the average irradiance of 5.5 would need to generate 3600 watts per hour. You can then divide the system wattage by the output of your photovoltaic panels to figure out the number of solar panels required. In our example, if the panel is rated at 150 watts, you will require 24 solar panels.