Consumers need to have a good idea of how much solar energy they will be producing once they have commissioned a system. Most Solar Companies will provide a production report during the bidding phase. Most production reports will be a ball park estimate using a solar production software such as PV Watts. PV Watts is very accurate but in no way can be 100% accurate in the actual production of a solar photovoltaic system. The energy produced from solar panels is dependent on the size of the array, the orientation of the array, the magnitude of sunlight, and the time of year.
Solar Array Size
Notice the cloud coverage between 2:00 and 3:00 PM. The production continues to follow a bell curve pattern once the cloud has passed.
The easiest concept to understand is that larger capacity solar arrays yield more energy production than small solar arrays. The wattage size of a solar module is rated by the amount of wattage produced during a flash test in a testing laboratory. Solar Modules are wired together to accumulate the total solar array size. For example, 10 solar modules rated at 250 watts will deem a 2,500 Watt Solar array. In real time production, a solar array will start producing energy in the morning and slowly increase production until it reaches the optimum production time when the sun is
directly facing the solar modules. From this point, solar energy production will slowly decrease until the sun sets. The peak of solar energy output usually represents 85 – 90% of the name plate rating of the system.
Orientation of the Solar Array
This graph represents 2 different solar inverters. One Solar Array is facing eastern and the other is facing western. Notice that the eastern array produces better in the morning while the western array produces better in the afternoon.
Solar panels in Austin, TX should be oriented in a southern azimuth between due east and due west. The amount of solar production can vary 8 – 12% depending on the type of solar module and orientation. Some solar modules, such as Panasonic, have better production efficiency in less optimum azimuth angles. Due south is the optimum for solar production and all around production. Eastern azimuths will deem extra production in the morning while western arrays will produce more energy in the afternoon. Western facing solar arrays are becoming more popular in areas with peak demand pricing.
Magnitude of Sunlight
The measure of power per unit area provided by the sun is called irradiance. Solar irradiance varies by the time of day, time of year, and cloud coverage. Solar energy production has a direct relationship with solar irradiance. Stormy weather and overcast conditions can greatly reduce the amount of solar being produced.
Cloud coverage can significantly reduce the amount of solar energy produced.
Time of Year
As seasons change, so does the angle of incidence between the earth and the sun. Central Texas has a solar incidence spanning between 80 degrees in the summer and 45 degrees in the winter. This means that the angle of the modules will also affect the amount of solar produced. Most solar modules are mounted directly to a roof and the angle is determined by the slope of a roof. However, commercial solar power systems and groundmounts can be custom designed at a set angle. A preferred angle for annual production is 30 degrees. Lower sloped solar arrays will produce better in the summer while steeper arrays produce better during the winter. Summer production is often a better investment because of the heavy summer loads and consistent sunlight.
The solar production on a winter day is less than the summer production when the solar panels are at a low slope in Austin Texas.
The low slope orientation of the solar panels optimizes production during the winter when the sun is at a higher incidence angle.