Calculating Solar Energy System Output

The amount of electricity a home or commercial solar electric system will produce depends on three factors: its orientation (known as azimuth), tilt, and attendant losses. The table below shows the expected annual outputs for a sample 1 kilowatt (kW) system at different tilts and azimuths.

  • Tilt: the vertical orientation of a system relative to the horizontal
  • Azimuth: the horizontal orientation of system relative to true south

In the northern hemisphere, the optimal vertical orientation (tilt) of a solar energy system is the latitude at the location (around 39 degrees in this area), and the optimal horizontal orientation is due south.

1 KW     Derate: 0.82     Zip code: 20886
Projected Annual Output (kWh)
Array Azimuth Array Tilt
-300 -150 390 +150 +300
-300 1,250 1,318 1,327 1,239 1,148
-150 1,257 1,338 1,351 1,297 1,179
180 1,259 1,346 1,358 1,302 1,183
+150 1,257 1,338 1,351 1,296 1,181
+300 1,236 1,286 1,314 1,276 1,170
% Deviation from Maximum Output
Array Azimuth Array Tilt
-300 -150 390 +150 +300
-300 -8.0% -2.9% -2.3% -8.8% -15.5%
-150 -7.4% -1.5% -0.5% -4.5% -13.2%
180 -7.3% -0.9% 0.0% -4.1% -12.9%
+150 -7.4% -1.5% -0.5% -4.6% -13.0%
+300 -8.0% -2.9% -3.2% -6.0% -13.8%

 Additional Factors

A number of additional factors discussed below also affect your solar energy system’s output. All of these factors are summarized in the derate factor. The derate factor is a percentage value that is applied to your home or business’s expected solar energy output, given the system’s tilt and azimuth, to estimate the real-world output you will experience.

Insolation

The more sunlight a solar panel absorbs, the higher the solar energy system’s output will be. However, many factors affect the amount of sunlight that solar panels absorb:

  • Intensity of the sun
  • Angle of the sunlight relative to the solar panel
  • The solar energy system’s tilt
  • The east-west orientation of the your home or business’s roof
  • Seasonal variations in the sun’s path across the sky

Dirt and Dust

Solar panels that have collected dirt and dust can experience up to a 7% loss in production. The good news though is that this loss can be avoided if you clean your solar panels regularly. A good hose is often all you need.

Electrical Conversion Losses (DC to AC)

An inverter is used to convert the DC power generated by your solar panels into common household AC power. Up to 14% of the power generated by your solar energy system can be lost in this conversion process.

According to the manufacturers, inverters commonly used in residential solar energy systems have peak efficiencies of 92% to 98%. However, the inverters are measured under well-controlled factory conditions that are different from your home. Solar energy efficiency in a residential home or commercial business is typically 88% to 92%.

Temperature

A solar panel on your home’s roof can reach inner temperatures of 122 to 167 degrees F or more. As the solar panel’s internal temperature increases, its output can fall by as much as 12%. You can outfit your solar energy system with panel materials such as thin films that are not as sensitive to temperature.

As a rough rule of thumb, a 100 W crystalline solar panel will typically operate at about 85 W in the middle of a spring or fall day in full sunlight.

Solar Panel (or Module) Mismatch

The maximum power output of an entire solar array will usually be less than the sum of the maximum output of the individual solar panels that make up the array. This is due to slight performance inconsistencies from one solar panel to the next. This phenomenon is known as “module mismatch” and results in a minimum 2% loss in system power.

Wire Losses

Your home or business’s solar energy system must be connected to your electrical meter. This wiring results in electrical losses of 3% to 5%, depending on the length and gauge of the wires.

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