Power Produced by a Solar Electric System

Solar panels are assigned a rating in watts based on the maximum power they can produce under the ideal sun and temperature conditions. You can use this rated output to estimate the number of panels you'll need to meet some or all of your electricity needs. However, the exact amount of energy produced by a solar electric system also depends on roof orientation and tilt, as well as other factors such as shading, dust, panel conversion, and wire losses.

PTC vs. STC rating

PvUSA (as part of the National Renewable Energy Laboratory, provides the "real world"  performance rating for all eligible solar panels allowed to be used in Hawaii. For instance, a typical system will produce 204.4 watts of AC power at PVUSA test conditions (PTC), (PTC is 1kW/m2 irradiance, 1m/s wind speed, 20 degrees ambient temperature).

In contrast, the same PV module would have a rating of 230 Watts STC. The dc output of solar modules is rated by manufacturers under stand test conditions (STC). These conditions are easily recreated in a factory, and allow for consistent comparisons of products, but need to be modified to estimate output under common outdoor operating conditions.

It is best to design to the real world (PTC) in sizing and implementing the correct PV modules (solar panels) and associated inverters. A note on this is that the electric companies will typically use the PTC rating to determine the system size for net metering and this will be the expected output to the grid.