I'm a big fan of solar power. But as with anything, I like to know exactly what I'm getting. One of the big unspoken issues in the solar sector is the difference between the rated or estimated potential output of a solar system--and the actual production of kilowatt-hours. A range of factors from the margin of error in the modules, to temperature, dust and losses from wiring, conversion to AC power and any batteries all can contribute to as much as 30 percent lower actual power production--even in the first year.
Compounding this problem in my mind is that in California only about a third to half of our solar installations are actually independently monitored, according to one of my friends at Fat Spaniel, one of the leading monitors of solar systems.
The California Energy Commission did some good thumbnail analysis of solar in the real world several years ago.
Here's the punch line from their analysis:
"So the '100-watt module' output, reduced by production tolerance, heat, dust, wiring, AC conversion and other losses will translate into about 68 watts of AC power delivered to the house panel during the middle of a clear day (100 watts x 0.95 x 0.89 x 0.93 x 0.95 x 0.90 = 68 watts)." From A Guide to Photovoltaic System Design and Installation (PDF) by the California Energy Commission. If you are interested in solar, you need to read their report.
But this 68 watts is only part of the story. If you have battery storage on the system they say it could reduce the power another 6-10 percent. They then stated that poor installation layout problems--including shading can take an additional toll. Another big issue is the angle of the roof and the direction it faces (in California, where your roof faces can affect the power output up to another 15 percent for many roofs). And interesting enough, for all the talk about making windows out of solar in what is typically described as Building Integrated Photovoltaics (BIPV), a vertical installation can reduce the power output up to about half all by itself!
Their bottom line: if the system is perfectly installed under perfect conditions the best case scenario for San Francisco would be 1,724 kwh, or electricity per year for each kilowatt installed and for Los Angeles would be about 1,758. But that's before all the "real-world" adjustments. When you make all those real-world adjustments--take another 25-30 percent or more off the top, even for a well designed system. This fits with our best San Francisco benchmark, our major 675 kW rooftop solar facility in the San Francisco at Moscone Center, which produces around 1,200 kilowatt-hours per year per rated kilowatt installed.
So when it comes to solar, let's make the right choice for solar power, but make it with our eyes open to the real world.