The system’s performance met my software estimates fairly closely. Before installing, I modeled system performance using the National Renewable Energy Laboratory’s (NREL) Solar Advisor Model (SAM) software. Unlike NREL’s PVWatts or other web-based calculators, SAM can perform a more detailed analysis using specific system components and historical irradiance data from near my home. This software is not easy for the layperson to use, but if you are willing to learn, it is not insurmountable.
I took SAM’s output, applied the monthly shading estimate from the Solmetric app, and then compared that to the actual performance. Our system’s yearly generation was about 2% less than SAM predicted, which I consider quite close. Month-to-month variations from the estimate have ranged from -39% to +19%. Any number of factors could explain the variation—dirty panels, snow cover, winter pollution, wiring losses, solar radiation different from the average, etc. But from May to August 2011, I had four straight months of negative meter readings, essentially turning the meter clock backward—a satisfying feeling.
Other than the city permitting process, the design and installation went smoothly. Because of my solar experience and the unique design and location, I took on a burden that the average homeowner shouldn’t need to—the installer normally will design the system, apply for the permits, handle the interconnection application and incentive paperwork, get utility interconnection approval, and perhaps even set up the data monitoring system. Just remember that the lowest bid usually won’t give you the best service. Make sure you clearly discuss what is and isn’t included in the package price with your proposed installer.
Mike Taylor is the research director for the Solar Electric Power Association (www.solarelectricpower.org) in Washington, D.C. A thermal energy audit and retrofit, with solar space or water heating, is his next personal energy challenge.
Alpenglow Solar • www.alpenglowsolar.com • PV installer
Solar Advisor Model (SAM) • www.nrel.gov/analysis/sam • Performance estimator
Solmetric iPV • www.solmetric.com • Shading analysis iPhone app
SolarAnywhere • www.solaranywhere.com • Satellite irradiance data
Database of State Incentives for Renewables and Efficiency • www.dsireusa.org • RE incentives
Kaco New Energy • www.kaco-newenergy.com • Inverter
SunPower • www.sunpowercorp.com • PV modules
The Energy Detective • www.theenergydetective.com • Monitoring
Unirac • www.unirac.com • Rack