PV Pergola: Page 3 of 3

By:

Mike Taylor

Issue Date:

December / January 2012

Intermediate

Inside this Article

Once the structure was completed, the PV rack rails were installed.

Taylor Batteryless Grid-Tied PV Pergola System

This Kaco inverter has the required input voltage, closely matches the array size, and can work with positive-ground SunPower modules.

The TED monitoring system uses input from transducers clamped around the cables at the main breaker box.

Once the rails were up, the modules were mounted in a landscape configuration.

The city’s planning department allowed the PV pergola by terming it a “garden structure.”

Intermediate

Performance

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.

Take-Aways

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.

Access

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

System Components:

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