My solar career was launched 10 years ago, when I helped start Minnesota’s first PV incentive program (see “A Tale of Two States: Small Solar Rebates—Steady Success,” HP111). Today, I do market research on electric utility engagement in the solar industry for a national association. Ironically, in all of those years, I didn’t have the right situation to install my own PV system. But after moving to northern Utah when my wife got a new job—and buying what I hope is our last home—it was finally time to make a solar commitment.
My goals were simple: maximize solar’s impact on our electricity consumption, minimize the physical size of the system, and design an attractive system. It was time to apply 10 years of advising others to my own PV installation.
The common siting choices did not work for our house: the backyard is significantly shaded and in a flood plain; and the roof is moderately shaded and flat, creating snow and wind-loading issues that I wanted to avoid. Using Solmetric’s iPV iPhone app (see “Siting App” sidebar), I analyzed the shade in the backyard (40%–50% shading annually), the roof (15%–25% shading), and the front yard (4%–6% shading) which was shown to be the sunniest location. Although my house is located in a residential neighborhood, it sits 125 feet back from the street and the front yard contains 11 fruit trees, a 1,000-square-foot vegetable garden, and the rest has perennial flowers—it’s not your typical lawn.
I designed a garden-appropriate wooden pergola for the array, based on a photo I found online. Unlike an industrial-looking pole mount or a metal-framed ground mount, the wood matched the existing fence and allowed us to walk under and through the structure (rather than around), softening the visual impact from the street and maintaining usable space underneath it. The pergola was built using Douglas fir, and to ensure longevity, the main wooden posts were set in slightly elevated Simpson brackets attached to the concrete footers, which keeps them well drained and away from soil contact. The entire pergola was sealed using Penofin, a UV-resistant stain, and all horizontal joints and penetrations were sealed with caulk.
I wanted to keep the pergola as small as possible but have an array big enough to have a measurable impact on our electricity use. That meant focusing on the modules with the highest power density—the greatest number of watts per square foot. I narrowed it down to SunPower SPR monocrystalline modules with rated efficiencies of about 19% and a power density of up to 18 watts per square foot—the most efficient modules on the market.
I compared different quantities of modules in various landscape and portrait configurations, looking at both the estimated annual performance and the resulting size of the pergola. I wanted to make sure we would offset at least 50% of our consumption but also not get too close to 100%—right now, the utility pays for excess annual generation at half the retail rate. I resurrected my high school trigonometry to play the tilt angle against the height and depth of the structure, including making sure that the fruit trees behind the pergola wouldn’t be shaded from May through October. I’m not an engineer and just overbuilt to cover any possibility for wind and snow loads, figuring the city engineer would tell me if the structure wasn’t up to par. My final design was nine 230 W modules, placed in a landscape orientation three wide and three high, and tilted at 30°.
One problem I found was that wiring the nine modules in series created the potential for high open-circuit voltages in the winter. Only a few inverters will accept 550 volts and closely match my 2 kW array size. SunPower modules also require a positive ground, which the inverter must match (most equipment uses a negative ground). Kaco’s Blueplanet 2502xi inverter was slightly oversized for my system, but would work fine within these requirements. I could have chosen microinverters, and a number of people in my area are doing so, but I’m not yet convinced of their longevity in the field under harsh temperature and moisture conditions—it all depends on your risk tolerance for using newer technologies.