Decades before “going green” was hip, my dad, architect Jerry Moore, designed and built our house in Boulder, Colorado, using energy-efficiency principles. I didn’t fully appreciate those energy-saving features until I became involved in promoting sustainability while living off-grid in Durham, North Carolina. Inspired by my experience, I attended Solar Energy International’s Photovoltaic Design and Installation Course and later joined Southern Energy Management as a PV installer and designer.
I was excited when my dad called me to install a PV system on my childhood home. His decision was driven by available rebates and tax incentives, a needed roof replacement—and the discounted “family” rate for labor and materials. For the job, I gathered a powerful crew of women including two other SEI graduates and instructors: Rebekah Hren and Lauren Craig. It turned out to be an interesting installation with a few technical twists!
The home’s 28- by 36-foot, south-facing roof was a great match for solar, with an azimuth of 15° west of south and a pitch of 33.7°. When the house was built in the 1980s, the roof had been designed for a future PV system. However, over the last 30 years, a large mountain willow on the southwest portion of the property had grown considerably. A Solar Pathfinder shade analysis revealed that the tree shaded part of the roof primarily after 3 p.m. throughout the year. We did a more sophisticated analysis by uploading photos into the Solar Pathfinder program, which allows you to precisely trace tree lines and even account for scattered sunlight through deciduous trees when the leaves have fallen. Based on the location, orientation, slope, and shading factors, the program calculated 81% of usable solar irradiation for the site.
Grid-tied PV system sizing is not always determined by household energy consumption. System sizes are usually limited by available roof space or budget constraints. My father’s goal was to maximize generating capacity. With module efficiencies as high as 18%, SunPower’s E18 series modules were a first choice, compared to others in the 13% to 15% range. Since SunPower modules are slightly shorter and slimmer than other modules of the same power rating, there was room to squeeze in an extra module per row.
With the southern roof facing the street, aesthetics were also a concern. With black cells and a black frame, the SPR 225s provided an attractive solution. In addition, the systems are offered in scalable packages, including modules, inverter, rails, and a monitoring kit. All the equipment was delivered in a convenient portable storage unit, which served as on-site material and tool storage. However, the increased performance, style, and convenience does come at a premium—$0.50 to $1.00 more per watt.
The rooftop could accommodate 30 modules, resulting in a total system size of 6.75 kW. Our rack choices were either Unirac’s SolarMount or SunFrame. The SolarMount does not require as much precision, since the rails are underneath the modules. SolarMount also allows the use of grounding clips, which ground the modules to each other and to the rails, offering easier installation than using ground lugs on each module and stringing bare copper #6 wire throughout the entire array. We chose the SolarMount option for these reasons and because the flashed standoffs for the L-feet and rails could be installed by the roofing contractor. Using Unirac’s installation manual and local wind and snow loads, I calculated the maximum distance to be spanned by the L-feet. With that and row placements, my dad provided the roofers with a precise layout of the standoff locations.
I chose SMA America’s Sunny Boy 6000-US inverter because of the company’s reputation, performance, and reliability. Inverters can be loaded from 100% to 115% of their AC rating. Due to system inefficiencies and performance under real weather conditions, the 6.75 kW array infrequently produces more than the inverter’s 6,000 W AC power rating.
The last part of the design was the connection to the grid. The inverter specification for maximum AC output current dictates the size of the breaker needed to backfeed into the main service panel. The Sunny Boy 6000-US has a maximum continuous output current of 25 A at 240 V. For NEC requirements, a safety factor of 1.25 resulted in a 35 A breaker (25 A × 1.25 = 31.25 A, rounded up). However, our mains panel only had enough capacity left for 25 A (see “PV Breaker Calculations” sidebar), so instead we used a supply-side connection as the easiest and most cost-effective solution.