The home’s triangular roof had a skewed edge, so instead of a ladder we used a scissor lift for worker access. (See the “Roof Safety” sidebar for other challenges and solutions.)
Attaching the feet and rails to the standoffs installed by the roofers was straightforward due to our design work. With the SolarMount system, the first module’s placement is crucial. If the first module is even slightly askew, the discrepancy will be magnified going down the row. I always start the installation on the bottom, so that once a row is in place, there’s somewhere to rest the next row of modules.
To minimize the effects of late-afternoon shading, we wired the modules in three sections, in columns, so that there was a western, middle, and eastern string configuration. As the tree’s shadow creeps onto the western part of the roof, it will take the strings out one by one. If the strings had been organized by rows, the entire array would have stopped producing as soon as the shadow line crossed the first column of west-most modules. This also had safety benefits by allowing us to work on one side of the roof at a time—we did not have to constantly switch anchor points for our ropes to prevent swinging.
To make the wiring transitions to the attics, I typically use SolaDeck’s flashed junction box, which can be hidden underneath the array. This allows the penetration to be hidden under a module and still comply with the NEC article 690.34 requirement of being “rendered accessible directly or by displacement of a module.” But since this wiring couldn’t be located under the array due to attic access, we painted the low-profile combiner box to match the roof and positioned it 2 feet below the array to make a penetration into an accessible garage space where the inverter was located.
Since we opted for a supply-side tap for interconnection, we no longer had to make our way from the inverter to the main service panel, but rather to the outside of the house near the meter location. After coordinating with a licensed electrician and the local utility, we ran our inverter output outside to an AC fused disconnect. We then added a junction box to tap into the service lines between the meter and the home’s main service panel. We installed a ground rod for grounding the module frames and inverter, and buried a copper conductor and tied it to the existing electrical service ground rod.
After making the last connections on the roof, we ordered the final inspection by the local electrical inspector. Following their approval, the utility company replaced the existing electric meter with a new bidirectional meter and officially commissioned the system for service. At last, we reinstalled the DC fuses in the inverter, engaged the DC and AC disconnects, and started producing home power!
System performance was predicted using the Solar Pathfinder software and NREL’s PVWatts online calculator. PVWatts uses system size, location, historic weather data, orientation, and tilt to estimate production. You can get even more precise by manipulating the calculator’s derate factors. We changed the shading factor to 81% to coincide with the Solar Pathfinder’s results, and inverter efficiency of 95%, which resulted in a 0.643 DC-to-AC derate factor. PVWatts predicted this system would produce about 8,020 kilowatt-hours (kWh) per year. The system’s first year of production, from January 1, 2009, through January 1, 2010, was 9,175 kWh—14% more than the estimates. The system’s energy production continues to exceed household energy consumption.
Kyra Moore is a 2008 graduate of SEI’s women’s course. She joined Southern Energy Management (SEM) as a solar technician soon after graduating. Now a NABCEP-certified solar installer, she is a commercial systems designer. Kyra also instructs classes on solar electricity for the North Carolina Solar Center.