From my experience, this load was on par with that of a highly efficient, all-electric home with passive solar design and solar hot water, and in which occupants practice energy conservation. In our case, besides space heating, we power one laptop and four desktop computers, a photocopy machine, a security system, and a dozen or so 15 W compact fluorescent lights in the office and showroom. We also have a total of six T-8 fluorescent tubes (32 W each) at task-lit stations in the production area. On a typical day, there are also six cordless-tool battery chargers plugged in.
Once we determined the load, it was time to size the system. Peak sun is a standard measurement of the sun at its brightest, and varies depending on the location’s weather, latitude, and other factors. One peak sun-hour is equivalent to 1 KWH per square meter per day. In other words, a 100% efficient solar module that’s 1 square meter in size would produce 1 KWH per hour under the best circumstances—on the brightest day at high noon, with the PV module perpendicular to the sun and at a time of year when the sun is passing through the least amount of atmosphere. Of course, PV modules are really in the 11% to 19% efficiency range, but you get the picture.
Most inhabited places have between 3 and 7 peak sun-hours per day. To calculate peak sun-hours in our location, we used the University of Oregon Solar Radiation Monitoring Laboratory’s data. According to their analyses, average peak sun-hour equivalents for a fixed PV array in Portland are 3.9 hours per day. This assumes a true-south orientation, no shading, and a 30-degree tilt.
We elected to mount the modules at a 15-degree tilt to aesthetically fit them to the space we had. The shallower angle reduces performance by about 4%, but the concession for the array’s more streamlined appearance was worth it. We also recognized that there would be about 5% in losses to convert DC to AC through the inverter, and an additional 1% voltage drop through the wires carrying the solar-spawned electrons. This added up to 10% in reductions for our planned layout, leaving us with the equivalent of 3.5 peak sun-hours (90% of 3.9 peak sun-hours).
It could be argued that a 15% correction factor might be in order, allowing for temperature derating, soiling, and other factors. However, we felt 10% would be a reliable number, given Portland’s moderate average daytime temperature (62°F), plenty of rain to keep modules clean, and an awning-mounted system with excellent backside air circulation that keeps module operating temperatures lower than if they were mounted parallel to the roof.
To calculate the size of the PV array we’d need, we divided the average daily KWH load by adjusted peak sun-hours to get the peak kilowatt rating of PV modules needed to reach net zero-energy. Although calculations showed that we’d need a 6.57 KW array, it’s good to oversize slightly. As it happened, 33 Sharp 208 modules in landscape layout perfectly fit the awning area we planned for the PV array. This configuration gave us a DC-rated 6.86 KW and a 4% margin of error in our estimate to reach zero.
To verify the sizing, I ran another calculation based on the actual annual production of installed PV systems in the area. I had good data from several customers. One in particular had installed a 3 KW system three years prior. He found that he was getting an average of 1,214 KWH per year for each peak KW of PV on his roof. Using his real-world numbers, I multiplied 1,214 and 6.864 (the size of our PV array) to get 8,333 KWH for the year. Dividing 8,333 by 365 days, I got 22.8 KWH per day—almost exactly what we needed.
The array is comprised of three, 11-module strings, feeding a single SMA America Sunny Boy 6,000 W inverter. The SB 6000US features a handy and time-saving extra—a built-in, four-pole disconnect, so both the AC and DC can be turned off with one switch. As required by Energy Trust of Oregon, we installed a Centron permanent KWH meter to keep an uninterrupted record of the system’s AC output in the event that the inverter might have to be serviced.
Before installing the system, we hired an engineer to make sure the steel awning on the front of the building would support a half-ton of additional module and racking weight. Once we received the thumbs up, we got to work, mounting groups of three modules together on the ground, which allowed us to precisely set two parallel underside rails to match the modules’ bolt-hole pattern. Because someone had recently stolen modules from a similar awning mount system on a nearby building, we riveted cover plates over the bolts as a theft deterrent.
Using a boom lift, we raised each three-module group and positioned it on the frame. Four assistants put up the whole PV array in a day. We had already installed the inverter, had an electrician do the AC wiring, and ran conduit while waiting for the engineering report. It just took one more day to complete the DC wire run and then the system was ready to go.