Given the falling price of PV modules and Shawn’s ability to install the system himself (he’s a solar installer with True South Solar, based in Ashland, Oregon), we felt it made more sense to design a solar-electric system to meet our homestead loads. Like an SHW system, a grid-tied PV system would produce energy year-round and have its highs and lows. However, the big difference would be that we could use all of the energy it produced. How? With a net-metering agreement with the utility that would credit any surplus energy our system generated, “banking” it for our future use. In summer, when we could hang our clothes on the clothesline and use our solar oven more, our system would almost always be generating more electricity than our homestead uses, and we could draw upon this surplus in the cloudier winter months.
We lived in the house for almost a full year before installing the PV system, so we were able to use our past electricity bills to help arrive at a system size. In many cases, we had duplicate loads, since the workshop, which includes an electric range, space heater, and water heaters, was often occupied. The spa tub (admittedly, an energy hog) also ran during this assessment time, as did the radiant floor system. (The spa tub is no longer in use, and we use the radiant floor system only as a backup to the wood heater, which is a backup to the passive solar.) With the extra loads, that year, our monthly consumption for the homestead averaged about 1,350 kWh—or about 45 kWh a day. Ouch! With an average of 4.7 daily sun-hours and a 77% system efficiency, we’d need a 13.4 kW system to zero-out the homestead!
However, we knew we could—and would—do better in decreasing our electricity use. Some of that “extra” electricity use during the year had been running some big tools during the home’s construction, such as compressors, saws, drills, heaters, dehumidifiers, and the like. Some of that usage was due to the inefficient spa’s draw, and some due to testing (and initially “charging”) the radiant floor system. We guesstimated that a system that was under 10 kW would do the trick—and then some.
The PV system’s design would have been straightforward, except for one thing—my knowledge of a sleek, sexy new PV module that was just being introduced. I had seen Lumos Solar’s glossy frameless modules at a solar conference earlier that year and I was smitten. The marriage of form and function—“art” that made electricity!—was just too tempting for me. I also thought it would be a great opportunity for my solar-installer husband to showcase a product that was a little different from other modules on the market. The company he works for could potentially use our home to show the frameless module option to clients.
The modules didn’t have the power density of some other brands, but we had a big roof and could fit more modules to meet our design goals. However, there was a learning curve involved in the installation process (see “Working with Frameless Modules” sidebar).
The PV system officially went online on December 13, 2011. As of January 22, 2013 (406 days later), the Fronius inverter showed 14,177 kWh production—a daily average of 35 kWh. PVWatts uses a system efficiency of 77% as the default, when our real-world efficiency is better than that. If you plug in the derate values of the actual components we used (0.97 for power tolerance for Lumos LSXs, 0.955 for the Fronius inverter’s efficiency) and assume 100% availability, the DC-to-AC derate value goes to 83%, and 34.7 kWh per day predicted—very close to our daily average.
Since June, our monthly electricity bills have been $10.12—the “basic charge” for having a utility meter. As of December 13, 2013, we had used most of our accumulated surplus electricity, but several exceptionally sunny, cold days in January were replenishing our bank of energy again.
Home Power managing editor Claire Anderson is keeping a watchful eye on her homestead’s electricity use and PV system’s production with a TED 5000 energy monitor.