Side-By-Side Solar

Beginner

Inside this Article

The installed PV array
The installed PV array.
The author's rental property.
The author's rental property.
The author with the three sets of meters and disconnects
The author with the three sets of utility meters, performance meters, and PV system disconnects.
Three Xantrex GT5.0 inverters
The three Xantrex GT5.0 inverters feed three different homes, each with its own utility service.
A portion of the 9.12 KW PV array.
A portion of the 9.12 KW PV array.
Detail of the roof-to-array mounting system.
Detail of the roof-to-array mounting system.
The workshop with its three independent arrays.
The workshop with its three independent arrays.
Sandy Lawrence
Sandy Lawrence
The installed PV array
The author's rental property.
The author with the three sets of meters and disconnects
Three Xantrex GT5.0 inverters
A portion of the 9.12 KW PV array.
Detail of the roof-to-array mounting system.
The workshop with its three independent arrays.
Sandy Lawrence

Some landlords might scoff at the idea of adding a solar-electric system to a rental property. When it comes to investing time and money into any kind of improvement for rentals, “less-is-more” tends to be the favored approach. While my wife Barbara Schickler and I may err on the frugal side when making superficial upgrades, we never hesitate to do whatever it takes to improve the energy efficiency of our buildings—such as adding high-efficiency furnaces and windows. We see it as a win-win-win: for our tenants, the environment, and us. So when the opportunity arose to add solar-electric systems to our rental properties, we jumped at it.

Our situation is somewhat unique. We run a bed-and-breakfast that is part of our 4,600-square-foot home in the far northwest corner of Washington state, and we also own three adjacent 5-acre properties. Two of these have freestanding rental homes, and the other has a greenhouse that we use to raise seedlings for our tree farm. Altogether, our four properties occupy a 20-acre parcel in Whatcom County, about 15 miles from the Canadian border, at 48.9 degrees north latitude.

A year or two ago, we decided to add a workshop. Our plan was to build to a 2,000-square-foot structure on the parcel with the greenhouse. The new building would house a carpentry workshop, exercise area, and gear locker, as well as provide a south-facing roof space for a solar-electric system that could help power our home and two rental properties. As a self-taught carpenter with several projects under my belt, I felt confident that we could save on labor costs by doing the work ourselves—except for some help from subcontractors for the foundation, insulation, drywall, and plumbing.

Nailing down regulatory approval for the system was the first step. With its long history of involvement in RE, our utility, Puget Sound Energy, was open-minded about my idea for a grid-tied solar-electric system. What gave them pause was not the PV installation, but that I wanted to mount the electric meters for three separate residences on the wall of a workshop on a fourth property—one without a residence. They finally gave us the go-ahead with the explicit understanding that the meter bases would need to be transferred back to the other homes if we ever parted with one of the rental properties. While that was certainly a valid point, we weren’t too concerned since our plan is to retire here and maintain our rentals for income. But should we ever sell our rental properties, the system can be reconfigured and the meters can be moved to their respective homes.

Solar-Ready Roofing

Given the constraints of our wooded property, we ended up paying an arborist to remove several large cottonwood trees to clear a solar window. Though we hated to see them go, we had more than adequately counterbalanced their loss by planting several thousand trees on our property over the previous decade. Construction of the workshop spanned 12 months, with several weeks devoted to readying the roof for solar.

Since I had no interest in climbing up on the roof to adjust the tilt of the PV arrays several times a year, I choose a fixed-mount racking system. To avoid roof penetrations, I routed my conduit from the arrays through the vertical clerestory wall. In retrospect, I wish that I had included three risers coming through the roof—one at the foot of each string—but I had not yet sized the modules and strings, and wanted to leave some flexibility. Conduit runs would have been shorter with vertical penetrations closer to the inverter room inside the workshop. I overcompensated by using #6 wire in the conduit. Only late in the process did I learn that the conduit between the PV array and the DC disconnect must be metal—not PVC—for its run through a wall cavity. This National Electrical Code requirement is an effort to better protect the wiring during any future construction, and reduces the risk of fire within the wall if a ground fault develops in the wire run.

System Assembly

Component Selection. After exploring various options and seeking bids, we decided upon Evergreen 190 W PV modules. We wired 16 modules in series for each of three arrays, and mated each string to a Xantrex GT5.0, 5 KW inverter. Each array is rated at 3,040 watts at operates at 427.2 volts at maximum power. Based on the historical low temperature here in Bellingham of 4°F, a maximum of 14 ES-190 modules in series is recommended by the inverter manufacturer. Hopefully we won’t see temperatures below the historical figure, or damage to the inverters could result. We chose the larger capacity inverter to allow for future pole-mounted modules at the shop.

Inverter Setup. Celt Schira, my consulting electrical engineer, contacted Xantrex tech support and received a strong recommendation to site the inverters close to the utility’s meters due to voltage drop concerns. Because the inverter has to have a higher voltage than the utility to export power, it is extremely important to minimize voltage drop on the AC side. This meant running the underground conduit to the more distant east end of the building, rather than to the adjacent west end of the shop as originally planned. If I had consulted with my electrical engineer before construction, I could have built the inverter room closer to where it needed to be. I also learned from the engineer that utility crews strongly prefer 3-inch conduit—as opposed to 2.5-inch—which would have made pulling the wire much easier.

Inverter Mounting. Though these inverters are designed for outdoor mounting, I wanted to coddle them a bit and walled off a space from the rest of the shop to keep out sawdust and clutter. In an urban setting, this placement would also protect them from vandalism or theft. I installed additional bracing behind the drywall to support the inverters’ weight, and then painted and finished the wall so they would better blend with the building.

I brought my cable runs from the roof into a large junction box recessed halfway into the wall to both accept the behind-wall conduit runs from the roof, and to allow outer conduit into the inverters, as opposed to using the knockouts in the back of the inverters. This will allow easier future changes to the system. I also added conduit runs below the inverters for the future PV arrays.

Module Layout. The arrays on the south-facing roof lie below a row of clerestory windows that provide daylight to the north half of the structure. I had the thought that light reflected from these windows might enhance the production of electricity from the top string, the array dedicated to our house. Within an interval around solar noon, this is actually so. I can see reflected light hitting the upper modules, and the readout indicates that the array is producing an additional 30 to 40 W. But the unintended consequence of butting this string up to the clerestory wall is that early morning and late afternoon shading in the summer months leads to about 4% less net production from this string over the first eight months of operation.

Testing 1-2-3

Having three parallel strings allows a unique opportunity for comparing nearly identical systems in an actual installation. Since our systems went online in July 2007, each of the three arrays has averaged about 6.7 kilowatt-hours (KWH) of production per day. In eight months of operation, the net production of these systems reached 1,510 KWH for the upper array, 1,578 KWH for the middle array, and 1,575 KWH for the lower one. Though I came to expect that shading from the clerestory overhang would diminish production on the upper array, I was pleasantly surprised that the difference between all three arrays was small—and even more impressive, the difference between the middle and lower arrays (without the summer shading problem) was only 0.2%. These figures are, as I would argue, testimony to the quality of engineering that goes into these photovoltaic modules and inverters.

The average electrical load in our 4,600-square-foot home is about 10 KWH per day—well below the national average for a detached single-family residence—largely because propane provides the fuel for space and water heating, cooking, and occasional clothes drying. Our primary electrical loads are refrigeration, lights, electronics, a well pump, and small pumps for the heating system. Our PV systems provide about two-thirds of our own home’s electricity, and probably a higher fraction for each of the two rental homes, which are less than half the size of our home/B&B and have fewer people. After I expand the PV system with pole-mounted modules to the east or south of the shop, we should have enough production to meet nearly all our electrical needs.

A system of this nature is not for everyone—or even every landlord—but we made it work. As a do-it-yourselfer, I was able to obviate one of the big costs of building the workshop: labor. As a landlord, I circumvented the classic economic problem of split incentives. My tenants have lower electric bills but are still motivated to conserve to reduce their bills even further. Meanwhile, I can justify raising the rent a bit, and the tax credits and depreciation schedules help pay for the project over time. Even without all the financial incentives, I would have moved forward with the project—because, in the end, it’s about saving resources, not dollars.

Access

Sandy Lawrence, M.D., was a professor of medicine at the University of California at Davis before retiring to upstate Washington. His wife, Barbara Schickler, is a certified nurse and midwife. Between the two of them, they have delivered 7,000 babies and raised three of their own.

Celt M. Schira, P.E. is a licensed professional electrical engineer and LEED-accredited green building consultant. She has 21 years of engineering experience in electrical design, telecommunications, and renewable energy. She owns Schira Consulting in Bellingham, Washington.

Dana Brandt, P.E. • www.ecotechenergy.com • Incentives consultant

Jeffree Utter • Installation consultant

System Components:

Evergreen Solar Inc. • www.evergreensolar.com • PV modules

Xantrex Technology Inc. • www.xantrex.com • Inverters

UniRac Inc. • www.unirac.com • PV mounts

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