The Metzlers live in a 3,500-square-foot log home high in the mountains. While Helmut and Suzy are the only permanent residents of their home, visiting family and friends increase the energy impact to the equivalent of at least three full-time people.
Before installing the PV system, an audit identified the need for several energy-efficiency upgrades. They sealed some problem spots for air leaks, including exterior doors, some of the log beams, and crawl space entrances; and reduced their use of refrigerators. Far from the grocery store but with frequent guests, the Metzlers use several refrigerators and freezers. Now they shuffle food from one or more and unplug the empty ones when it’s just the two of them.
While most of their home runs on electricity, a wood-fired boiler is used for space and water heating in the winter. Wood is locally available and inexpensive in this mountain town. Electric baseboard heaters take over when the Metzlers are out of town and can’t keep the boiler going. Two electric tank-style water heaters are used in the summer and when they are not using the wood-fired boiler.
Based on an average annual electricity use of about 16,800 kWh (before the efficiency improvements) and the available garage roof space, a 10 kW PV system would hit the mark, offsetting almost all of their utility electricity use. Since the utility only pays the avoided cost (the lowest wholesale price of electricity, often only $0.01 or $0.02 per kWh) for excess energy generated each year, there was no financial incentive to make the system any bigger.
The garage roof faces southwest, and although its 22° pitch was lower than optimal for shedding snow, it was the best place on the property for a PV system. After matching PV modules to the roof space and inverter input voltage windows, the final design was 10.34 kW, or forty-four 235 W modules. The National Renewable Energy Laboratories PVWatts version 2 calculator estimates annual production from this PV array to be 14,246 kWh.
The extremely low ambient temperature of this region (down to -24°F, per ASHRAE data) makes choosing an inverter difficult, as it must have a wide voltage window to accommodate high PV voltages as well as module degradation, which will lower an array’s operating voltage over time. Because of these voltage concerns, and a small amount of shading during the winter months on one corner of the array, a Power One Aurora inverter was selected. These inverters have a wide input voltage window—200 to 530 V—and also have two separate inputs, so shading on one series string will not affect the other. The inverters were mounted on an inside garage wall to avoid getting buried by snow, and to keep them above their minimum rated operating temperature of -13°F.
The low roof pitch was a concern for winter production loss, as the roof will not shed snow easily. In this area, a 10:12 pitch (about 40°) would have been much better. Helmut was willing to manually clear the snow after big storms to ensure the array kept producing.
To accurately measure production, a revenue-grade kWh meter was installed on the combined AC inverter outputs. The grid interconnection was done with a supply-side connection in the AC meter main panel. While many net-metered residential PV systems are connected to the utility grid through a back-fed circuit breaker in the home’s main service panel (a load-side connection), there are limitations to how much power can be connected in this way. In a supply-side connection, the inverter output is wired to the utility grid between the main service panel and the utility’s meter, without the limitations imposed on load-side connections. Because of the large PV system and the location of the utility meter and main service panel, a supply-side connection made the most sense.
After a year of production, Helmut reports that the PVWatts estimates were a little low. Last year was sunnier than average, and his system produced 15,576 kWh—a 9.3% increase over the PVWatts estimate of 14,246 kWh. The system produced as much energy as the Metzlers consumed that year, saving them an estimated $1,744 and putting them well on their way to achieving their 5.4% rate of return.
Lena Wilensky co-owns Nunatak Alternative Energy Solutions, a small Colorado RE company. She is a Solar Energy International instructor and a NABCEP-certified PV installer, and is certified by ISPQ as a PV Affiliated Master Trainer. She is also the proud mother of a future solar sister.