Design with PV in Mind

A great solar site has always been at the top of my homestead “want” list, so when my husband Shawn Schreiner and I stumbled on a piece of property for sale that had a wide-open solar window and a building pad that was ready to go, we pounced on it with our PV and passive solar dreams.

Both Shawn and I spent months scouring the Web for passive solar home plans that provided a small, efficient footprint and that would be relatively easy to build ourselves. We initially fell in love with a modern, simple design that would have suited our passive solar plans well, but scrapped it due to its north-facing roof.

So we went back to the drawing board—Shawn spent lots of time drawing in SketchUp, free modeling software that proved very handy, since we could observe how the various designs worked with solar gain. Besides having a passive solar home, we knew we wanted to use renewable fuels exclusively, so designing a large, uninterrupted south-facing roof space for a PV array and, potentially, solar hot water collectors, was essential.

The design we finally arrived at was a modified, “modernized” ranch—two long rectangles joined at a common wall. A south-facing roof measuring 60 feet by 20.6 feet would give us more than 1,200 square feet for PV modules and solar hot water collectors. With only one penetration for a stove pipe in the uppermost part of the roof, we had lots of room for renewable energy collection.

The building pad wasn’t facing true south and redoing it wasn’t an option—we needed to work with what was there. At a 218° azimuth, the house favors the south-southwest. We also deviated from the “default” roof pitch (equal to the location’s latitude: 42°) specified by PVWatts, and the “optimal” orientation specified for our location, which is 7:12 or 30°. Instead, we went with a 3:12 (14°) pitch for ease of building and aesthetics. How much would this compromise solar generation? Surprisingly, not very much—compared to an “optimal” orientation and tilt, our potential solar production would only take a 5% efficiency hit (see graph).

Load Goals

Our main goal was to use renewable energy systems for our homestead, which includes a small, separate office building and a 576-square-foot cabin, and make as much electricity as we use each year. Except for space heating, which would typically be served by our wood heater, all of our loads would be electric.

Although I hand-calculated the potential heating contributions from our passive-solar design, the figures were untested until the house was built and occupied. We sized a wood heater to provide backup heating, but building codes required an auxiliary system, so we opted for an in-floor hydronic system, fueled by an electric water heater. Our original plans called for using solar thermal collectors for both domestic hot water and hydronic heating. However, winters in southern Oregon are often overcast—when we’d need space heating the most. The sunny summers would’ve guaranteed plentiful production from a SHW system, but with a system also sized for space heating, we would have had to make sure our overheating protection was robust and design in some sort of diversion load.