I’ve found that the most interesting aspect doing solar assessments is that they frequently remind me of the importance of asking the right question in the first place.
Consider a solar PV (electric) assessment I did for a homeowner in rural Dane County. The home is a passive solar design from the ’70s, with two stories of south-facing, floor-to-ceiling windows and a substantial amount of tile flooring to absorb heat all day. It has two backup heating sources: a wood-burning stove and some electrical resistance heating recessed into the floor in front of the first-story windows. The house is on-the-grid for electricity, but has no natural gas or LP supply.
The homeowner described their heating situation as primarily passive solar, with a fair amount of wood-heating, augmented with some electrical heating. Sounded like a pretty good design…
As always, the first thing I did was to download their utility records for the past couple of years. If you haven’t done this for your own home, I strongly recommend it: MG&E has a great on-line service that gives you tables and graphs of your energy consumption, and lets you download the data for further analysis. The first step to an energy-efficient home or business is knowing how much energy you use and when.
As soon as I looked at the numbers, I realized that the homeowners’ description of their heating strategy didn’t exactly match the data. Their modest-sized house was using over 15,000 kilowatt-hours per year, nearly double the state average and triple what I typically see for houses of similar size and age!
Of course, being an all-electric house meant that they did have to cook and dry clothes with electricity, but 15,000 kWh/year is still a lot of electricity.
The owner was a bit puzzled at how high their electrical consumption was, as he knew they burned quite a bit of wood and the house was, after all, a passive solar design that generally felt pretty comfortable except during long spells of especially cold and cloudy weather.
A key step in any solar assessment is to measure the building’s “solar window”. The amount of solar energy available to be collected on the roof of a building depends on a number of variables. Some are determined by the latitude and climate of the location (how long the days are, how much cloud cover the area has in a typical year). Others are specific to the building itself (the slope of the roof, the direction it faces, any shading from surrounding tress and buildings). I use a special camera system that photographs the sky from east to west, and then use software that takes the photos and computes the amount of solar energy available in a typical year and the amount that would actually reach a collector on the building’s roof.
And that’s when we learned the punch line to the story: When the house was built 30-odd years ago, they had planted a row of pine trees as a wind-break and privacy screen. The trees are a considerable distance to the south of the house, but had obviously grown quite a bit over the years. The solar window analysis showed that the trees had grown so tall that they were blocking the sun during most of the middle of the day (prime solar heating time) during most the the mid-winter months!
So the data showed, to everyone’s surprise, that they really had a wood- and electrically-heated house with a passive-solar assist. We had started out trying to see if solar PV panels could help reduce their electric bill, when the real issue was trees impairing their passive-solar system.
I also noted that a number of the south-facing windows showed signs of fogging, indicating that the thermopane windows had lost their vacuum. This meant that they were letting less sun in and more heat out than they had originally.
So my assessment report advised that, Yes their roof could be a suitable location for solar panels. But the highest priority improvement in which they could invest is to restore the passive solar design by trimming the trees and replacing the failing windows!
I love the idea of generating carbon-free electricity on the roof as much as the next person, but as a solar assessor my job is to give homeowners objective, factual advice about how to get the most benefit for their investment of time and money. It doesn’t make sense to invest in expensive solar PV panels, and then throw the electricity away through leaky windows. First things first: doing conservation measures first, then adding renewable energy hardware is almost always the best policy…