Roof Evaluation for PV

While working with the nice folks at Taza Chocolate in Somerville, MA as part of a class project, I was able to go up on the roof of the 561 Windsor Street building and evaluate it for photovoltaics (note it is sunny because I did this in November and am a bit slow putting the pictures up on the blog). The building has a large flat roof with relatively no mechanical equipment. There are also no tall buildings to the south that might shade the roof. Though it was obvious that the building received direct sun most of the year, I used a Solare Eye camera to verify the path of the sun in relation to the building and determine what times of the year, if any, the roof would be shaded. The picture above (taken with a timer) shows me on the roof, leveling the camera and orienting it due south. Since I’d already entered our location into its computer, it knew our latitude and longitude and could overlay the path of the sun (as seen in the image at the end of this entry).


According to the Solar Eye results, the roof receives direct sunlight 99% of the time sunlight is available. Assuming the 21,761 square foot roof can hold at least 600 SunPower 305 watt panels (shown to the right), which are 62” by 41” each, the PV Watts program from the National Renewable Energy Lab predicts that the roof would produce 210,251 kWh per year in Somerville, Massachusetts. The first cost for this project would be $2,196,000 at an estimated $12 per watt. Assuming a 30% tax credit of $658,000, this brings the project down to a 19 year payback. This payback would be significantly improved when the accelerated depreciation and any potential revenue from renewable energy credits are included. The state of Massachusetts is expected to unveil a new set of renewable energy incentives in January, which will further improve the payback, which would likely be less than ten years when all revenue sources are considered. While Taza does not currently have roof rights to their building, if they are considering a long-term lease they may elect to negotiate access to the roof. Be sure to check out Taza Chocolate if you're in the mood for some great chocolate. Taza is a small bean-to-bar chocolate maker, and the only producer in the US of 100% stone ground, organic chocolate. If you're purchasing in bulk and live within 5 miles or so of the factory, they'll even deliver it with one of their custom tricylces.