You can now estimate with great detail the solar electric potential of any roof in Cambridge, MA by just typing in an address on a webpage, the Cambridge Solar Tool
(http://cambridgema.gov/solar). For instance, the double triple decker in which I live has six apartments and a total roof area of 2,781 square feet. 1,136 of those sq ft have high PV (photovoltaic) potential. This could support an 18kW solar electric system providing 22,945 kWh per year, enough to power about a third of the electricity used by those six apartments, if each apartment uses the rough US average of around 11,000 kWh per year (my own annual electric use is around 1,600 kWh/yr).
The estimated savings per year for such a PV system are $9,081. The total cost is $101,720. With the Federal tax credit of $30,516 and a MA state tax credit of $1,000, the final cost to the owner would be $70,204. In addition, the Solar Renewable Energy Credits (SRECs) of 27¢/kWh could produce $6,212 per year (at least that's my reading of the MA SREC program, but I could be wrong). Such an investment would pay for itself in about 8 years with a return on investment (ROI) of 12.93%, a better return than gold (10.19%) or the stock market (Dow Jones average: 5.50%). The solar electricity would replace other fuels that now spew 12 tons per year of carbon into the atmosphere.
If the owner did not want to put any money down, they could opt for a Power Purchase Agreement (PPA), buying electricity from a third party which owns, installs, maintains, insures, and monitors a PV system on the roof of our double triple decker at a long term, generally 20 years, fixed and lower cost than what is paid now for power.
Cambridge is the first city in the world where you can go online, type in an address, and get such detailed information. The Solar Tool is a project of the Cambridge Community Development Department (http://www.cambridgema.gov/cdd.aspx), the Sustainable Design Lab of MIT (http://mit.edu/sustainabledesignlab/), Cambridge Energy Alliance (http://cambridgeenergyalliance.org), Modern Development Studio (http://www.modeonline.com) and others. Christoph Reinhart, of the Sustainable Design Lab, says that other cities including London and Singapore are interested in this kind of solar tool and that it is comparatively easy to model PV potential on an urban scale. The Sustainable Design Lab also has a daylight simulation tool, DaySim (http://www.daysim.com/), that "models the annual amount of daylight, glare and electric lighting use in and around buildings."
The Cambridge Solar Tool was first publicly demonstrated at the Cambridge Library on Wednesday, October 3. Besides the representatives of the city and the developers, a solar installer walked the audience through the process, a homeowner related his experiences in installing 5.64 kW PV system on another Central Square triple decker, and a vendor explained the solar leasing model or PPA. As the majority of residents of the city are renters, there was a mention of green leases for renters and condo owners. There is even some planning by the city to establish a solar buying club or coop to reduce costs to individual homeowners and real estate entities. A new group, SunUp, Cambridge, plans to canvass the top-rated solar sites and connect interested owners of such sites with contractors for energy audits and solar assessments with a discounted pricing system. The Home Energy Efficiency Team (HEET) (http://www.heetma.com), a group which does weatherization barnraisings in Cambridge, the Boston Area Solar Energy Association (BASEA) (http://www.basea.org), and the Cambridge Energy Alliance are cooperating on this project. Cambridge also has a house by house infrared mapping of heat loss done in February 2011 through another MIT project headed by Mechanical Engineering Professor Sanjay Sarma and I expect that information will also be used to pick the most likely customers for solarization and energy efficiency.
Before and after the meeting, there were table exhibits by seven local solar and renewables companies. That evening's presentations will be online at Cambridge Community Development and Cambridge Energy Alliance.
The estimate is that rooftop PV in Cambridge could supply about a third of the electricity that the city and its residents now consume. With higher levels of energy efficiency, that portion could rise. This solar contribution does not include solar hot water and space heating, south-facing walls and windows or food production.
Cambridge is not the only city doing interesting energy initiatives. NYC has finished its first round of energy benchmarking all its buildings over a certain size. The results are now available at http://www.nyc.gov/html/gbee/html/plan/ll84_scores.shtml