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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
(  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 (, the Sustainable Design Lab of MIT (, Cambridge Energy Alliance (, Modern Development Studio ( 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 (, 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) (, a group which does weatherization barnraisings in Cambridge, the Boston Area Solar Energy Association (BASEA) (, 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.

Installers present: - installer speaker - PPA speaker

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


More cities with Solar Tools?

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Comment Preferences

  •  Tip Jar (16+ / 0-)

    Solar is civil defense. Video of my small scale solar experiments at solarray.

    by gmoke on Sun Oct 07, 2012 at 10:13:21 AM PDT

  •  Los Angeles County has one as well (2+ / 0-)
    Recommended by:
    gmoke, KenBee

    I think it's going through another round of beta testing.  It doesn't seem to like my browser.

    Why is it that a 3% tax increase for the wealthy is considered "socialism" and an 8% wage cut for the middle class is "doing your part"? MartyM

    by delphine on Sun Oct 07, 2012 at 10:41:10 AM PDT

  •  Your total cost of $101K (3+ / 0-)

    is pretty high unless you have a very unusual roof. For an 18K system you should be looking at from $4.50 to $5 a Watt, installed.

  •  I was at the rollout of this tool Wednesday night (2+ / 0-)
    Recommended by:
    gmoke, KenBee

    It looks great!  I'd love to adapt it state-wide so I can find the best roofs for solar, since solar power is paying my bills these days!

    Follow our efforts to turn Southern Worcester County (MA) blue! Greater Blackstone Valley Dems

    by AnotherMassachusettsLiberal on Sun Oct 07, 2012 at 11:24:19 AM PDT

    •  No F2F! (1+ / 0-)
      Recommended by:

      And you didn't say hi.  

      Cambridge had the LIDAR data that MIT could use to develop the map.  Whether the state has the same kind of data for the whole Commonwealth (G@d preserve her, hat tip Charles Pierce) is another question.  There is a state GIS facility which would be the place to ask.  If you do want to pursue this, I may still have an old colleague who works there and my name might be a foot in the door (to mix the metaphors).  I can also introduce you to Christoph Reinhart at MIT if you want to go forward with this.

      Solar is civil defense. Video of my small scale solar experiments at solarray.

      by gmoke on Sun Oct 07, 2012 at 11:54:21 AM PDT

      [ Parent ]

  •  very cool (2+ / 0-)
    Recommended by:
    KenBee, gmoke

    I'm a little disappointed that they really did stop more or less at the Cambridge line. I'm on the Somerville side, I see my house but there's no data :<(


  •  How latitude dependent is this? (1+ / 0-)
    Recommended by:
    gmoke way of saying: isn't this (just) latitude dependent and couldn't something like this be made simpler by latitude, or is there some cloud cover % mixed in? A localized weather index x the latitude factor = solar gain/sq ft?..or something.

    Please excuse my ignorance and the four letter word.:>

    Thanks for this, each community needs this and probably already has the beginnings of it.

    This machine kills Fascists.

    by KenBee on Sun Oct 07, 2012 at 12:11:24 PM PDT

    •  Solar Potential (2+ / 0-)
      Recommended by:
      KenBee, Calamity Jean

      This level of detail takes into account shading from trees and other buildings.  There are solar maps that work purely by latitude and insolation (the amount of sunlight that falls on the ground) based upon measurements and observation.  Google "solar maps" and you can see what it out there.  Usually, they give the insolation numbers in watts per sq ft or sq meters per year.

      Germany, which has probably the largest installed solar base of any country, has the solar equivalent of Seattle, a notoriously rainy part of the world.  Almost all of the USA sees more sun than Seattle.

      Solar is civil defense. Video of my small scale solar experiments at solarray.

      by gmoke on Sun Oct 07, 2012 at 12:26:34 PM PDT

      [ Parent ]

  •  Calif SASH and MASH and GRID programs (1+ / 0-)
    Recommended by:

    In Calif there is the MASH (multiple) and SASH (single) programs giving solar a chance to succeed for low income housing.
    A big component of the program is the state resources (iffy) CSI, the Calif Solar Initiative.
      There is a group in the bay area, GRID and training for volunteers and new careers. Some low income houses qualify for substantial price reductions.
      (I post about them here now and again hoping someone will pop up about their experiences with them etc. I have none.)


    GRID Alternatives' mission is to empower communities in need by providing renewable energy and energy efficiency services, equipment and training. We believe making energy choices that are good for the environment can go hand-in-hand with improving the lives of those living in low-income communities. GRID Alternatives works collaboratively with communities and local organizations to identify specific needs and to develop renewable energy solutions that are environmentally, socially and economically sustainable.

    Currently, GRID Alternatives' core program is the Solar Affordable Housing Program, where we train and lead community volunteers and job trainees from all walks of life to install solar electric systems with low-income homeowners.

    Can't argue with that.

    This machine kills Fascists.

    by KenBee on Sun Oct 07, 2012 at 12:13:47 PM PDT

  •  the most recent GRID posting: a house pioneering (1+ / 0-)
    Recommended by:

    for the neighborhood..

    As Vice President of the local homeowner’s association, Mrs. Moore wanted to pioneer the process of going solar with GRID for the entire neighborhood; “I hope that everyone on the street calls in and makes an appointment to set up an interview.”
    Cause that's what it's going to take, door to door.
       I'm looking at the large flat roofs around me, all bare, all sunny most of the time, perfect candidates for the MASH program...except the landlords are short sighted short term gain type people with long term investments. The tricky part is that the tenants each have their own meter and the landlord will want to make them pay of course, hmmm, have to look into the GRID MASH program more as well...

    This machine kills Fascists.

    by KenBee on Sun Oct 07, 2012 at 12:34:06 PM PDT

    •  Green Leases (2+ / 0-)
      Recommended by:
      Calamity Jean, KenBee

      Getting solar for renters is a problem.  I have a couple of small panels in a south-facing window that I use but there aren't many who would do that.  Green leasing arrangements are now being worked on and Cambridge Community Development Department can give you some information on that.  Purchase power agreements can give a landlord incentives to solarize and pass on some of the savings to tenants but I don't know if that's happening anywhere yet.

      I plan to send my landlord a print out of our roof's solar potential.

      Solar is civil defense. Video of my small scale solar experiments at solarray.

      by gmoke on Sun Oct 07, 2012 at 01:13:19 PM PDT

      [ Parent ]

      •  Just allowing the landlord to sell his (3+ / 0-)
        Recommended by:
        KenBee, gmoke, JeffW

        PV-generated power to the electric company might do it, if the initial cost can be brought down enough and the electric company was required to pay a reasonable price for the power.  That way a six-apartment building would have seven income streams.  

        Renewable energy brings national global security.     

        by Calamity Jean on Sun Oct 07, 2012 at 03:46:34 PM PDT

        [ Parent ]

  •  Looks like your electrical useage is about (1+ / 0-)
    Recommended by:

    15% of average! Do you have any tips for us?

    •  Tips (1+ / 0-)
      Recommended by:
      Mother Shipper

      I use only CFLs and will probably start switching to LEDs as the CFLs wear out;  every vampire load I can find is on a power strip which is turned off when not in use;  my refrigerator is a highly efficient model from Denmark, smaller than almost all US models;  a couple of small PV panels in a south-facing window in my bedroom which powers LED lights for night reading;  no air conditioning in the summer, only directional fans;  small heater in the winter for the room I'm in.

      I live alone or with one apartment-mate.

      Solar is civil defense. Video of my small scale solar experiments at solarray.

      by gmoke on Sun Oct 07, 2012 at 07:53:42 PM PDT

      [ Parent ]

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