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Let there be light
by -- May 25th, 2015

Costs have dropped, so more and more homeowners are interested in installing photovoltaic solar power on their rooftops.  While the up-front investment is still significant, the “pay-back” time has dropped considerably.  Many people are in the “clear” in as little as 10 years.

It is worth evaluating the decision to go solar from another perspective.  When one installs a solar array, the decision creates demand for the manufacture of solar panels, the frames that hold them, and a variety of incidental connections.  All these are made from materials extracted in mines and made in factories that use energy, often natural gas or electricity from traditional sources that emit carbon dioxide to the atmosphere.  So, one might logically ask how long it takes to pay back the “carbon debt” to the atmosphere as a result of the installation of a new solar panel on your rooftop.

Several assessments are available with a wide range of answers depending on the type of photovoltaic that one chooses, where it is made, how it is displayed, and where you live.   The manufacture of some panels, for example mono-crystalline silicon, is more energy intensive that others.  The production facilities in some countries are more efficient than others.   And, of course, where you live affects the amount of solar energy that you receive, the energy you generate, and the length of time it takes to pay off the “carbon debt.”  The energy payback time for mono-crystalline silicon panels is about 1.7 to 2.7 years,  whereas the carbon debt for some types of thin-film panels is paid off in as little as 0.7 years.  Energy payback times have dropped considerably during the past decade, reflecting more efficient manufacturing processes and the benefits of volume.

So, from the perspective of the atmosphere, your decision to go solar is beneficial in about 2 years—less than the time it takes to pay back the monetary cost your initial investment.  Last year, we installed 6.2 kW of panels at our house in Maine, and we have not been disappointed.


Garbinet, S., S. Belboom and A. Leonard. 2014.   Life-cycle analysis (LCA) of photovoltaic panels: A review.  Renewable and Sustainable Energy Reviews 38: 747-753.

Peng, J., L. Lu, and H. Yang. 2013.  Review on the life cycle assessment of energy payback and greenhouse gas emissions of solar photovoltaic systems.   Renewable and Sustainable Energy Reviews 19: 255-274

Stoppato, A. 2008.   Life cycle assessment of photovoltaic electricity generation.  Energy 33: 224-232.


  1. Curt
    May 26, 2015

    Love the blogs. We are thinking of adding solar to our mt home. how much does 6.2 Kw cover? Or do you sell just back to the grid?

    • William H. Schlesinger
      May 26, 2015

      We are engaged in a net metering program with our local power supplier. Our solar array covers more than our needs from April through October, so we sell it to the grid. It supplies less than our (much smaller) needs during the long, dark winters in Maine, so we depend on the grid to help us at that season.

  2. Charles Adair
    May 27, 2015

    Great article! We recently worked with students, faculty, and staff (through a Bass Connections in Energy project) to put together a set of resources for Duke University employees to help with the decision making process of going solar. In addition, we partnered with a local Solarize program to provide Duke University employees access to group purchasing discounts! See here for more information:

    While this information applies specifically to North Carolina, it should still help with general questions regarding rooftop solar.

    Hope this helps and please feel free to contact me if you have questions!


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