Here in New England, there is no shortage of trees. And, as you can imagine, trees can be an enemy of solar energy. On a regular basis, I recommend to our customers that they remove trees to improve solar production. Why do I care enough about one customer’s production to suggest cutting down valuable natural resources? Because the more solar energy your system can produce, the greater its financial benefits to you and its environmental benefits to the planet.

The more solar produced by your system, the lower your electric bill and the more income you can earn by selling the “solar value” of your electricity in the form of Solar Renewable Energy Certificates (SRECs). In Massachusetts, your solar system has to operate at at least 80% of ideal system for you to qualify for the Commonwealth Solar Rebate. And to be that productive it needs minimal shade.

Trees or, more specifically, shade from those trees, reduces the productivity of your solar array. However, as you no doubt know, when you cut down trees, you eliminate a valuable carbon dioxide (CO2) capturing structure. Is putting up a solar array worth the tradeoff of destroying the carbon absorbing trees?

I wanted to know, before I recommended this sometimes drastic step to our customers. Here’s what I found with my “tree math”.

1.  How much carbon dioxide does a single mature tree absorb? Different sources offer different numbers – no surprise in the constantly evolving world of carbon sequestration analytics. I’ve seen estimates ranging from 18 pounds per year per tree to more than 50.

I ended up going with this source, which says a mature tree absorbs 271,580 pounds of CO2 per acre over its first 20 years. (Mature trees absorb more than younger trees. Makes sense.)

2.  How many trees are in an acre? I couldn’t narrow this down to Massachusetts so I went with a New England figure. According to a report from the U.S. Dept. of Agriculture and the Forest Service, New England has 4,816 trees per acre.

3.  So, trees in New England absorb around 50 pounds of CO2.

271,580 pounds of CO2 per acre / 4,816 trees per acre = 56 pounds

4.  How much CO2 does electricity production create?  According to the U.S. Dept. of Energy and the EPA, 1 kilowatt-hour (kWh) of electricity produces 1.34 lbs of CO2. In the northeast, that number is probably closer to 1.2 because we don’t rely as heavily on coal to generate electricity as the rest of the nation does.

5.  How much CO2 does a typical solar electric array offset? A 5,000-watt solar electric array on a roof that is 80% of an ideal site in terms of output, will generate about 4,800 kWh per year. Therefore, this solar energy will prevent 5,760 pounds of CO2 pollution from going into the atmosphere, every year.

4,800 kWh generated per year x 1.2 lbs CO2 per kWh = 5,760 lbs of CO2 offset per year

6.  What’s the tradeoff between trees and solar? The 5,000-watt solar system eliminates 5,760 lbs of CO2 per year. That 5,760 pounds correlates to the carbon absorption capability of more than 100 trees:

5,760 pounds of CO2 / 56 lbs per tree = 102 trees per 5,000 wattsStack of recently removed trees

From a carbon offset standpoint, the solar array is a big win. If you are considering cutting down fewer than 100 trees to get the most out of a 5,000+ watt solar electric system, don’t feel guilty. On a net environmental basis, you are doing the right thing.

If you still worry about cutting down trees, you can always plant new ones elsewhere in your yard. And you can appreciate the other benefits of less shade, like New England Clean Energy customer Debby Andell of Acton, who took down 11 towering pine trees to make way for solar. As Debby points out, “we now have a more open back yard with renewed sun on our vegetable garden, and no more worries when wind storms are forecast!”