On December 13, 2012, I published my original “Tree Math” article here on The Energy Miser. Nearly three years later, it continues to be our most visited blog post. But much of the source data for my original analysis has since disappeared from the “Inter-Tubes”. So, allow me to present “Tree Math 2” with updated and, I think, improved calculations.
Here in New England, there is no shortage of trees. And, as you can imagine, trees can be an enemy of solar electric systems. On a regular basis, I recommend that our residential customers remove trees to improve solar production. Why is it worth cutting down valuable natural resources to improve one system’s output? Because the more solar energy your system produces, the greater its financial benefit to you and its environmental benefit to the planet.
Trees or, more specifically, shade from those trees, reduces the productivity of your solar energy system. However, as you no doubt know, when you cut down trees, you eliminate a valuable carbon dioxide (CO2) capturing organism. Is putting up solar worth the trade-off of removing 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”.
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. In the first edition of this article, I calculated a per tree annual sequestration number using average pounds of CO2 sequestration per acre of forest. I then divided by the estimated number of trees per acre of forest. While this provided a ball park number, it was not as precise as I would have liked, so I am updating my analysis using a carbon sequestration methodology from the U.S. Department of Energy (DOE).
In its report, “Method for Calculating Carbon Sequestration by Trees in Urban and Suburban Settings”, the DOE offers a method for – what a surprise – calculating carbon sequestration by trees in urban and suburban settings. The DOE provides data for many types of trees, of different ages. Since the amount of carbon a tree absorbs from the atmosphere over time depends on the tree’s species, size and growth rate, this source can be used to make very exacting calculations. For this article, I’ll use a carbon sequestration rate representative of a 30-year-old eastern white pine (Pinus strobus), which is very common in New England back yards. Using the DOE methodology, I estimate that:
How much CO2 does electricity production create?
According to a 2010 study by the U.S. EPA, in New England:
I’m using the non-baseload figure, rather than total (which would be 0.722 pounds), because rooftop solar installations don’t generally affect baseload electricity generation (such as coal and nuclear), but rather reduce the use of electricity from peaking power plants (usually natural gas fired), as described by the EPA here (in the Electricity Reductions section).
How much CO2 does a typical solar electric system offset?
First, we need some assumptions for the “typical solar electric system”. In my original Tree Math calculations, I used a 5,000-watt solar energy system. Today’s average residential system is closer to 7,500 watts, so my imaginary system will have 24 SunPower 327-watt panels, for a total capacity of 7,848 watts.
Next assumption: the roof. A roof’s viability for solar is quantified as a percentage of the ideal, which would be 100%. In New England, a system facing due south with a 37 degree tilt and no shade is ideal. From my experience, the typical New England roof is 80% of ideal, so let’s go with that.
Next up, let’s determine what our 7,848-watt system on an 80% roof will produce throughout the year in New England (more specifically Worcester, Mass.), using NREL’s PVWatts tool:
Using the carbon intensity factor of 1.106 lb CO2/kWh, we can calculate that this system will offset 9,606 pounds of CO2 every year:
What’s the trade-off between trees and solar?
The 9,606 pounds of CO2 that our solar system offsets every year is equivalent to the carbon absorbing capability of about 50 trees:
From a carbon offset standpoint, the solar array is a big win. If you are considering cutting down fewer than 50 trees to get the most out of a 7,500+ watt solar electric system, don’t feel guilty. On a net environmental basis, you are doing the right thing. (Fifty is what the numbers say. I expect most of us would think twice before truly cutting down 50 trees, but you get the general point.)
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!”
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