Tree Math: Solar Panels vs. Trees, What’s the Carbon Trade-off?
This article by Mark Durrenberger of New England Clean Energy appeared originally on his blog The Energy Miser, and is re-posted here with permission.
We recommend that you install high or remove trees or branches to improve solar production before taking down trees. But on a net carbon basis solar is greater than tree's in mitigating carbon in most places.
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 as well as a whole host of other benefits from cleaner water, wildlife habitat and moisture retention. Is putting up a solar array worth the tradeoff of destroying the carbon absorbing trees?
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 watts
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.