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, I wrote “Tree Math 2” with updated and, I think, improved analysis. You can find it here.
Tree Math: Solar Panels vs. Trees, What’s the Carbon Trade-off?
December 13, 2012
21 Responses to “Tree Math: Solar Panels vs. Trees, What’s the Carbon Trade-off?”
I’ve always wondered about this math. Thanks for laying it all out so clearly!
I did a similar analysis about a year ago. I found data that gave a similar order of magnitude for the amount of CO2 that northeastern forests absorb, and found that the amount of CO2 emissions avoided due to solar were well over 10x that (e.g. if you replaced an acre of forest with an acre of solar, you’d avoid a lot more emissions than the forest would have absorbed).
I just cut three large trees in my yard that were storm risks. I wasn’t happy about it, but I have a lot of firewood for next winter, and am that much closer to having a good roof for solar!
I’ll never prefer cutting a trees just to ensure the most amount of sunlight on my solar panel. Instead of cutting trees later, people should install solar panels on higher or open areas. Along with saving money, we need to be aware about Eco sustainability as well.
Tony, totally get your point. In a perfect world, it wouldn’t be an either-or choice. The good news is, more and more, builders and home-owners are thinking of roof orientation and suitability for solar as they design new homes and surrounding landscaping.
I like the premise of this article. But there seems to be something wrong with the math. An acre is 43,560 square feet, and the source is quoted as giving 4816 trees per acre. This means each tree gets to grow in just over 9 sq ft (3ft x 3ft). Just how big are these so-called “mature trees?!” It doesn’t add up. Please do a reality check.
Hi Alec – Page 7 of the U.S. Dept. of Agriculture/Forest Service report (link provided in section 2) states that there are 4,816 trees per acre in New England. However, this figure includes dead standing trees, seedlings and saplings. You can fit a lot of seedlings into a 9-square-foot space! I re-ran the numbers using the report’s estimate of 202 live, actual trees per acre, which suggests each tree has 215 square feet to itself, and that the solar electric system would offset 4.3 trees. I believe the “truth” is probably somewhere in between the two scenarios. Obviously, there is a lot of extrapolation going on in this analysis. In the end, I believe the net environmental benefit justifies the solar, although the number, age and size of trees in question should be factored in. Thanks for raising your question.
You don’t seem to have taken into account the increased cooling cost of eliminating shade trees. Most homeowners will install panels on the roof, and clearing trees to expose the panels to full sun will heat up the house more.
Brond Larson, thanks for your comment.
A solar array might actually be a better way to cool a roof than trees. Here’s why:
First, let’s assume we’re dealing with an ideal roof, one pointed due south. For trees on the south side of a home to actually cool that home would require a canopy completely overhanging the roof, because the sun is so high in the summer sky. Most people in New England don’t want that many branches hanging over their roofs (leaves clogging the gutters, broken limbs during storms, etc.).
We install solar panels 6-8 inches above the roof, which shades the roof, allows air flow under the panels, and cools the home, not to mention extending roof life and speeding up snow shedding. We haven’t found hard data on this (would love to see some if anyone has it), but we do have considerable anecdotal evidence from our customers whose air-conditioning bills have dropped after installing solar. The effect of solar on a Franklin animal hospital’s attic temperature is even discussed in this case study: http://bit.ly/15hVb54. So don’t worry, taking down trees to go solar isn’t going to increase your home’s heat gain. It will probably lower it.
Because much summer heat gain is through the east and west windows, which let in more light and heat than your roof does, the better option is to plant deciduous shading trees to the east and west of your house to stop the greenhouse effect.
So, how can we discover if our array is shaded part of the day, other than setting up a time lapse camera?
By analyzing detailed production data from each inverter, it should be possible to discover when shading is an issue and how much of an issue it is.
Can you interest Enphase and other inverter suppliers in doing this and putting the results where customers can see it?
Rich
Rich, first, there is no need to worry about shade on your system, unless you have trees that grow a lot after your installation was done. When I recommend taking down trees, it’s before installing a system, in order to make solar economical. We don’t sell and install systems on roofs that have shade issues, unless we’ve discussed it beforehand and the customer insists.
As for using your Enphase monitoring to identify shade issues, the screen images (the colors on the panels that change throughout the day) will show you when production dips, and you could then confirm whether or not it’s shade on your panels or simply a cloud by looking at your roof. But that would require you to be there. Unfortunately, I don’t think Enphase is interested in monitoring customer systems and flagging potential issues.
Your calculation assume 4816 trees per acre. That is a large pretty impossibly large number of trees. Please recheck your assumptions.
While the science and math in this study are sound, I would like to point out that you are promoting cutting down trees. Yes, it might be efficient for our current energy model, but solar energy is a means to reduce CO2. It is illogical then, to reduce systems one has in place now. 1 acre of tree absorbs about as much CO2 as a vehicle produces in 26000 miles. If it were up to me I would offset a lot more carbon with trees, make it a responsibily to reach net neutrality and move towards a carbon negative world.
My opponion is, stop making crucial decisions off what a single study may prove. The perioscopic view one has while looking at strictly a single study creates a loss that would otherwise be gained in having a full spectrum of view.
There is one issue you left out in your promoting of clear cutting land.
Wildlife habitat. There are hundreds, if not thousands of gravel pits, landfills, abandoned lots, parking lots, rooftops and super-fund sites suitable for solar. The clear cutting of land should always be a last option. Sorry, but i don’t believe you should destroy the land to save it.
Clear cutting land? The thought gives us shudders. We never said anything about clear cutting land to put up solar. This article addresses taking down a few trees to make rooftop solar work. That is all. We share your concern for the land. That’s why we’re in this business in the first place.
I know Debbie, and she’s a wonderful person. In her case, her choice makes sense, and for home solar, clearing some trees often makes sense. Whether a tree is deciduous or evergreen, and whether is is located north or south of the home matters greatly in total heat and cooling savings. The math in this article suffers from some logical gaps.
By the math propose here, one acre of trees traps as much carbon as 40 5-kilowatt solar arrays. The math is misleading, however, and the number actually needs to be divided by 20, in which case an acre of forest still equals 2 5K solar arrays, and that is in terms of trapped carbon only. The cost-benefits analysis given here does not account for the habitat, water conservation, cloud-forming, economic and other benefits of trees which solar arrays do not yield. In addition, trees are never environmentally injurious, but solar materials are toxic, an area which needs improvement.
Solar is great. So are wind and other resources. Better home design to save energy, greater use of passive solar, and practical seasonal attire can address the energy problem as much as anything. Transportation and commercial energy waste are huge pieces of the energy-use pie.
Over 4800 trees per acre? That is impossible. Are you sure you didn’t mean 480 trees? I have trouble placing any credibility in this article for this reason alone. Your estimate is way off.
Please see the link at the top of this article to access an updated version, which relies on different methodologies (although I stand by the original figure – that includes trees of all sizes, not just mature. You can fit a lot of saplings into a an acre; check out reports from the US Dept of Agriculture).
What about the energy required to produce solar panels? I know it takes a significant amount of power to refine silicon used in solar panels and the life cycle of a panel is only about 20 years at best. You have to include that in your analysis.
While the sources are hard to come by, it is estimated that it takes just a few years for a solar panel to offset it’s own carbon impact. Furthermore, the expected life on a panel is more than 25 years (the warranty is 25 years). While you may be concerned about the carbon footprint of solar panels, keep in mind that an oil or natural gas generator can NEVER offset it’s carbon footprint. Thanks for commenting — Mark
According to this guy’s “math” we should cut down all the trees and replace them with solar panels to better offset our carbon emissions. 1st off, NO, your math is wrong: besides eating carbon, trees produce valuable byproducts like food, lymber, paper, etc, not to mention beauty. Solar panels require land destroying mining operations. You should never cut down a forest to produce solar power. Instead, put solar panels in places where trees don’t grow very easily, like on your roof, in a desert, or on a satellite.
Sigh. I never suggested cutting down a forest. I never said the first choice for a solar installation should be in a forest clear-cut to make way for panels. A roof or existing structure that requires no tree removal is obviously always the first choice. Read the article more carefully before jumping to judgement please. — Mark
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