Should I cut down trees that shade my solar panels?

If you're thinking about installing solar panels but have trees that shade your property, you'll need to figure out how much a problem it is. Here's a guide, and some tips to minimize the impact of shading.

Photo of trees shading a home.

If you want to get the maximum performance out of solar panels, it’s important that they have full sun for as much of the day as possible.

One of the most common issues that a homeowner will face when deciding whether to install solar panels is dealing with shading from nearby trees.

This is a particular problem if shadows fall on any south-facing section of your roof in the middle of the day when power production is at its peak. Because the middle of the day is when incoming sunlight is the most intense, midday shading will have a much worse impact on your system then shading early or late in the day.

Still, if you have some shading from trees, it might be possible to install panels and get a reasonable financial return.

How much will shading affect your solar panels?

When it comes to planning a solar array, trees basically block sunlight completely. Even though a little light might filter through leaves and branches, it’s not enough to generate a meaningful amount of electricity.

To see the effect of shade on your solar panels, try The Solar Nerd calculator with different values for shading, from none to partial and major shading. You can quickly see that shading has a seriously detrimental effect on how much electricity a solar array will generate.

And the issue isn’t just complete shading: panels that are only partly covered by shade can suffer a severe or complete loss of power generation. This is because the individual cells in a solar panel - there are often 60 cells - are wired into strings, much like Christmas lights.

When one bulb in a string of Christmas lights goes out, the rest of the string can be affected. Solar panels are a little like that. The solar cells are wired into multiple strings, so shading a single cell can cause power output to drop in the rest of the string. Having multiple strings means that not all of the power output may be impacted, but any partial shading can cause a big reduction in power output.

This is why shade trees have such a negative impact on solar panels. Solar panels really full sun to work their best.

Shading early or late in the day isn’t that bad

Time of day is perhaps the most important consideration when deciding whether shading is going to be a problem for your solar array.

Here’s a graph of the power output for my system on a sunny day:

sample graph of solar power output

The maximum output is in the middle of the day, as you would expect. The system still produces power early in the morning and in the late afternoon, but the output is relatively low.

This means that it might not be a major concern if shading from your trees only falls on your roof early or late in the day.

The only exception is if you have a time-of-use (TOU) plan from your utility company, as is the case with all new net metering customers in California. If you have a TOU plan, net metering customers get credited at the rate in effect at that time. This means that the electricity you generate during peak hours will earn a higher credit than off-peak times.

The upshot is that if you have trees that shade your roof late in the day, you will lose electricity generation when its more valuable, because utility peak hours are typically in the late afternoon and early evening. If your solar array has its panels pointed west, late day shading could be a major problem for you.

Choosing an inverter that works better in the shade

If you have a location on your roof where you have to contend with partial shading, you can help improve your power output by choosing what are known in industry lingo as “module-level power electronics” (MLPE). For the consumer, MLPE means microinverters or string inverters paired with power optimizers.

With MLPEs, every solar panel either gets its own complete DC-to-AC inverter (as is the case with microinverters) or has a power optimizer, which a package containing a “maximum power point tracker” that helps to minimize the impact of shading on a string of panels.

Microinverters are the best at handling partial shading, because every solar panel/microinverter pair is completely independent of the others. Power optimizers provide some of the same advantage, but if too many panels in a string are shaded, the entire string can still be affected even when power optimizers are utlized.

Inverters are a pretty important topic, so you can read our guide on solar inverters to learn more.

Half-cut solar panels work better in the shade

In addition to choosing an inverter system that works better when you’re dealing with shade from trees, you can also choose solar panels that handle shade better.

Half-cut solar panels have a particular advantage when they are covered by partial shade. Half-cut means that the individual cells that make up the solar panels have literally been sawn in half (often with a diamond wire saw), doubling the number of connections within the panel.

Remember earlier how we said that panels are made of individual strings of cells? With a half-cut panel, the number of strings are doubled. With more strings, there’s less impact when shadows fall across the panel because each string is smaller and contains fewer cells.

Many manufacturers are incorporating this technology into their panels, including popular brands such as REC and Q.Cells (Hanwha).

You can read our article on half-cut solar panels to learn more.

Isn’t it bad for the environment to cut down trees to install solar panels?

If helping the environment is part of the reason why you want to install solar panels, you might wonder if it makes any sense to do that if it means cutting down trees.

Short answer: almost certainly yes. It comes down to how your local supply electricity supply is produced.

To find out how dirty your electricity is, enter your zip code into the EPA’s Power Profiler tool. It’ll tell you how much CO2, SO2, and NOx pollution is created by your local electricity supply.

Estimate your emissions
Screenshot of the EPA Estimate Your Emissions Tool

After entering your zip code, enter your average monthly electricity usage in kilowatt hours. If you don’t know what that is and you’re too lazy to check your latest electric bill, just use the national average of 1,011 kWh.

Once you plug that in, you’ll get a report showing how much pollution your electricity usage generates. It’ll look like this:

Estimate your emissions report
Sample report from the EPA Estimate Your Emissions Tool

Under the Annual Results section of the report, you’ll see some interesting facts, one of which is the number of trees you would need to plant every year to sequester the carbon dioxide that results from your electricity use. Here’s the report for my usage in upstate New York:

It would take 26 seedlings grown for 10 years or 1 acres of forests in one year to offset those CO2 emissions.

My electricity usage is lower than average - about 600 kWh per month. And my grid electricity is also cleaner than average. But even so, I would need to plant 26 trees every year to offset my grid electricity usage. Multiply that by the 25 year expected lifetime of my system, and that’s equivalent to about 650 trees.

This means that if I had to cut down down a couple trees it possible to install solar panels, it would still be a major net benefit for the environment.

A professional solar installer will give you a shading analysis

Solar installers have software that can do an sophisticated estimate of the potential solar generation of your property, including using satellite images to model the shading impact of trees and other nearby objects.

This shading will change from winter to summer, so if you’re discouraged by shading from your trees, you might find that your roof is clear at another time of year. Get a quote from a qualified solar installer to get an accurate analysis.

#Shading #Environment

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