Question

Which is better—shade or solar?

Asked by Clay

Is it better to have a home shaded by mature trees (and therefore protected from the hot sun), or in full sun and using solar panels? Can you use solar panels on a home that is shaded by mature trees?

Answer

Mick Dalrymple

Answered by Mick Dalrymple

Scottsdale, AZ

Eco-Friendly Building Center

January 10, 2008

These trade-off decisions are where the fun and challenge lie in greening your home.

You could have your cake and eat it too by planting trees and buying green power from your local utility or by buying RECs (Renewable Energy Credits).

Otherwise, if you want to look at on-site renewables, let’s first look at the benefits of trees as an energy-reduction strategy:

  1. They're natural.
  2. They're inexpensive, providing high value for your energy-reduction dollar.
  3. Their shade and coverage are adjustable (through trimming and if they are deciduous).
  4. They have a very low operating cost (water, if necessary, and maintenance).
  5. They bring evaporative cooling effects (evapotranspiration).
  6. They are aesthetically pleasing.
  7. They are multi-functional (energy reduction, beauty, carbon sequestrations…).

This topic always reminds me of William McDonough at the Cradle to Cradle home design competition. It seemed as though every entry we would come to he would say something to the effect of, “This design would be fantastic if it just had trees all over it! It needs more trees!”

And the downsides of trees:

  1. They can possibly cause home damage from limbs.
  2. You need to avoid roots growing near foundations.
  3. Trees may obstruct views.
  4. Non-deciduous trees limit seasonal adjustments.
  5. Non-native trees may consume significant transported water.

The positives of solar are numerous, with the primary negative being the upfront capital investment (unless a solar leasing company has entered your market). If you are building a new house and wrap the upfront capital into the mortgage, you can actually experience a positive cash flow each month. (People frequently ask me the payback on solar PV and I ask them the payback on a jet ski. Jet skis bring adrenaline rushes. Solar PV brings peace of mind because you don’t really care what happens to energy prices… and there are all the other benefits solar provides.)

My recommendation is starting with native trees to make shade. Also, make your home as energy-efficient as possible. It’s the same as the decision to look at the miles-per-gallon gas usage of your car before you consider whether to buy ethanol to power it. Priorities. Shading is best on the east and west and best if it is provided via deciduous trees to allow sun in during the winter. Then, find a place for solar water-heating panels. Solar water heating can pay for itself in as little as three years in some markets, and you need relatively little well-exposed roof space to make it happen. Then, look at solar PV.

Here’s what you want to consider with solar PV and trees:

  1. Any reasonably responsible solar PV installer will bring a meter or other tool to your rooftop and be able to determine if trees or any other obstructions are going to interfere with your panels, as well as when the shade will occur (time of day, seasons, and now or future). There are some pretty cool tools out there to do this.
  2. If you cannot avoid shade on your panels, minimize it. Try to make sure panels are shaded only in the early morning or late afternoon, not during their peak production time. Also, keep trees trimmed to minimize the shadows they cast or, better yet, choose trees that are going to grow to an appropriate size.
  3. Panels are wired together in a series and parallel electrical circuit combination. Think of it as multiple lanes on a packed freeway. If one panel (or portion of a panel) is shaded in a series row of the array, the power output of that entire row will be diminished greatly. The equivalent is a tree falling in front of a car in one lane of the highway, blocking all the traffic in that lane. This would suggest that, if you are concerned about shade, you want to wire as few panels in series as possible and as many in parallel as possible, instead (more lanes). However, inverters (the equipment that converts the electricity from DC to AC) are made such that they operate within a certain range of high voltage and current, so your options may be limited.

Remember: Efficiency first; renewables second. Go forth and plant!

Tagged In: tree shade

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