US shade trees cut bills and emissions

Shade trees on the west and south sides of a house in California can reduce a homeowners’ energy bills by about $25, a study has concluded.

The survey, involving 460 homes in the Sacramento area, is described as the “first large-scale study to use utility billing data to show that trees can reduce energy consumption”.

“Everyone knows that shade trees cool a house,” said co-author Geoffrey Donovan. “No one is going to get a Nobel Prize for that conclusion.

“But this study gets at the details,” he added. “Where should a tree be placed to get the most benefits? And how exactly do shade trees impact our carbon footprint?”

Dr Donovan is a research forester with the Forest Service’s Pacific Northwest (PNW) research station, compiled the findings with economist David Butry of the National Institute of Standards and Technology.

Some of the study’s key findings are:

  • Placement of a tree is the key to energy savings. Shade trees do affect summertime electricity use, but the amount of the savings depends on the location of the tree.
  • Trees planted within 40 feet of the south side or within 60 feet of the west side of the house will generate about the same amount of energy savings. This is because of the way shadows fall at different times of the day.
  • Tree cover on the east side of a house has no effect on electricity use.
  • A tree planted on the west side of a house can reduce net carbon emissions from summertime electricity use by 30%.

The report, The Value of Shade: Estimating the Effect of Urban Trees on Summertime Electricity Use, has been submitted for publication to the journal Energy and Buildings.

The researchers said they chose to do their study on homes in Sacramento because of the city’s hot summers and the fact that most people use air conditioners.

The Sacramento Municipal Utility District, in partnership Sacramento Tree Foundation, operates an active tree planting programme, in which residents are eligible for up to 10 free trees each year.

Source: press release

Date: 05/01/2009

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