Source versus Site and Zero Energy

The United States Department of Energy (DOE) recently release a definition of “zero energy building”. This is good news, because it clarifies when a building, campus, or community can call itself “zero energy”. To date, there has been some amount of gray area with the term. Probably the most significant outcome of this new definition is that it uses source energy, not site energy, as the criterion for energy production and consumption.

Buildings consume and produce energy. The energy they produce comes from things like solar panels. The Zero Energy definition makes clear that those solar panels have to be on the property that contains the building. In other words, a building cannot buy renewable energy credits from a distant site that has solar panels. Some organizations, like college campuses, have been balancing their energy equation by buying power from off-site renewable energy sites. This type of trade-off is not allowed to meet the definition of Zero Energy.

Energy consumption is largely the consumption of off-site generated energy. Energy is generated somewhere, such as a a power plant. That power plant might be miles from the building site. Let’s say the building uses 10,000 btu’s of energy in January. How much fuel was consumed to produce that energy? If it is electrical energy, the average answer is that the the amount of produced energy required to allow someone to consume 10,000 btu’s is 31,500 btu’s. In other words, electricity producers produce 3.15 times more electricity than is actually consumed. The balance is lost in production and transmission.

This large difference is the difference between source energy and site energy. A building might only use 10,000 btu’s on site, but it took 31,500 btu’s to produce that 10,000 btu’s.

Interestingly, the difference between source energy and site energy for fossil fuels is much closer to 1. This suggests very little energy is lost in the production and distribution of fossil fuels compared to electricity. To get a quantity of electricity to a building, over three times that energy needs to be produced. To get a quantity of fossil fuels to a building, not much energy needs to be produced.

The definition of net zero, using source instead of site energy, places a premium on both the conservation and production of electricity. Fossil fuel efficiency is important, but not nearly as important as electricity efficiency.

One factor does not seem to be addressed. What about wood used as a fuel, particularly wood from the property that contains the subject building? The DOE definition of zero energy does not contemplate the use of site-generated wood fuel sources. Here in Maine, we burn a lot of wood for heating. A lot of this wood comes from the very property that consumes the wood. Does a home that heats itself entirely from site-derived wood have an advantage regarding the DOE definition of Zero Energy? I don’t know, and I have written to the Energy Efficiency and Renewable Energy part of the Department of Energy to find out what they are thinking.

A Common Definition for Zero Energy Buildings, September 2015, prepared for the U.S. Department of Energy by the National Institute of Building Sciences.
A Common Definition for Zero Energy Buildings, September 2015, prepared for the U.S. Department of Energy by the National Institute of Building Sciences.


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Principal at Sealander Architects, Ellsworth Maine. Revit guru. Married with 3 children. Avid gardener. Lived in San Francisco for nine years. Master in Architecture from Columbia University Bachelor of arts in religious studies, Wesleyan University. Graduated Staples High School, Westport CT. Hope to spend some time in Hokkaido before all is said and done.