On the Zero Net Energy Homes discussion board, Marc Rosenbaum clarifies… when it comes to electricity, IS source energy the same as site energy?



Student 1: The (course) book says that for electricity, source energy equals site energy… but, there is a small difference. [According to the U.S. Energy Information Administration] About 6% of the electricity is lost through transmission and distribution.

Marc Rosenbaum: Student 1, the book is unclear on this point and it’s great that you bring it up. What the book is saying is that if the fuel used is electricity, then the basis of calculating what is zero annual net energy is the same whether you count it as site or source, because a kWh in equals a kWh out. I’m not sure I agree with this on a strict basis, because a net zero house typically exports some solar generated energy and imports some grid energy during the year. The question of whether these are truly equal in terms of carbon emissions is a good one, and it’s quite complex. It depends what the carbon loading is of the imported energy vs. the exported energy, and these may not be truly equal. Usually exported energy coincides with the utility’s peak load, and so it may offset peaking energy that is more or less carbon intensive.

What the book also says, which is a fundamental point, is that the primary (source) energy factor for electricity nation-wide is 3.37 – that means that overall 3.37 units of energy are being used to deliver one unit of electricity to a site. So electricity is generally (if made using fossil fuels) a high carbon fuel to use on site compared to natural gas, which has a primary energy factor closer to 1.12. So, using electric resistance heat where the electricity is made centrally using a natural gas generator, then transmitted over the grid with its associated losses, is more carbon intensive than using the natural gas on site for heating directly, even including the efficiency of the gas-burning heating device.

Having said this, my opinion is that in a case where a house has PV but imports more than it exports (and therefore is not net zero), then the net imported energy needs to be counted as source energy for the purpose of counting carbon emissions. So, if I import 2,000 kWh/yr, my source energy, and therefore carbon impact, is 3.37 times that.

If you are interested in the relative emissions of different grid regions, this EPA site has a lot of info.

You can see that the grid where you are has over double the CO2 equivalent emissions per MWh than the grid where I live in New England. This is representative of the mix of sources used to make electricity – coal, nuclear, oil, gas, hydro, renewable. So your net zero efforts have double the impact of mine!

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About Marc Rosenbaum – Director of Engineering, South Mountain Company

Marc Rosenbaum, P.E. uses an integrated systems design approach to help people create buildings and communities which connect us to the natural world, and support both personal and planetary health. He brings this vision, experience and commitment to a collaborative design process, with the goal of profoundly understanding the interconnections between people, place, and systems that generate the best solution for each unique project. Design practiced at its highest level goes beyond efficiency and conservation to create places that regenerate and nurture the natural world and all of its inhabitants. Current areas of concentration include Passive House design, Deep Energy Retrofits, and Net Zero Energy buildings. Projects of his have won awards from the Northeast Sustainable Energy Association (NESEA – four times a winner), ASHRAE (twice a winner), and the Energy Efficient Building Association (EEBA). Three of his projects have been on the American Institute of Architects (AIA) Earth Day Top Ten list. The French Wing for the Society for the Protection of NH Forests earned a LEEDTM Gold certification, the first LEEDTM certified project in New England. In his practice of sustainable design consulting, he has worked for institutional clients such as MIT, Vermont Law School, Yale, Dartmouth College, Cambridge School of Weston, and Middlebury College; non-profit clients such as the Society for the Protection of NH Forests and the Woods Hole Research Center; commercial clients such as Stonyfield Farm, Inc., Tom’s of Maine, and the Hanover Consumer Co-op; cohousing groups such as Pioneer Valley, Pine Street, Island Cohousing, Peterborough, Cobb Hill, and Alchemy Farm; with many architects including William McDonough Architects, Maryann Thompson Architects, Kaplan Thompson Architects, Solar Design Associates, Bruner Cott Associates, Coldham & Hartman Architects, Moore Ruble Yudell, and Payette Associates.

Marc is also teaching: