Call me crazy, but I want to get this straight once and for all. Here is how I understand zero energy for a residential building. Please feel free to way in with comments.

Step 1) Zero Site Energy
energy produced on site = energy used on site

Step 2) Zero Energy (edit: Zero Source Energy)
energy produced on site = primary energy used at provider to produce site energy
(assuming all systems are grid-tied and all incoming energy comes from the grid, all outgoing energy goes to the grid)

Step 3) Carbon Neutrality or Zero CO2
energy produced on site offsets CO2 in the amount of CO2 used in primary energy production at provider to produce site energy
(assuming systems are grid-tied and all incoming energy comes from the grid, all outgoing energy goes to the grid)

In reality, I think the Zero Energy and Zero CO2 standards can be achieved without offsetting all theoretical primary energy and primary energy production CO2, since that amount of energy is never fully utilized from the provider—some part of it is directly produced and consumed on site.

Step 4) Carbon Offset & Net Positive Energy
enough energy is produced on site to offset additional carbon and add additional energy to the grid 

It appears to me that the only way to make a dent in energy consumption and CO2 production by buildings is to build a bunch of “Step 4” buildings. Feel free to disagree with me in the comment section. 

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  • jesper kruse says:

    Hi Tim,
    Check out EBN Oct.2005 for their definition of all of the above.

  • Ed Shank says:

    Hi Jesper and Tim,

    For those of us w/o a subscription to EBN, can you post a synopsis of their definitions of zero energy? Also, I believe the nomenclature for the two types of building energy are “site” energy and “source” energy, the latter being what is generated off site at the primary generator (in electrical terms). The source energy typically includes the transmission losses in evaluating its respective carbon footprint. If I have misstated this, someone please correct my posting. See you in Duluth, Tim. I’m looking forward to the conference and the dialogue with fellow PHIUS “pholks.”


  • The NREL (National Renewable Energy Laboratory) site has a paper that defines the terms pretty definiteively. This may be the paper that EBN refers to (I don’t have that copy of EBN handy).

    To read the paper, go to:

  • Tim Eian says:

    Here is in short how the NREL paper that Paul posted defines the four categories:

    • Net Zero Site Energy: A site ZEB produces at least as much energy as it uses in a year, when accounted for at the site.

    • Net Zero Source Energy: A source ZEB produces at least as much energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to generate and deliver the energy to the site. To calculate a building’s total source energy, imported and exported energy is multiplied by the appropriate site-to-source conversion multipliers.

    • Net Zero Energy Costs: In a cost ZEB, the amount of money the utility pays the building owner for the energy the building exports to the grid is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.

    • Net Zero Energy Emissions: A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources.

  • Christina Snyder says:

    Thanks for helping to iron these definitions out.

    Interesting that an off-grid building still brings all these definitions together down to one number, provided the home doesn’t import fuels to back-up their Renewable Energy systems. I suppose any combustion back-up could take such a home away from true net zero energy, but designing an RE system to meet all energy demands in a very cold, cloudy climate like MI can be unaffordable. We are going to see how close we can get for how little $, however.

    Using combustion of current biomass from our homesite as emergency backup is acceptable to me, even though it releases CO2, because the biomass would be releasing the same amount of CO2 over the next decades if it was just left to rot, and even with our woodstove we currently accumulate huge brush piles from a partially wooded lot each year. We definitely have the current technology to reshape our building stock to kick the fossil-fuel habit and run on RE & current biomass as back-up energy thanks to the work of the Passive House Institute.

    For the grid-tied building, the four definitions of types of zero energy assume relevance, defining levels of being guilt-free of negative environmental impact, I suppose. I agree it is important to be aware of the full impact of our choices by accounting for impacts incurred at the source as well as at your home meter, but I don’t want these terms to incur snob-appeal for wealthy folks to be able to boast about how green their mansion is – they’d do better to build tiny instead and donate their excess to PHIUS to make more Passive Houses.

    I think the Passive House Institute is right in treating Zero Energy/ Carbon Nuetrality as icing on the cake and focusing on the cake instead. We will accomplish far more for the planet if we use a limited pool of money to build/retrofit most buildings to Passive House Standard, than we do by creating a few guilt-free Net Zero Energy Emission buildings.

    Of course, demonstration ZE buildings do have their place in helping to capture the public’s imagination – when they show up to see the wonder building it gives us a chance to tell them that they can build or retrofit to P.H. standard for a small fraction of the cost of getting to Z.E., and that puts them on track for being able to add RE and get to ZE at some future date. That’s my justification in trying to push towards ZE in my climate (plus I also want to get my seasonal heat storage concepts out of the theoretical realm and into the real world where we can data-monitor them).

    I try not to push clients toward ZE, except to make sure that my designs are taking advantage of available solar resources and are planning for the addition of future RE systems. Building solar-ready P.H. buildings with surfaces oriented & located for max. solar collection, and chases from collector areas to equipment rooms saves huge amounts on implementing future RE systems without adding much in up-front costs, so that is what I strive to include in all projects.

    Towards our solar future,

  • david white says:

    the definition of zero net site energy surprises me. i’m a little confused as to the reasoning behind the definition. my first guess is that it simply avoids the philosophical hassle of determining to what extend it’s OK to put back 3.37 units of fossil energy taken with 1 unit of PV electricity.

    i guess this definition works more easily as the profile of energy types produced matched that of those consumed. however, as that point is reached, all definitions line up.

    2nd, regarding the source energy definition, i think there may be a mistake in the NREL paper. it claims that 1/3.37 units of electrical energy must be produced to offset 1 unit of natural gas energy. i would increase that requirement to 1.12/3.37, since 1 unit of natural gas use means 1.12 units of source energy.

    even more interesting that nowhere in the paper is mentioned the issue of distribution alone. the wikipedia article drew only 1 difference between site and source: accounting for transmission. this would mean that you pay the “postal charges” (which were measured in the 1990’s at something like 7%) for borrowing and returning electricity to and from the grid. perhaps this is excluded from NREL because (i think that) the grid-returned electricity can be locally used up, so transmission losses may be quite low.

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