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Building Science

The Department of Energy Chooses a Definition for Net Zero

They also have a preferred name for buildings that produce as much energy as they use

A zero energy home produces as much energy as it uses. Sounds like an easy thing to determine, but the devil, as usual, is in the details. The U.S. Department of Energy now has decided on a definition and how to qualify buildings for zero energy status.
Image Credit: Image #1: Energy Vanguard

A few weeks ago, I wrote about whether homes that produce as much energy as they use should be called net zero energy or zero net energy homes. Several readers offered up another choice: zero energy homes.

I’ve also written in the past about four different ways to define net zero energy (the term I’ve preferred). Now, the U.S. Department of Energy (DOE) has weighed in on both of these debates. Do you know what they decided?

The name

The name of the DOE report is called A Common Definition for Zero Energy Buildings. That gives away their name preference, which they’ve actually been using for a while. They dropped the “net” altogether and assume people will be able to figure out what it means. I’m OK with that. The scientist and grammarian in me wants a more precise term, but I get it.

Sam Rashkin, the Chief Architect in the DOE’s Building Technologies Office, has been presenting on clearing up our language in the energy efficiency community for the past couple of years. I just heard him again recently in Chicago, where he gave the closing keynote speech at the 10th annual North American Passive House Conference.

He always starts by talking about how no one wanted to buy the delectable fish that was called Patagonian toothfish. But when they changed the name to Chilean Sea Bass, it became a big hit. Words matter.

So, zero energy homes it is. I’m OK with that, even if it’s a bit inaccurate (because all homes use energy).

The definition

In the introduction of the DOE report, they write, “A zero energy building (ZEB) produces enough renewable energy to meet its own annual energy consumption requirements, thereby reducing the use of non-renewable energy…

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  1. Ted Cummings | | #1

    Just a couple Comments and a
    Just a couple Comments and a Question.
    In Duke territory, and presumably others, one can use net metering and reach net zero Electrical energy using PV panels; but, one cannot be compensated for PV generated power in excess of Electrical power used. There are very few people who would put those extra panels on the roof to cover minimal uses of Natural Gas energy, used in say cooking or a decorative sealed combustion fireplace, just to be able to say that their home is a "zero energy building" (ZEB). They would be expending a fair amount of money for that extra PV capacity and would be giving away that excess Electrical energy to the power company. Until power companies in general compensate PV panel owners for the excess Electrical energy they generate, I would expect to see very few ZEBs except in all electric homes. Yes, I know that I could give up that decorative fireplace, but....
    In places like Wisconsin (and in Indiana, where it's being considered) where the reimbursement for all Electrical energy produced on site is at a significantly reduced rate, the use of PV panels with net metering could virtually disappear, greatly affecting the national conversion to solar power. With current net metering, I'd have to put about 1.25kW of additional panels on the roof to generate the additional Electrical energy to offset that additional expected Natural Gas use.
    I'm not complaining about the justification or accuracy of the term ZEB but, like the "toothfish", "almost zero energy building" doesn't have the same ring and will be discouraging to many.
    Q. I don't believe that appliances such as a decorative fireplace are included in HERS index calculations so that from a HERS point of view one can have a decorative fireplace using Natural Gas and still obtain a HERS index of ZERO, which I intend to do in a home that I'm ready to begin. But, at the same time, because of that decorative natural gas fireplace, I will not have a ZEB. HERS Zero, but not ZEB. Do I have that correct ?

  2. User avater GBA Editor
    Allison A. Bailes III, PhD | | #2

    Response to Ted Cummings
    Ted, you are correct. The HERS Index doesn't include decorative fireplaces. It also doesn't include driveway snowmelt systems or heated garages. You could indeed have a HERS Index of 0 and not qualify as a ZEB under the DOE's new rules.

  3. User avater
    Dana Dorsett | | #3

    Issues with the conversion factor for electricity
    I'm wondering what data set they used to come up with a national average of 3.15 source energy to consumption, and how old that set is?

    In New England almost all of the 25-35% thermal coal power generation has been replaced with ~50% combined cycle gas. That power accounts for roughly half the power going onto the grid, with maybe a third of the total electricity pie going to nuclear, and another good slice going to hydro and non-hydro renewables. The quasi real-time grid mix on the ISO-NE grid is updated every 5 minutes in this chart:

    Note- power generated on the customer's side of the meter only shows up as missing load in the ISO-NE snapshot. A gigawatt of rooftop solar would be real power, but doesn't appear since it's not separately metered and accounted for in real time. Commercial and utility scale wind and solar as well as landfill gas, biomass, and waste-to-energy power sources are included in the non-hydro slice.

    In the Pacific Northwest it's almost all hydro, with a bit of nuclear and a growing fraction of wind, with next to zero thermal fossil burners.

    These areas are nowhere near a 3.15 conversion factor, and with the rapid deployment of both wind and combined cycle gas in the midwest (including Texas) over the past decade it's unlikely that it's still a credible conversion factor there either.

    In Wyoming or West Virginia where it's almost all 30-35% efficiency coal (with a few 40%+ supercritical coal plants) a conversion factor of 4-ish is perhaps credible, but where are all of the other sources that bring the national average up to 3.15? I simply don't believe it. The national grid source mix is evolving rapidly, and that conversion factor seems woefully out of date.

    This is related to some of the nits to pick with PassiveHouse carbon accounting, where they go to great lengths to reduce energy use even in areas with nearly carbon-free grid, and don't discount for site-sourced energy (unless it's actually Net Zero), with the dubious assertion that those measures are reducing greenhouse gas emissions more than the embodied carbon emissions of the materials used for the last 30-50% of R value. Saving 5,000 kwh/year in Tacoma or Eugene is not at all comparable to saving 5,000 kwh/year in Casper or Wheeling in terms of lifecycle greenhouse gas emissions or source fuel energy.

  4. KEVIN ZORSKI | | #4

    Wood Heat?
    How does wood heat fit into this scheme? In Maine, many people heat with wood. Wood stoves are losing favor in tight, energy efficient homes in many areas, but will probably remain popular in northern climates where it remains an inexpensive, and one could argue renewable source. What if all the wood comes from the homeowner's property? Does pv have to replace this power to remain "zero energy?

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