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 in the building sector.” As I wrote in my article on the 4 possible definitions, things get a little squirrelly when you start talking about how you’d measure the energy produced and consumed. And a couple of the definitions aren’t actually based on energy; one uses energy costs and the other uses emissions.

The two big contenders, though, are whether you base the definition on site energy or source energy. First, let’s just define what these two things are.

Site energy – This is the energy used on-site, as measured at the meter. For electricity, just look at the kilowatt-hours you get billed for; for gas, the therms or BTUs.

Source energy – This is the energy used on-site plus the energy that went into getting that energy onto the site. For electricity, it’s the kilowatt-hours you get billed for plus the kilowatt-hours of energy that are “consumed in the extraction, processing and transport of primary fuels such as coal, oil and natural gas; energy losses in thermal combustion in power generation plants; and energy losses in transmission and distribution to the building site.” That quote is from the DOE report, A Common Definition for Zero Energy Buildings, released this month.

Now, how do you qualify a zero energy building? Using the site energy definition, every kilowatt-hour (kWh) of electricity you import to the site has to be offset by an equal number of kilowatt-hours produced on-site and exported to the grid. Likewise, if you burn natural gas on-site for heat, for example, all the BTUs you burn would have to be offset with an equivalent number of BTUs produced in the renewable energy production system on-site.

Using the source energy definition, you have to account for the energy used at the source and in transmission and distribution. Doing the calculations for source energy is a bit less straightforward when the home uses fuels other than electricity.

In the new DOE report, they came down on the side of source energy. Here’s their fleshed out definition:

An energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.

They also put forth an expanded definition of source energy:

Source Energy: Site energy plus the energy consumed in the extraction, processing and transport of primary fuels such as coal, oil and natural gas; energy losses in thermal combustion in power generation plants; and energy losses in transmission and distribution to the building site.

I say it’s expanded because they’re including the embodied energy of the fuel burned at the power plant.

In addition to this definition, they also define something new called Renewable Energy Certificates (RECs), which they define as “tradable instruments that can be used to meet voluntary renewable energy targets.”

If you want to learn more about all this, go to the DOE page on this new Zero Energy Building definition and download the 22 page pdf. They show how to do the calculations, qualify zero energy buildings, and use Renewable Energy Certificates.

One of the best things about this paper is this qualifying factor from page 10:

The designation Zero Energy Building (ZEB) should be used only for buildings that have demonstrated through actual annual measurements that the delivered energy is less than or equal to the on-site renewable exported energy.

It doesn’t matter how it models. What matters is how it actually performs. That’s not exactly a common way of thinking in Washington, D.C., where saying one thing and doing another is a way of life.

Calculating the zero energy balance

Of course, you have to define the site boundary, which could be the perimeter of the building or the property line. Any energy that crosses the boundary is counted as delivered or exported site energy.

I wrote about these calculations a little bit in my article on the four definitions of zero energy use, but it wasn’t as clear as it could have been. This time I think you’ll get it.

The DOE is going with source energy as the basis for zero energy buildings, so let’s see how that works. The first thing you need to know is how to go from site energy to source energy. For that, all you do is multiply by the appropriate conversion factor. The table shown in Image #3 (at the bottom of the page) is given in that latest DOE document on zero energy buildings and come originally from ASHRAE standard 105.

What confuses a lot of people is that second line. For example, if a home uses 10,000 kWh per year of electricity delivered to the home, it takes 31,500 kWh of source energy to make that happen. So you might think that if you put photovoltaic modules on your roof, you’d need to export 31,500 kWh of electricity to the grid to offset the source energy of the electricity delivered.

Not true! Every kWh that you send to the grid really offsets 3.15 kWh of source energy because that kWh can be delivered to another home instead of the electricity from the power plant, with its 3.15 kWh of source energy. Hence the second line in the table above. Imported and exported electricity both get multiplied by the same conversion factor.

Using a fuel like natural gas on-site can be confusing, too, but if you just use the table above, it’s pretty easy. Let’s put some numbers in an example:

Imported energy

Type	Site Energy	Factor	Source Energy
Electricity	6,000 kWh	3.15	18,900 kWh
Natural Gas	40 therms (1172 kWh)	1.09	1277 kWh
		Total	20,177 kWh

To qualify as a zero energy building, this home would have to offset 20,177 kWh of source energy. Because exported electricity gets multiplied by the 3.15 conversion factor for electricity, that gives us:

E x 3.15 = 20,177 kWh

E = 20,177 kWh ÷ 3.15 = 6,406 kWh

What that means is that you’ve got to produce 6,000 kWh of electricity on-site (renewable only – diesel generators don’t count) to offset the imported electricity and another 406 kWh to offset the natural gas use.

Make sense?

If you’ve got other energy types or fuels in the mix, you do the same thing. Just pick the appropriate source energy conversion factors from the table and calculate the imported and exported energy numbers.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.