4 Ways to Define Net Zero Energy Use in Buildings

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ASHRAE is meeting in Denver this week, and my friend Robert Bean, the mastermind behind the Healthy Heating website, tweeted the other day that they're going on a tour of NREL's new net zero energy building. I'd love to go—both to the meeting and the tour—but will have to wait for another opportunity. I did check out the NREL page on their Research Support Facility and am impressed.

While there, I found some links to other resources, including a paper from 2006 titled, Zero Energy Buildings: A Critical Look at the Definition (pdf). I've been on a bit of a net zero energy (NZE) kick here lately, and wrote a bit about the controversy over the definition in my article about Matt and Amy's NZE home in Asheville, North Carolina.

In that article, I gave a brief introduction to the issue of differing definitions, including the role of hippie electricity and off-the-grid homes. The NREL paper, however, does a much better job of it than I did there, so let's take a quick look at their four definitions for a net zero energy building. (They call it a zero energy building, but I don't like that term.) First, though, let's address the issue of where the renewable energy is generated.

On-site vs. off-site

Before defining net-zero, the report goes into the boundary issue and determining what counts as on-site production of energy. The table below shows their list of supply-side options. Note that energy efficiency is first.

net zero energy building on site vs off site

In the discussion of this issue, the paper states, "Typically, the only area available for on-site energy production that a building has guaranteed as 'its own' over its lifetime is within its footprint." Anything you put on the ground could be lost to development.

1. Net Zero Site Energy

In the first definition, an NZE building produces as much energy on-site as it uses on-site over the course of a year. If the building uses 10,000 kilowatt-hours, it's got to produce 10,000 kWh or more, ideally on-site within the footprint of the building.

This definition doesn't account for the type of energy used, though. Where this becomes important is if the building uses natural gas, propane, or some other fuel besides electricity.

Let's say your home has a natural gas furnace, and it's 95% efficient. For every 95 kWh of heat that your home needs, your furnace is burning 100 kWh of natural gas. (Pardon the units, but since the energy produced is in kWh, I'm going to stick with that.) That means you've got to provide 100 kWh of site-generated electricity.

If, on the other hand, you had a heat pump, it would need maybe 40 kWh of electricity to move 95 kWh of heat into your home. Rather than having to produce 100 kWh of electricity, then, you'd only need to produce 40.

As you can see from this example, this definition of net zero favors on-site electricity use. It's also the simplest of the four definitions.

2. Net Zero Source Energy

A net zero source energy building produces as much energy as it uses, but you account for the source. For an all-electric building, the site and source definitions are the same.

With other fuels, however, you have to account for what happens at the power plant to do the conversion. One kWh of electricity delivered to your home from the utility company's power plant, for example, was originally about 3 kWh of heat at the plant, leading to a multiplier effect of efficiency when you save electricity on-site.

That multiplier effect also works the other way. As I said in the last section, the definition based on site energy favors electricity over gas. With the definition based on source energy, you have to account for the energy lost at the power plant and in transmission when calculating the amount of electricity you need to produce to cover the natural gas burned on-site.

So, instead of having to produce 100 kWh of electricity for each 100 kWh of natural gas consumption, you'd divide that by the site-to-source conversion factor. In the paper, the authors cite a national average for electricity of 3.37, which means that for each 3.37 kWh of natural gas burned, you'd have to produce 1 kWh of electricity.

This definition clearly favors natural gas. It's also not so simple because the actual site-to-source conversion factor for a given location is likely to be different than the national average. It also changes hourly, depending on which plants are operating.

3. Net Zero Energy Costs

A net zero energy building based on energy costs is one in which the building earns as much money from selling electricity produced as it pays for energy bought. It's an appealing definition to accountants, I suppose, and of course everyone would like to be net zero or better in energy costs.

The problem is that, "as utility rates can vary widely, a building with consistent energy performance could meet the cost ZEB goal one year and not the next." In addition, utility companies often pay you their avoided cost rather than the same amount you pay them for the kilowatt-hours you buy. That means you'll need more on-site production than you would in the definitions based on site or source energy.

4. Net Zero Energy Emissions

A building that achieves net zero energy based on emissions, "produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources." How easy it is to do this depends greatly on the type of energy used and the location. A building in the Pacific Northwest, which has a high percentage of electricity generated by hydroelectric plants, can reach achieve net-zero energy emissions without much difficulty.

Clarity matters

The NREL paper (pdf) does a good job of elucidating the issues around the definitions of net zero. I think the most common definition in use is the net zero site energy building. It's certainly the simplest, even if it isn't quite fair to fuels other than electricity.

Another important issue that I haven't really addressed here is what constitutes site-generated energy. Photovoltaic modules (a.k.a. solar panels), wind turbines, and solar thermal count. Fuel cells, which turn fossil fuels to energy on-site, do not.

The definition of net zero energy in buildings may not be quite as complex as "what the meaning of the word is is," but it's not as as simple as you might imagine either.

 

Related Articles

The Electricity Multiplier Effect for Home Energy Efficiency

NIST Takes on the Net Zero Energy Home Challenge

A Really Cool Net Zero Energy Home in the North Carolina Mountains

 

Footnote

Yes, I use a lot of acronyms and initials here. If you'd like to have a handy dandy list of common acronyms and initials used in the field of green building, energy efficiency, and building science, you're in luck. I put one together and am constantly updating it. As of right now, it has 140 terms, and you can download it here.

 

Photo of NREL's Research Support Facility used with permission of NREL under the terms of their End User Image License Agreement.

Comments

Allison Bailes

Curt K.: Good point. They also didn't raise the issue of transportation energy, what David Goldstein calls transportation efficiency. When you do start factoring in all the variables, the definition gets more and more unwieldy, though. With the current state of our buildings and our energy system, I think a building that achieves net zero by almost any definition is a good thing.

Curt Kinder

My own home is a case in point: 
 
I really enjoy its energy performance - average of $30 per month, but it is constructed of ICF and closed cell sprayfoam, both of which have high embodied energy. 
 

Leigha Dickens

Great post and good definitions, it is indeed important to distinguish between them as we push for this whole "net-zero" thing to become more popular. We're trying to ramp up our sales message on net-zero homes and my perception so far is that potential net-zero customers care most about net-zero energy costs. Some do care about net-zero site energy use, as well. I think folks can care about net-zero source energy too but it will be a much harder thing to sell.  
 
A point of confusion in the article, you state that in an all-electric home, site energy and source energy are the same thing. Then you go on to explain why we need to account for far more source energy with electricity than any other fuel because of the losses at power plants and through transmission. Wouldn't you mean to say that for an all-electric building, the site energy and source energy are very different?

Ryan Shanahan

If the construction industry can focus on NZ site energy (def 1) and the utilities can focus on creating source energy renewably and increasing the efficiency of the grid we're one giant leap closer to the goal. Sometimes I think we try to take on too much as an industry... CA's Net Zero by 2020 building code uses def 1 correct?

Allison Bailes

Leigha D.: Yes, this is a very confusing point, and I figured someone would raise it in the comments. First a clarification.  
 
You wrote: "...you state that in an all-electric home, site energy and source energy are the same thing." That's not quite what I said. What I really said was that whether you apply the site or source energy definitions of net zero to an all-electric home, you end up with the same result. That is, you need the same size PV system to reach net zero either way. 
 
I had to sort through this when I came to it in the NREL paper, too, so let's take a look. If the building is all-electric, each kWh of electricity you generate displaces 1 kWh of electricity from the grid. In the site energy definition, you're generating 1 kWh on-site for each kWh you use on-site. If you use 10,000 kWh per year of electricity, your PV system has to generate 10,000 kWh of electricity.  
 
The source energy definition says you account for the source of electricity. According to the paper: "To calculate a building’s total source energy, imported and exported energy is multiplied by the appropriate site-to-source conversion multipliers." For electricity generated on-site replacing electricity imported from the grid, the multipliers essentially cancel each other out. 
 
Why does it cancel? Because every kWh of electricity you generate replaces 1 kWh of grid electricity. The grid kWh started off as 3 kWh of source energy (on average), but the PV electricity is replacing all 3 kWh because that was 3 kWh of source energy that didn't get used for that end use. 
 
Make sense? It takes a while of thinking about it to really wrap your head around the reason for this, but it does work out. 

Leigha Dickens

Oh yeah, that makes sense. We're saying that simply because we are moving to renewable energy generation on the site of the building, we're eliminating a bigger energy burden on society than is immediately obvious just because we're no longer generating it in a way that makes tons of excess heat and losing a bunch of it on the way through transmisison.  
 
At least when the sun is shining. Without battery storage on site, you still draw from the grid sometimes, so truly net zero with respect to source energy in a grid-tied all-electric home would mean producing enough power to offset all of that hidden energy you expend when you pull from the grid, would it not?

Leigha Dickens

Nevermind, I see the part about canceling each other out. That does make sense.

Robby Schwarz

I would propose that a fifth definition is being used as well. Zero energy as defined by the HERS index score. (a HERS Index of zero) This one is problematic as it encompasses many of the definitions you describe. In addition, it is often not in alignment with how a utility is defining Zero energy. In Colorado, for example, the utilities/PUC’s DSM program will only allow 120% of the expected electrical energy use in the house to be offset with PV. However, you may need more than that to achieve a HERS Index of Zero. This is a real problem and caused because of the misalignment of definitions.