Comments on: 4 Ways to Define Net Zero Energy Use in Buildings

By: Robby Schwarz

Robby Schwarz — Mon, 01 Jul 2013 07:21:16 +0000

I would propose that a fifth
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.

By: Leigha Dickens

Leigha Dickens — Thu, 27 Jun 2013 17:12:44 +0000

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

By: Leigha Dickens

Leigha Dickens — Thu, 27 Jun 2013 17:01:07 +0000

Oh yeah, that makes sense. We
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?

By: Allison Bailes

Allison Bailes — Wed, 26 Jun 2013 21:44:59 +0000

Leigha D.:
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.

By: Ryan Shanahan

Ryan Shanahan — Wed, 26 Jun 2013 20:51:17 +0000

If the construction industry
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?

By: Leigha Dickens

Leigha Dickens — Wed, 26 Jun 2013 20:13:00 +0000

Great post and good
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?

By: Curt Kinder

Curt Kinder — Tue, 25 Jun 2013 05:49:27 +0000

My own home is a case in
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.

By: Allison Bailes

Allison Bailes — Mon, 24 Jun 2013 16:43:28 +0000

Curt K.:
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.