Getting to Zero

Graham S. Wright, Senior Scientist & Product Program Manager

The PHIUS+ 2015 passive building standard is a high-performance building standard that has three main types of requirements:

  1. Limits on the design heating and cooling loads, both peak power and annual energy.
  2. A limit on the overall annual design energy use, on a source-energy basis.
  3. Prescriptive quality and commissioning requirements.

The prescriptive requirements include those of the U.S. Department of Energy’s “Zero Energy Ready Home” program (ZERH), thus, PHIUS+ certification earns the ZERH designation, as noted on the Certification web page.

The limits on heating and cooling energy were determined by life-cycle cost optimization, and these have a close connection with passive measures such as insulation, shading, solar gains. PHIUS+ was developed with the idea of supporting a cost-optimized approach to designing a net-zero-energy building, with the heating and cooling criteria indicating “where to stop” investing in passive and envelope measures, and instead turn to equipment efficiencies and renewable energy generation.

For going beyond “Zero Readiness”, there is (as stated in the Certification Guidebook section 3.10.2) an add-on badge available, for projects that include enough on-site renewable energy to actually get to zero, on a net, annual, modeled, source-energy basis. This is the PHIUS+ Source Zero designation.

The first part of this article will attempt to clear up the difference between PHIUS+ Source Zero and the more common Site Net Zero concepts.

Among passive house practitioners there does seem to be a general awareness that the source energy use of a building is 2.X to 3.X times as large as the site energy use, because a lot of this energy is electricity. Most electricity is generated in combustion power plants where two-thirds of the energy in the primary fuel is dissipated as waste heat.

This may give a vague impression that getting to Source Net Zero would require 2-3 times as much PV generation as for Site Net Zero. That impression is incorrect, because the leverage works both ways! Every kWh of renewable electricity generated on site also offsets 2-3 times as much primary fuel at the source. Energy savings at the “downstream end” have a magnified benefit, as Amory Lovins at Rocky Mountain Institute has been pointing out for years. (See for example from 2008.)

Even for the worst case in terms of PV generation requirements for Source net zero, (an all-electric building) the array size is the same as for Site net zero.

Here is a (hopefully) clarifying example of a source net zero situation. Suppose a building uses 10 kWh of electricity for every therm of gas.

Site usage Source usage
Electricity 10 kWh = 10 kWh x 3.16 = 31.5 kWh
Gas 1 therm = 29.3 kWh x 1.1 = 32.2 kWh
Total use = 39.3 kWh = 63.6 kWh
Site generation   Source offset
PV 20.2 kWh x 3.16 = 63.8 kWh

Notice that the renewable energy generation to get to source zero (20.2 kWh) is less than that needed to get to site zero (39.3 kWh). This is a somewhat exaggerated example, and for annual numbers those are closer to the scale of a home fish aquarium than the home itself. The point is that the on-site PV can be lower for source zero than for site zero because of the additional measure used to reduce the source energy, that is, directly using a primary fuel with a low source energy factor (1.1 for gas).

The PHIUS Source Zero badge happens to align almost exactly with the source zero definition released by the US Dept. of Energy in September 2015 (just a few months after the launch of PHIUS+ 2015.) The logic of using source-energy accounting rather than site energy, or net annual energy dollar cost, is that source energy represents the impact on society. As stated in the DOE report:

“Most building managers are familiar with site energy, the amount of energy consumed by a building as measured by utility meters. Site energy consumption can be useful for understanding the performance of the building and
the building systems, but it does not tell the whole story of impacts from resource consumption and emissions associated with the energy use.”

In particular, PHIUS Source Zero badge aligns with DOE in that Source Zero Campuses can also be recognized, and with respect to the use of national average source-site ratios. Again, as the DOE report puts it:

“The Zero Energy Building definition uses national average ratios to accomplish the conversion to source energy because the use of national average source-site ratios ensures that no specific building will be credited (or penalized) for the relative efficiency of its energy provider(s).”


The second part of this article will take a step back and review some thinking about how society as whole gets to either Zero Emissions, or 100% renewable energy to put it another way, and what the building sector’s contribution to that should be. The “Net Zero” concept is very popular I believe for two reasons: one is that it is easy to understand and explain how much Zero is – as soon as you get into terms like Low, or Nearly Zero, or Partial Zero, then you have to understand and explain units of measurement, Low compared-to-what, and so on. But more importantly it seems intuitively fair – “my building has put out as much energy as it used so we are all-even, square deal.” The second part of this article will examine whether that idea of fairness, and the current PHIUS approach to source energy, will hold up over time, or at scale.

For the time being though, net-zero buildings are obviously hugely helpful to the cause and deserving of special recognition; please submit more of them for certification!