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Net zero-energy PV or PV sized for the A/C load?

Matt_Salkeld_PEng | Posted in General Questions on

A rather big picture question – 

Net Zero Energy building standard is ramping up in Canada. It is obvious to many however the target should instead be Net Zero Carbon with the inclusion of a Life Cycle Analysis of the building.

Anyway, the Net Zero Energy standard includes a PV array to generate as much electricity annually as the house consumes in energy (of all fuel types).  In Canada the vast majority of the solar electricity is generated over six sunny months. This might have implications or a poor match for some electricity grids that use the most electricity in winter. 

How does the size of the NZ PV system coincide with the needs of the local utility, their grid and the seasonal demands?  Is the NZ PV sized array more arbitrary than practical?

Would it be a better idea to size a PV to meet the a/c demand or load of the house? Seems practical for summer a/c peaking grids. The PV would be two to three times smaller and cheaper. The PV could displace the gas-fired peaking power and therefore significant GHGs. I am thinking of local optimization compared to a crude blanket applied approach nation wide (does that sound familiar right now??)


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  1. Jon_R | | #1

    > the target should instead be Net Zero Carbon

    NZC is somewhat better then NZE, but you really don't want to draw a box around the building and optimize it for net zero anything. You can do more for less $ if you optimize over a larger scale. For example, utility scale PV is 2-3 times more cost effective than the building scale PV that is often used in NZ buildings.

    Unfortunately, there are many policy makers that don't understand this and are causing money to be wasted. Solar panels on houses is the wrong solution (when a grid connection is available).

    1. maine_tyler | | #3

      >"PV is 2-3 times more cost effective than the building scale PV that is often used in NZ buildings"

      Jon, would it be fair to say most of the extra cost goes into labor? Or are there other variables like panel rates.

      1. Jon_R | | #4

        You may find that info below. Note that I was being conservative - residential PV might cost 5X more than utility scale.

  2. maine_tyler | | #2

    >"Is the NZ PV sized array more arbitrary than practical?"

    My current feeling is that the entire concept of net-zero is somewhat arbitrary. Partially for the reasons Jon mentions and the reasons you mention with grid balancing issues, etc.

    I'd be curious to hear other thoughts about how to best 'spend the money.' By some metrics, as big a PV system as can fit would be best, but I assume there comes a time when we're looking at cost balancing for other investments in the structure.

    The analysis also needs to be sure to include the 'embodied' energies and carbon of the PV system.

    The only issue I see with Jon's notion of utility scale-PV is that environmental externalities may be missing. In other words, utility PV may be better $/CO2e, but not necessarily better for planet impacts/CO2e. This of course depends on the specific utility project. Some may be just fine. If they start developing in what would otherwise be growing space/habitat, etc, there are concerns.

  3. Matt_Salkeld_PEng | | #5

    Using building roof areas should be a resource advantage over fields or treed areas. ?

    Homeowners finance their PV with a low cost mortgage making the numbers okay and harnessing private wealth. Your point is taken - if we can harness private wealth into solar farms it could go much further.

    Would it be possible the have a system that is not grid tied? This for example could run the air conditioning with or even without an inverter? Would eliminate hassle for the utilities. I wonder if utilities actually welcome the idea of tens of thousands of supply interconnects?

    Another idea is a couple PV to supplement an electric hot water tank wout tying in to the main panel. No inverter required. DC elements are I believe available.

    The Net Zero Carbon opportunity is storing carbon sequestered in biomass in the building for a hundred years. This compared to either burning that biomass for power and releasing its carbon or using conventional insulation that produces significant GHGs.

    1. maine_tyler | | #6

      You may find utilities won't always applaud connected solar because it presents some challenges once a critical mass of solar is reached. That said, things are changing and it is likely to soon become a reality regardless (it, meaning managing a grid with lots of variable renewable energy sources: VRE's). This is a big topic unto itself , sometimes getting a bit controversial and certainly is a rabbit hole of uncertainty.

      You could choose to not tie into the grid, but I don't see what advantage that brings. The grid allows you to share your excess generation when you're not using it, and draw extra power when you need it. Sharing is good. You can think of the grid as storage (although its more like a bank in that the money/energy isn't actually sitting idly in wait; its just getting moved elsewhere to where it's needed.

      Sizing solar to meet year-round energy demand strikes me as a way to basically 1) feel good about your personal project, or 2) 'optimize' your dollars based on other set parameters, like the size of the house and the cost of the kitchen counters.

      Not all net-zero energy houses will pay-back their carbon debt in the same time-frame. This, perhaps, points to a fallacy with the NZE concept. There is increasing interest in the embodied carbon you mention.

      This isn't all to say solar shouldn't go on the roof. It's to say one should perhaps simply put up as big a system as desired / can afford (or as Jon suggests invest in community solar, though I agree with you that we ought to use our roofs since they are there looking up at the sky anyways). Land use issues will become ever more present as the grid transitions more fully to renewables.

    2. maine_tyler | | #7

      >"Another idea is a couple PV to supplement an electric hot water tank wout tying in to the main panel."

      Thermal storage could prove valuable. But I would still argue we're better off with all the systems tied together. The larger the balancing area, the less likely we are to curtail renewable generation or demand other supply sources.

    3. Expert Member
      BILL WICHERS | | #8

      You don’t need an AC or DC element. The definition of RMS AC voltage is the AC voltage that will produce the same amount of work as the same DC voltage in a resistive load. Since a heating element is a resistive load, a 240v AC element fed with 240v DC power will deliver exactly the same thermal energy in watts.

      The issue you have is that DC is more difficult to switch on and off due to the lack of zero crossing to help quench arcs across switching contacts. That basically means the heating element in your water heater will be fine running on DC power, but the contactor (fancy name for a beefy relay) probably won’t handle it well. You’d need to replace the control components to convert an AC water heater to DC operation, not the heating element itself.

      Just use the solar system for peak shave (load reduction during the time when solar is producing power), and call it good. Peak power is both most expensive, and usually also least efficient, so using solar power to reduce peak loads provides maximum benefit at minimal cost.

      I’ve said before too that I hate the concept of carbon being synonymous with energy — it is not. This is especially an issue in Canada where the vast majority of all electrical power consumed was produced by hydroelectric generation with no carbon emissions. Stick with energy, or even embodied energy in physical things, and don’t try to work everything out in terms of carbon.


      1. maine_tyler | | #12

        >"I’ve said before too that I hate the concept of carbon being synonymous with energy — it is not. "

        I think that's the point Bill, and why people are using carbon in place of energy for discussions that pertain to climate specifically. It's good not to equate them. But if someone's goal is reducing carbon, then the discussion should revolve around carbon (really CO2e). I would think anyways.

        Sometime energy and carbon may be a decent proxy. Other times way off. You are right and it's a salient point.

  4. Matt_Salkeld_PEng | | #9

    Thanks Bill, what about using the enphase micro inverter at the PV panel - would that solve the problem of having to upgrade the hot water tank switch?

    That is also my thesis to focus on shaving peak power production on summer afternoons as this should have multiple benefits at the lowest cost. Again in Canada we typically have really low emissions power barring AB SK and NS.

    I heartily disagree about embodied carbon. There is a terrific opportunity to sequester atmospheric carbon and store it in buildings using biogenic materials. The rate things are moving we are obligated to do this ASAP for all new buildings.

    1. Expert Member
      BILL WICHERS | | #11

      Any inverter that is providing you with AC power to run things will be fine with the water heater as long as it has enough capacity to run it. Resistive loads are the easiest loads to power. It’s typically reactive loads (motors, a few other things) that can be tricky sometimes.


  5. Matt_Salkeld_PEng | | #10

    Hot Spot has the non grid tied solar powered air conditioner of heat pump. Lennox has a solar integrated heat pump system but it is grid tied.

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