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Community and Q&A

R. Carter Scott’s Latest Methods

Kevin Dickson, MSME | Posted in Green Building Techniques on

In these days of cheaper and cheaper PV, huge arrays make more and more economic sense:

18KW ! Wow. Does anyone agree with me that this is a stronger trend than Passivhaus requirements for new construction?

One design decision I would question is the propane water heater. Propane may soon become more expensive than electric resistance water heating. If a commercially proven inexpensive air-to-water split heat pump water heater were available, I’m sure they would have gone with it.

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  1. User avatar
    Mike Eliason | | #1

    i'll say negative, and here's why: i enjoy a good back and forth :)

    seriously, though - in essence it already is a passivhaus (HRV, superinsulated, compact form).

    for the region, wall and roof insulation could be about right (some of our projects model out at less on the wall - with cellulose, not SPF).

    if it were to be a grid connected all electric net zero PH - the max site consumption would be...
    1,800sf x 0.92 (TFA factor) x 14.07kBTU/ft2a = 23,300 KBTU/a, or 6,828 kWh/a (that's including DHW)

    18kWp on this as a PH would let them drive two nissan leafs ~22,500 apiece (which is probably more important anyway).

    as for what would be needed to achieve PH? better insulation under slab, better windows (potentially), and a more efficient HRV (probably)... but it would allow them to drop one, if not two, of the heat pumps.

    frankly, if net zero is a req't - then i'll take these stunning net zero passivhaus rowhouses (@ $129/sf) over the suburban saltbox.

    'These are zero energy with zero premium, so there should be zero debate. Why would you build it any other way?' - tim mcdonald

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Low PV prices, good PV incentives, and favorable feed-in tariffs allow homeowners to consider the installation of a very large PV array -- and the investment has a very quick financial return. If you can afford it, why not?

    Of course, there really isn't any need for such a huge array -- and most of us can't afford a Chevy Volt or Nissan Leaf.

    When it comes to Passivhaus certification -- it certainly seems optional or unnecessary if you already have a net-zero-energy house. Why bother?

    Finally, on propane: I think that Carter Scott switched from natural gas to propane when he realized that the natural gas hook-up and meter-reading fees cost more than the gas... so propane is cheaper than natural gas. Obviously, an all-electric house would make sense from an environmental perspective, however.

  3. Kevin Dickson, MSME | | #3


    I agree that urban infill is much more sustainable than the suburbs... it would be much easier to own just one car in a walkable neighborhood like in Philadelphia. And just think how much better our cities will be if developers concentrated on rebuilding obsolete houses instead of greenfields.

    Thanks for the insight on the propane... it also could be used for emergency heat with a millivolt propane fireplace. If you light the pilot only when needed, propane has zero standby losses. If you believe that the grid will be less reliable in the future, having propane or natural gas on site is a good choice.

    Speaking of the grid, today's grid tied PV arrays don't function when the grid is down. I like Carter Scott's idea of using the Prius for backup electricity, but he won't have a way to recharge it without using gasoline when the grid is down. With the right switches installed, he could. Just something to remember when buying a grid tied PV system. I'm not sure it's allowed by code yet.

  4. Jack Woolfe | | #4

    Speaking of the grid, today's grid tied PV arrays don't function when the grid is down. I like Carter Scott's idea of using the Prius for backup electricity, but he won't have a way to recharge it without using gasoline when the grid is down. With the right switches installed, he could. Just something to remember when buying a grid tied PV system. I'm not sure it's allowed by code yet.
    Answered by Kevin Dickson, MSME

    You probably remember seeing Alex Wilson's blog "Gas Lines Point to a Need for Resilience" where he said:

    With the net-zero-energy house my wife and I are currently building (rebuilding) in Dummerston, Vermont, we’re thinking of installing a fairly conventional grid-connected solar-electric (PV) system, but using a new inverter that is coming out early next year that allows you to plug a load into it when the sun is shining — even when the grid is down. (Most grid-connected PV systems can’t operate when the grid is down, though the sun may be shining brightly.)

  5. User avatar
    John Semmelhack | | #5

    This isn't so much to do about Passive House vs. PV. It's not like Carter Scott is building shitboxes. He's building super-insulated saltboxes...just a notch below Passive House efficiency. He's found a sweet spot that seems to really work for him, and I'm a fan of what he's doing....especially in minimizing his HVAC install cost, while optimizing performance.

    At this point, the giant PV systems here seem to be just as much a function of distorted incentives as low PV prices. The slow, regulated utilities (financed by Massachusetts ratepayers) are over-paying for the "solar attribute" through high SREC prices and the state is kicking in unnecessary tax credits and rebates. Small, nimble businesses, such as Transformations, are stepping in to make some money while the gettin' is good. I would do the same in their shoes. According to the presentations slides I've seen, the relatively low sales prices are, in part, made possible by the homeowner releasing all PV-related tax credits, rebates and SREC income to the builder, or alternately, leasing the PV system from the builder. Brilliant strategy, I think. In addition, as a clever builder and business owner, Scott has integrated the PV installation into his own company, instead of outsourcing. Cheers to Carter.

    Side note - Mike's comment regarding the Onion Flats projects in Philadelphia points to a more important trend in new construction - higher density. The trend toward townhouse/rowhouses and multi-family construction means a lot less available roof area for PV!

  6. Sonny Chatum | | #6

    Kevin, you asked: Does anyone agree with me that this is a stronger trend than Passivhaus requirements for new construction?

    Throwing up PV is a stronger trend than ANY sort of substantial, residential energy footprint reduction, I would bet, at least in the new residential/retrofit mainstream. For my State, I have personally and literally seen stack upon stack of applications for SRECs. There's no way most of these people are substantially reducing their electricity demand (forget Pasivhaus) prior to silicon glazing their roofs. I would also bet that many of these people feel comfortable using MORE electricity after PV installation, since some of their previous bill has been offset by the PV.

    Sure, the PV helps the utilities meet renewable energy portfolio requirements, but it's drops in a bucket with holes in it.

    As many of us know, the residential energy retrofit opportunities are huge, but few are doing it. I believe that in most U.S. climates, if's a disgrace that most of the people that have appropriate roofs aren't using them to go net-zero with PV.

    Just think of the miraculous, but demanding, efficiency of nature, which we try to emulate in so many ways. Perhaps it is no mistake that it is possible to achieve energy net-zero in an "average" house with a "normal" roof area, but only by respecting every detail of energy efficiency.

  7. Dennis Heidner | | #7

    So I will bite the tag line... 18kW is an awful lot of power for one house. There are 50+ panels on the house, probably 260W each, setup at different angles. Houses like this would have required special approval from power company because it exceeds 10kW that is normally allowed for a neighborhood transformer. The house was already built to be super efficient.

    So that brings up a couple of obvious conclusions :

    1. Nice pool in back yard. That consumes some of the power that isn't going to the electric car.
    2 Since this is larger than normally allowed net metering agreements - the house was designed as with the SolarPV on the roof for the purpose of selling the power COMMERCIALLY back to the utility. Perhaps taking advantage of good feed in tariffs.
    3. Using the roof top as a source of revenue is actually a very clever and good idea.
    4. With all that roof area covered - no need for any of that special "super duper Thermo King" insulating paint to keep the roof cool :-) Roof is already shaded by panels.

    The solar on the roof was never about driving the consumption in the house to zero. It would be to compensate for the swimming pool, a car, and bring in extra revenue, etc.

    That also brings up a good question -- how does the Passive House standard work with swimming pools that might be wired in to run off the electric service for the house -- or perhaps has the pool heat plant attached to the house. Is that energy use included in the house energy?

  8. Kevin Dickson, MSME | | #8

    The author/builder doesn't mention the pool in the article. It could be that the pool doesn't have a heater. That is possible with the use of the right pool cover and a short 2.5 month season. Filtering still takes a lot of electricity though.

    He also doesn't discuss that sunspace in the back.

  9. Dennis Heidner | | #9

    Yes, I noticed the picture - but no discussion. I presume the picture was finished product...

    There were not a lot of windows on the south side, sunspace had to be the compromise to bring in daylight, pickup some solar heat during the winter.

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