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Community and Q&A

PV + Electric with or Without Propane

Callan _1 | Posted in Green Products and Materials on

I am the owner/designer working with a green-certified contractor on a multi-generational, green, family home (~4000 sq ft). We are wrestling with whether to take advantage of the ease of natural gas as a back-up system for our end-of-the road house or switch to all electric and improve our carbon footprint. The house will have PV but we experience many power outages (about 30 to 40 days a year, up to 3 days at a time). We plan to have a generator but easily available natural gas (propane) is tempting for back-up generator, heat and cooking given the cost of PV batteries.

THE QUESTION: should we go all electric (e.g. induction, electric fireplaces, multiple electric on demand water heaters, heat pumps) or combine electric (heat pumps) with propane (fireplaces, cooking, on demand water heaters). For a variety of reasons, geothermal is not a good option for us. Budget is an issue and the reduction in costs from not needing to buy/bury a tank or pipe the house, along with the lower costs of electric fireplaces compared to direct vent gas could be shifted into solar batteries but does not offset the increased costs. Nonetheless, the climate situation is critical and we are very committed to reducing our footprint.

LOCATION/BUILD SITE: The house is on 20 acres at the end of the road in Montana in a rural zone 6 area that gets about 300 inches of snow per year with about 60% cloudy days. The house is on a non-windy side of a ridge with trees protecting it below to increase available light and reduce wind. The 1/2 mile long driveway was put in place 30 years ago and remains in great shape. The vegetation is pristine with very few noxious weeds and abundant native vegetation. The build site is partly flat with a mild slope (~6 feet front to back) inviting a walk-out basement. However, about 3 feet down we discovered a huge bedrock seam of partly fractured, partly solid granite

BUILDING ENVELOPE: Given the difficulties with the bedrock, we adapted the foundation rather than destroy the site. Recognizing the carbon footprint issues with EPS, we still opted to use R-28 graphite-infused ICF blocks for the foundation because we could both insulate and manipulate to the uneven shape of the bedrock while minimizing construction disturbance. The underslab insulation was limited by the bedrock. All footers have at least an R-6 thermal break with underslab insulation of R21 (garage & crawl) to R4-40 (living). All of the foundation walls will be bermed except for the area immediately around the entry door and egress window for the bedroom.

The house walls are metal skinned PUR sips R-40 and the sealed roof and constructed of R-70 SIPs. The windows and doors are all triple pane, European style high efficiency situated to maximize solar gain in winter/minimize heating in the summer. There are few exterior penetrations and those that exist are grouped in sheltered areas to minimize risks for air infiltration. The entire house, foundation, walls and roof, will be wrapped in a stick-on membrane to increase airtightness and reduce the potential for water penetration and decrease the risks associated with SIP seam separation. The exterior will be clad (over a rain screen) with KWP Ecoside composite wood.

THE STRUCTURE The structure is 40 feet deep and 72 feet long with a continuous 6″ wall running the 72 feet from ridge beam to foundation.  The garage sits on the main level and the house sits over 1/2 crawlspace and 1/2 walk-out basement. Engineered floor (open web) and roof trusses hang inside the building envelope and rest on the 6 inch wall so no span exceeds 20 feet. With this, the only other load bearing walls are for the stacked staircases.  The garage flanks the “storm side” (north) as a buffer between the main house and the frequent weather that moves across the area. The garage is incorporated into the building envelope but there is additional insulation in the garage ceiling (under the upstairs living area) and the wall that connects the garage to the main living space.

HVAC: There is a foundation to ridge beam 3 x 3 central chimney adjacent to the 6″ wall to house the air exchange and other duct work.   The house is heated by a high SEER variable speed, 10 zone heat pump along with a multi-head minisplit heat pump for the over-garage ADU. The mini split system is being used (1) because the ADU will be occupied prior to completion of the main house and (2) it simplified the ducting system dramatically. There is top-end air exchange system that ventilates the entire house, including the basement, garage and crawl. Because we need a secondary heat source in extreme cold when heat pumps go into what we call “toaster mode” when they have to heat air to circulate, we have three strategically located fireplaces (wood, gas, electric???).

The house will have a solar array easily capable of supporting the entire function of the house. It is also on the grid with our local electric cooperative. On sunny days we should be able to send power to the Coop and run the house. On stormy/cloudy days we will need to use Coop power or stored power. There will be a whole-house generator (diesel, propane or solar???) integrated into the system for our frequent power losses. We must have the ability to store power for 2-4 days if we rely on solar because of the short days and heavy cloud cover we have in the winter. For example, our solar-power weather station goes off line after a two day storm because there isn’t enough sun to produce more power. We have three years of daily, on-site solar potential data and can predict the needs from this.

Thank you in advance for your opinions and advice.

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  1. Expert Member
    Zephyr7 | | #1

    You’ll want a generator. Solar alone isn’t going to do it in this situation, and the batteries are a major cost and a maintenance item. If you’re worried about greeness, remember that all battery chemistries have a pretty significant impact. There are always trade offs.

    Note that propane and natural gas ARE NOT THE SAME THING. Not only do the two gasses have different energy content (propane has higher energy per unit volume compared to natural gas), but propane and methane (natural gas) are not sourced the same.

    A propane fueled generator has a few advantages: all the standard residential standby generators can run on both propane or natural gas with minor adjustments or modifications (which are available as kits and are factory supported). Propane never degrades, so there are no maintenance issues with your fuel supply. Diesel’s advantages are that it has much higher energy density compared to propane (that means a smaller tank for the same runtime), and the engines tend to last a long time. If you’re considering propane for other purposes, that’s probably the way to go, but keep in mind that you need a pretty large tank to get several days worth of runtime, and note that outages tend to strike when your tank is something less than completely full.

    Diesel fuel can get contaminated over time, so it’s usually recommended to try to use the full tank worth of fuel within a years time. In my own experience, diesel fuel lasts quite a bit longer than that, but one year is the guideline.

    I recommend a liquid cooled generator, which means it will have a radiator and a coolant system. These units run at 1,800 RPM compared to 3,600 RPM for typical home standby generators. The lower speed combined with far better cooling means these liquid cooled generators are far more reliable, especially for longer duration power outages.

    BTW, polyiso isn’t a problem. XPS is the rigid foam with the biggest environment impact. ICFs are usually considered to be one of the worst insulating systems in terms of environmental impact too. Always consider the full system for things like this, do not overly concentrate on any one part of the entire system.


  2. Callan _1 | | #2

    Dear Bill,

    Thank you for your thoughts. They are very helpful. And, they reinforce our concern that whatever we do we need to think in days on the system, not hours. We have lived in the same neighborhood for years and always made it on stored water and "camping" type solar kits to charge up our electronics along with a propane fireplace to keep the central core of the house above 50. We now have elder-elders in our group so we need to be more mindful of low temperatures than we have been.

    I do know LP and Propane are different, having used both, but I did not stop to think about their extraction. I will go educate myself. I appreciate your point. I also deeply appreciate the points regarding the generator types. I have been so busy with the foundation and other details I have not gotten my head wrapped around the types of generators.

    You are right about the polyiso, my bad typo. I was mushing things together. I have a lot of anxiety about using the XPS but our alternatives under the foundation are rockwool, which may or may not degrade, depending on which memo you read but won't satisfy our inspectors or extruded glass imported from Europe or possibly from Vermont. The cost is high, as well as the transportation. I am well aware of the insulation issues with ICF, and plan to add additional insulation (e.g. rockwool inside, for example) if we need. Thankfully, there are not many blocks needed because most of the foundation is 2 to 4 ft high. One 40 ft run is 9 ft high. With the bedrock craziness we had to make a decision to either switch to pier and beam (my preference) but that made us give up 600 sq feet of living space and a large mechanical area we needed or extend the time it took to build (we have spent 3 months on the excavation already), blast (a risk with the mixed fractured rock and other houses on the ridge not to mention enormous construction disturbance requiring restoration) or build complex concrete forms and then add insulation (expensive in time and materials). Perhaps we have made the wrong decision but it seemed like the least bad option we had. We can still back up on those, but only if we decide immediately.

  3. Mark Nagel | | #3

    I'm all too familiar with a situation such as yours: I am rural; heat with wood; cook with propane; small (7.5kW) diesel generator for backup electricity (can't allow electric fence and freezers to be out of power; and, have a well, so need electricity to pump!).

    Only time I'd ever considered battery backup was for a completely off-grid build I was proposing at one time.

    Emergency situations are tough to prepare for, and expensive. One has to keep in mind that when one is in an emergency situation that one will ONLY be able to support KEY devices. (I'm fortunate in that our 7.5kW generator is able to handle all our needs w/o really sacrificing- I don't have to isolate circuits.) The greater the electrical demands/requirements then the larger the backup generator will have to be: no difference if relying on solar and banks of batteries- more electrical demands/requirements would mean a lot more of these, and one has to scale based on solar availability! Though I prefer diesel (I have equipment that requires diesel, so I have a tank of diesel on-site), if one is using propane then it might be reasonable to just look to go with propane for generator fuel: no matter the fuel, ALWAYS be topped off when heading into the times of the season when power outages are most likely to occur.

    A couple months back I came home to find my power out. A bit odd to have that happen around here in the middle of summer! Fired off the generator. I later found out a neighbor had dropped a tree on a power line! Been here over 10 years and that had never happened before. Fortunately, my generator is always well maintained to be operational at ANY time. My point here is that despite how well one can predict things there will be times when the "unforeseen" happens. Risk assessment!

    PV panels for grid-tie likely OK (there's the issue of snow for there): I am not doing this; I also probably won't be doing it with my new structure (if my utility Co comes around and would like to stuff panels on my garage roof I'd be happy to let them, but the value in paying to do this just ain't there, nor is the utility [does me no good during a power outage!]). Battery backup would be, in my opinion, a last option: I have space in my garage for a bank of batteries- I maintain options! Generator just makes sense, and in your case propane: for reliability, longevity and efficiency I'll take a diesel, but that's largely owing to my situation, and diesel generators are more expensive. NOTE: Critical businesses rely on generators for backup power- maybe there are those with PV panels and banks of batteries, but I'd figure they are few and far between.

    My electrical service is pretty much limited to 100 amps (huge cost to upgrade due to service coming in under a roadway). I'm accepting the fact that this will likely keep me locked into propane: it will be for my "approved," "primary" heating source [but once permits get signed off I'll be heating with wood, so back to minimal propane use]; keeping electrical load low means I will still be able to use my small generator, and, in a sense, I am designing around it- it's the best life-raft there is and I can guarantee it's operation. Be sure to size storage tanks for propane (or whatever fuel) sufficiently: propane folks are able to readily figure this (there's sizing issues based on temps and other, so not something that just anyone is going to be able to figure out).

    Select solutions that are readily maintainable and have good local support. Things break in the most inconvenient times! The more critical the system the better it needs to be- sometimes "better" is attained through having immediate support and parts available [simple systems are easier to meet this criteria].

    I have an Ambient weather station and never have had an issue with it's solar power: I'm in the PNW; 100" of rain- we see a LOT of cloudy days! The console, which also uploads data to the Internet, is plugged into a UPS (as are my computers and associated equipment): the UPS (have a couple) provides sufficient time to bring up backup generator power.

  4. Callan _1 | | #4

    Thank you for taking the time to share so many details. It feels like we have a neighbor in you! I am smiling about the PWS. Snow was always our biggest culprit and once I dragged out to the pole and removed the snow WAY up there, I could get us back online but we regularly got knocked off by the 300-400 inches of snow we get a year. You are right about that being an ongoing issue for us with the solar panels. Our local coop is pretty cool so the panels come with good local support..

    Which brings me to your best point "Select solutions that are readily maintainable and have good local support."

    I am often so engaged by cool stuff that I forget having someone else with the same thing means there are parts and service. There are about 15 families in our neck of the woods, all of whom have the same generators (we have always roughed it in the past). I will make sure to pick something serviced by the same person everyone else has.

    When we were younger we heated our house with what was back then highly efficient wood stoves. We have a lot of wood left from the build and general forestry cleanup on the 20 acres but I have been hesitant to add to the pall of smoke that lays in our valley when it gets cold. I know there are high-efficiency wood burning units, and I suspect we have enough wood to get me to assisted living, but I all that smoke bothers me both for breathing and for psychological concerns. I will re-examine adding wood as a component of our overall system.

  5. Bob Irving | | #5

    I build in Northern New England, and for the past decade have used air source heat pumps exclusively. There are low temperature limits that might preclude you from using them, but here, I'll never go back to that old technology. I cannot think of one benefit of any fossil fuel system that is superior to these units.

    1. Mark Nagel | | #8

      " I cannot think of one benefit of any fossil fuel system that is superior to these units."

      I'll go trip a breaker to help point one out :-) (I won't do as my neighbor did, or as nature more often does around me, and take out a power line with a falling tree to prove the point.)

      OP is asking about backup power. ALL systems relying on electrical power will need to be assessed for their ability to operate during times that main power source is lost. IF a system is using some other power source then there ought to be a backup plan/power set up for that as well.

      1. Bob Irving | | #12

        “Trip a breaker” and both the fossil fuel system (unless you’re burning wood in a stove) and the ASHP will stop working. I’ve often been asked, when I say we use electric heat - “but what if the power goes off?” I ask what happens to their oil burner when the power goes out?
        I will say that in a super insulated, tight house, quite often nothing happens. I know of two instances - one with one of my clients - where they went to Florida for a week in midwinter and turned off the minisplits. In both cases, it was in the mid ‘50’s when they returned. “Resilience.”

        1. Mark Nagel | | #15

          Yes, these are the points/questions that need to be addressed. As I noted, I make sure that I, for my household, have them addressed. And note that I am here, forum, and especially in this thread, for the reason that I too am trying to navigate a build that will ensure I have things properly covered (I have a very dynamic and unique situation, as does the OP).

          Right now if you were to cut my power at my service pole I'd be able to stay warm, cook, heat water (though, because it's electric, I'd have to watch other loads), pump water, protect my critters (electric fence) for a month straight, regardless of the weather (if things aren't better by then then I'm guessing we have much bigger problems, and by "we" I'm saying everyone!).

          As the OP expressed concerns over key systems operating in rural locations I am trying to point out the things that I KNOW to work (actually have lived with).

          That something may be/or is green is of little importance if one isn't able to provide other key things. Fixating on shiny things often distracts from core things.

          Having a more energy efficient building means requiring less energy. This is pretty self-explanatory. This makes it easier to size a backup system (which I have done, and which I am striving to continue to do with a new build).

          But I go back to another key point in all this and that's that if you're out in a rural location and encounter problems it's quite likely that others are also experiencing those problems. Getting a system repaired might not be as easy, in which case due diligence in selecting for this is a wise thing to do (and keep in mind that our current supply chain difficulties are potentially going to only worsen).

  6. Callan _1 | | #6

    Bob, you make me smile. We have had one in our old house (300 ft lower elevation, 1/2 mi from the new build) and I agree, they are quite wonderful. We saw a tremendous decrease in our electrical use for Jan/Feb when we intervened on really cold days by "warming up the air" in the house with a few hours of our propane fireplace. If the heat pump went into "emergency" heating mode (what we call toaster mode) whereby the air is heated by passing it by coils like an old style toaster, the couple of hour warmup from supplemental heat tipped the scale. It was also useful when we lost power and the house got down to 50 degrees after a couple of days. We had one "cold" room that was disconnected from the rest of the house and had a minisplit. If we didn't use it for a while or if the temps got really cold (near 0 degrees F) we had to toss in something for it to work with or it never warmed up. Tiny issues as far as I am concerned! It doesn't get that cold that often.

  7. Mark Nagel | | #7

    ANY system that you have a big reliance on ought to have a thorough understanding of how it is to be maintained, and who will do that maintenance work. The subject of parts availability is starting to become more talked about as our current pandemic is showing us that our supply chains are very shaky; complex systems will be more vulnerable because they tend to have more parts. I could drag things deep into a rabbit hole, but suffice it to say, better to think long and hard up front and know that you'd done so than to look back and ask yourself why you didn't think about something...

    I have a refrigerator, which I'm fearing is starting to get long in the tooth, that was "Energy Star" approved some 11 years ago when I bought it. I started having problems with it icing up, not defrosting. Ice would block the flow from the freezer to the fridge section and there would go a bunch of food. Lots of time manually defrosting and cleaning up the mess. Ran across talk from a repair guy saying that when he'd run across this situation -a known issue- (many people also had their nice wood kitchen floors destroyed), he'd do the manual defrosting work and then DISABLE the energy setting (reprogram) and KNOW that he'd never see that customer again. Thanks to that guy's tip I have never encountered the problem again. How much in materials -energy- and time did all that add up to vs how much energy was supposedly saved? I have no idea, but I DO know that we never seem to [try to] account for such. Parts might be cheap today but what about in the future?

    1. Bas Sommer | | #9

      Would it help to have a liquid cooled generator with a fan coil inside so that the house has some base heat? It would reduce Propan consumption if power is out. I guess to cook with that heat would not be worth the effort..

      1. Expert Member
        Zephyr7 | | #11

        That is a “combined cycle” system, and it can work, but tends to be expensive to install. What is needed is an extra heat exchanger (you don’t want to run the engine cooling loop into your home directly, so you use a heat exchanger at the generator to isolate the loop that carries heat into the home), and you need an extra “water to air” heat exchanger in an air handler inside for heating. A system like this can get you up into the 80+% efficiency range in terms of extracting all the energy available in whichever fuel type you end up using. I have been involved in the design of some midsize (~500kw or so) combined cycle plants at work. Fun projects.

        Regarding extended outages, you need to have maintenance supplies on hand too. This means you need to keep materials on hand for oil changes. The usual oil change rule is “every 100 hours or every year, whichever comes first”. If you run 4 days you’re almost at that “100 hours” mark. You don’t have to do an oil change exactly at 100 hours, but I do recommend trying to do it every 7-10 days of continuous generator operation. I also recommend the use of full synthetic oil which holds up better in extended runs. If you’re in a remote area, and you expect long outages, I’d keep at least one oil filter on hand along with enough oil to do at least one oil change. Remember you need a good jug to hold the used oil too so that you can take it somewhere that can properly dispose of it. I like to use one gallon plastic orange juice jugs for this purpose.

        Note that LP is “liquified propane”. It is basically the same as gaseous propane, just in liquid form for increased density. Propane is a liquid in the tank. Natural gas is methane. Methane has a different molecular structure from propane. Regarding differences in extraction, propane is primarily sourced as a byproduct from oil wells. Natural gas is primarily sourced from dedicated natural gas wells. Many Natural gas fields have the interesting, and not yet fully understood, ability to “recharge” over time — meaning some exhausted natural gas fields in some areas gradually recharge with new natural gas over time.


        1. Callan _1 | | #14

          Bill, thank you for your information. I found it very interesting. I would love to believe that capturing propane (methane) would engender some super creative person to come up with a method of using the byproduct well so that as we phase out fossil fuels we use what we have of them to the best advantage.

          Thanks, too for the tip on the good orange jug!

          We do have a heat exchange planned into the house HVAC system but I don't know how it is tied into the generator. I will talk with my HVAC team and we will see what we can do to maximize the function then. The 3 x 3 central core that rises from the foundation to the ridge line houses these systems so it is easy to manage both for installation and repair (you can get into the system on each floor).

        2. Bas Sommer | | #16

          Would it make sense to buy a combined cycle unit from the beginning? I guess not as Backup but primary energy source..
          Edit - it would make sense if that genset would be a part of your regular coop grid so that it is a prime system . You would use the heat and the coop the power - in power loss situations it would be all yours

          1. Expert Member
            Zephyr7 | | #17

            I’m not sure there are any commercially made packaged units made for residential scale. Everything I’ve worked with commercially has been well over 100kw. Typical residential generators are 25kw and below.

            You can get “remote radiator” packages that are setup for extended cooling loops. Another option is “city water cooling”, which means there is a heat exchanger included. You would still need a radiator though. An easy setup would be to run the cooling loop first through your water to air heat exchanger in your home air handler, then through an outdoor radiator. This way everything will just “work”, with your thermostat just cycling the blower in the air handler on/off when heat is needed. If you run two stage (so that a gas burner kicks in if the generator alone isn’t providing enough heat), you’ll probably need a bypass valve on the generator loop so that you don’t heat up the generators cooling loop.

            There are various ways this can be set up. If you don’t want to engineer it yourself, I would suggest getting a creative hydronic heating contractor and having a meeting between them and a generator dealer. I know Kohler has worked with on some oddball projects before, so a local Kohler power dealer is probably a good start if you want to pursue this. From a hydronic heating contractors perspective, the generator is just another heat source, so it’s relatively easy for them to integrate the generator into a hydronic heating system — it’s just “different”, so you need someone willing to work through something they’ve probably never done before.


  8. Trevor Lambert | | #10

    Use of natural gas is a drop in the carbon bucket compared to your choice of SIP walls and roof. Those have an enormous carbon footprint that will never get paid back in energy savings for the life of the house. Plus that carbon cost is immediate, when it matters the most. Carbon saved 50-100 years down the road, or even just 25 years down the road does nothing to help the current crisis.

  9. Deleted | | #13


  10. Callan _1 | | #18

    Edited Answer

    Trevor, you are correct, typical sips made of XPS are a carbon nightmare. I cannot say that what we chose is carbon gentle, as I have yet to find much in the conventional construction world that is easy. We considered hay bales and double wall wood framing, among other options. The materials costs, transportation weight, and risks of material loss due to cold, rain or fire, led us to select the steel skinned polyurethane SIPS.

    I have gone back to the drawing board to try to find ways to reduce the embodied carbon across the entire build. Thank you for raising the point, again. It should never leave our minds.

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