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Community and Q&A

Backup Generator versus Battery Powerwall

bauchin | Posted in General Questions on

We are building a 2300 SF “pretty good” house in Climate Zone 6 in northern Vermont using all-electric systems (heat pumps and in-wall electric heaters). Considering two alternates for backup power: 1) exterior pad-mounted propane backup generator 2) Tesla (or other) battery power wall units in conditioned basement. We know that the propane generator takes us away from our “no fossil fuel goal” but it is necessary to have backup power in VT.
Does anyone have a breakdown/cost comparison of these two options that they can share with us? Any/all feedback is welcome
Thanks in advance

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  1. gusfhb | | #1

    DO you have solar?
    Solar with battery storage seems like the primo solution
    Without solar, your battery would support the most common power outages, but not the ones that matter.
    The ones you care about are the 24 hour + ones and an unsupported powerwall is not going to do that in the winter.

  2. Expert Member
    Michael Maines | | #2

    We did a BS+Beer Show on the topic: The short version is that if you live in a place with power that never goes out for more than a day or two, you could get by with one or two battery systems if you can afford them. If you want to be prepared to lose power for longer periods, or if your budget is tight, a standby generator makes more sense. Working toward an all-electric home is a noble goal but one way we can best use the amazing power of fossil fuels is for backup power that is rarely needed. If you really want to save money, you can use a portable generator plugged into a transfer switch.

  3. Expert Member
    Akos | | #3

    I think even a PV supported battery will make it through a long power outage in a snow storm. I have a 10kW ground mounted array up north by the cottage and it is not uncommon to have less then 1kW of production in a day during a storm, couple of days of that and your battery will be flat even if not running house heat.

    If you are going to have long power outages, house backup heat is a must, so you'll have propane on site already. Hooking a generator to that makes the most sense. I would not go overboard with a big pad mounted unit. These are pretty loud and when run at typical house loads are very inefficient. A better option is a decent propane powered inverter generator. These are much cheaper, quieter and use way less fuel running a typical house.

    I would spend some time doing the layout of your electrical and split the backed up loads onto a sub panel. This way it is much easier to install an MTS/ATS or add battery backup down the road.

    1. maine_tyler | | #7

      "split the backed up loads onto a sub panel. This way it is much easier to install an MTS/ATS"

      Do you say this mostly for the automated version?

      I inherited a subpanel that's plumbed into a plug for the portable generator and I'm endlessly frustrated by what the previous owners deiced to not put on that sub panel. They left off lots of general lighting, which just seems dumb given how low load it is anyways. Obviously one can better plan what's on that subpanel, but in any case it lacks flexibility. If doing a manual transfer with a portable plug in, what's the advantage to the dedicated sub panel?

  4. walta100 | | #4

    If you must to run your heat pump to stay warm thru a week-long power outage your Battey will be huge and unaffordable.

    You might make a battery work with a wood stove or another heat source.

    As with all batteries the chemical reaction that makes them work will have a finite life span maybe 10 years. I also see a battery bank as a huge fire hazard simply plopping one in the corner of a basement seems like a bad idea. I doubt the nasty stuff in the batteries is as totally sealed as they would have you believe.

    If you are set on building a battery room maybe Bill will give you a few pointers.

    Assuming you are going to own a tractor I always thought a PTO driven generator would be a better option than a standalone generator.


  5. amorley | | #5

    All the feedback above is really good advice. Two things to add as considerations:

    Will your utility subsidize your battery system? Friends of mine are finishing building a house in VT right now and their utility is Green Mountain Power. They got a great deal on battery backup power in exchange for enrolling their battery in a demand response program.

    Do you have (or are you planning on getting) an EV with V2H capabilities? Having anything from a Nissan Leaf with 60kWhr battery to the new Chevy Silverado EV with its ridiculously oversized 200kWhr battery might change your calculus. Coupled with a pair of 13kWhr stationary batteries and a PV array, you might be able to ride out the worst case outage scenarios no problem. If your outage is very localized, you could even drive to where there is power, charge up your EV for an hour, and drive home with a pretty full battery again.

    But propane for backup heat and power is a pretty straightforward option too.

  6. Expert Member
    DCcontrarian | | #6

    If you decide to have a propane tank, it's a lot more efficient to burn that propane directly for heat than to use it to run a generator to power a heat pump. Like 4-5 times more efficient.

    1. Expert Member
      Michael Maines | | #9

      But you also need to run lights, fridge, freezer, well pump...

      1. Expert Member
        DCcontrarian | | #10

        Right, you need a generator for those things. But splitting off the heat means a much smaller generator.

    2. maine_tyler | | #12

      "Like 4-5 times more efficient."

      What generator efficiency and what heat pump COP are you assuming to get 4-5 times?

      1. Expert Member
        DCcontrarian | | #15

        I'm assuming the generator is around 20 % efficient, and that in the depths of winter in northern Vermont the heat pump COP is going to hover close to one.

  7. gusfhb | | #8

    For reference, I run a portable gasoline powered generator and we have lost power several times a year for the last 13 years, not counting nuisance outages.
    I started with a 3.5kw when 2 weeks after we moved in we lost power for 2 days. cheaper than the food in the fridge.
    Lasted about 9 years, not big enough to start my well pump
    It died suddenly during a statewide outage and I bought a used 5kw which would start my well pump, but only lasted a couple years.
    currently have a 8kw which will start my well pump while doing everything else
    All of them were wired to the whole house, but no using heat pumps or the oven. Lighting internet and furnace are not that big
    You notice I mentioned the well pump. You can go quite a while without power, but not without water. With all late model fixtures and a big pressure tank, we can go most of the day, but no laundry.
    So if you have a well, make sure you size it for that.
    If you end up with solar/battery, I would think a small generator would be enough to refresh the batteries and cheaper than built ins. OF course that means you will need it every 4 years or so, and will it start.....
    Almost all my neighbors have propane built ins, you hear them exercise once a week.
    REminds me time to go test my generator.....nothing is free

  8. Expert Member
    BILL WICHERS | | #11

    Battery systems are not practical for long-duration backup power, period. The energy density is just not there. I would not put in an entire off-grid sized solar system just to act as backup power, either, since you're putting in a LOT of additional cost for a system that won't often be used, and you're adding in all the risk and maintenance involved with a battery system.

    A conventional generator is a much better way to go for backup power over extended periods of time (more than a few hours). I assume you're using the heat pumps for most of your heating, which will make more efficient use of your generator power compared to using electric resistance heat. You're still probably better off with a propane fired heater overall, but if that means installing another furnace and propane piping, then the generator is likely to be easier to install, even if it's not the absolute most efficient option overall in terms of fuel use.

    Put in a generator that can run what you need, then put in a load management system to shed things like electric resistance wall heaters at times of heavy load. Load management will let you get by with a smaller generator, and smaller generators operating at higher average percentage of their rated capacity are more fuel efficient compared to larger generators running at lower average loads as a percentage of rated capacity. BE SURE your propane tank is sized large enough to "boil off" enough propane vapor at your lowest expected outdoor temperatures to be able to run your generator (your propane supplier can probably help you to determine that). If your tank is too small to provide enough propane vapor to run the generator, you starve the generator for fuel, which is bad for the engine and will also cause you issues when running on backup power.

    I do prefer an automatic transfer switch that runs the entire home to avoid "missing" something you want to run during an outage. Current code usually requires you to put in a way too big generator with such a system though UNLESS you also install a load management system like I mentioned earlier. Things to connect as loads that can be dropped are electric water heaters, electric resistance heat, things like that. You MIGHT be able to put your well pump on that too, since the loads are restored when capacity is available, so they don't stay locked out the entire time you're on generator. You do need to plan such a system properly to get things to work right though.

    BTW, I would not recommend installing batteries in your basement. Put them in a fire rated room in a garage or otherwise outside the home for extra safety. While the chance of a catastrophic failure of a battery system is fairly low, if anything does go wrong you have a VERY BIG problem VERY fast, so it's worth it to plan the installation to be as safe as possible, which means keeping the fire under control as long as possible to give you time to get out. In a basement, it's much more difficult to keep things contained if something goes wrong.


    1. Patrick_OSullivan | | #23

      > BTW, I would not recommend installing batteries in your basement. Put them in a fire rated room in a garage or otherwise outside the home for extra safety.

      No matter where they are installed, LiFePO4 batteries seem to be much safer than traditional Lithium Ion. My limited research indicates they are much less capable of going into an out of control exothermic reaction in the event of damage.

      1. Expert Member
        BILL WICHERS | | #25

        LiFePO4 batteries *are* safer. But ALL batteries carry some level of risk. This is similar to those with oil fired furnaces and an oil tank in their basement. The risk of a problem is low, but it's not zero.


  9. bauchin | | #13

    Thanks all for the wonderful and well considered feedback! After reading everyone's comments, I feel that the best solution for us is to pursue the backup propane generator with automatic transfer switch and load management system that prioritizes the heat pumps and refrigeration/freezer (not the resistance wall heaters or hot water heater). Thanks again for all the great feedback! Blake

    1. Expert Member
      MALCOLM TAYLOR | | #14


      That's the solution almost everyone around here has settled on.

  10. boxfactory | | #16

    We are approaching the beginning of building our house, also in Vermont. Our goal is to install solar panels on the roof, and the orientation of the building has been determined by the path of the sun.

    We will not be installing a battery. Instead we hope to install the latest electric car chargers, that allow the cars battery to power the home. When more power is needed, the goal is that someone could drive to a charging station, and pick up some electricity. Perhaps groceries as well.

    Is this a perfect solution that will cover every “what if” situation? No.

    Do we own an electric car at this time? Also no.

    I will say that I feel a whole lot better about not spending a bunch of cash on a battery, that will take up a fair amount of space in our (relatively) small build.

    Just my 2c

    1. Expert Member
      DCcontrarian | | #17

      Here's what I have to wonder about: just about every car built in the past 80 years or so has a generator on-board, in addition to a large gasoline engine. But you never hear people talking about using their cars as backup power for their houses. I really don't see why an electric car is fundamentally better-suited to this task than a gas-powered car.

      1. Expert Member
        BILL WICHERS | | #18

        It's not better suited, not really. The "go get some electricity" isn't necassarily a good option, since major outages tend to be wide spread, so where are you going to go get more electricity? The charging stations nearby are likely to be out of power too. You don't want to use too much of the power reserves from your car to run your house, because then you might get stranded with no way to go anywhere for anything. On top of all of that, it takes a mighty big battery to run a house for any appreciable amount of time, especially if you're running heat in the winter -- which, coincidently, is also the time when the battery will be least efficient, since they don't have as much available energy storage capacity when they're really cold.

        The "my car is my backup power source" is really not a good option for many reasons. It's much better to have a conventional generator available. Batteries are only really good for short term (hours at most) backup power. When you start getting into multi-day outages, batteries are no longer a practical option.


      2. gusfhb | | #20

        The largest alternator in a car is something like 200 amps for a police cruiser or maybe RV. ~2400 watts, at 13.8 volts. More normal is half that about 100 amps ~1200 watts, and that is max output, not available at idle.
        A big 12 volt inverter, you might be able to run a few critical systems for a while, but it is nothing like the ability of an EV to pound out current.

      3. Matt_T | | #21


        The alternator in a regular car isn't big enough to be much use as a generator. Only good for about 1kW at idle. I did it for a week, few years back after a bad storm, to run lights and refrigeration. Better than nothing but a poor substitute for a generator.

        A BEV is better in that it can support much higher loads. But that is time limited and "running out for more electricity" might not work too well. A multi day outage will effect lots of people so the closest public chargers that have power are likely to be very busy.

        A hybrid would be the ideal for backup power because it combines high output with the fuel advantage of gasoline.

  11. boxfactory | | #19


    You make some excellent points. Thank you for your response.

    It is my understanding that when one uses battery backup, one does not heat their home when a power outage is taking place.

    That being said, the CERV ERV is being considered due in part to its ability to act as a homes heat source in the shoulder seasons. I haven’t gotten around to looking at any numbers, but it occurs that as it makes a small amount of heat, the CERV likely draws less power than a minisplit. Perhaps its use could delay the time before a house became uncomfortable in a wintertime outage.

    As it happens, I lived in one of the dozen or so communities in VT that were isolated after their roads were washed away during tropical storm Irene. I’m mildly embarrassed to have not remembered this when thinking about driving to recharge an electric vehicle.

    Thanks again,

    1. Expert Member
      DCcontrarian | | #24

      I assume by CERV you mean the CERV2 by Build Equinox? That's the only one I'm familiar with. The CERV2 can either heat or cool, and it can vent or recirculate. It has four vents -- two that bring inside air in and out, two that bring outside air in and out.

      In vent-heat mode it draws in outside air, warms it and sends it into the house, then draws air out of the house and uses it to warm the heat pump and discharges the cooled air outside. This is the mode where you see the really high COP numbers, because the air being warmed is cold and the air warming the heat pump is warm, and COP is determined by temperature delta. But this is a little misleading, because it's serving the energy-recovery purpose of an ERV, which is where the high COP is coming from.

      In recirculate-heat mode inside air is warmed by the heat pump and recirculated, and outside air is brought in and exhausted to warm the heat pump. This is essentially the same as any other heat pump and has similar COP.

      The cooling modes are the converse of the heating modes.

    2. Expert Member
      BILL WICHERS | | #26

      >"It is my understanding that when one uses battery backup, one does not heat their home when a power outage is taking place."

      The issue you have then, is when that wintertime ice storm takes out your power for a few days, does your home freeze? Do you have to deal with water damage?

      If you're in an area that is prone to power outages and also has harsh winter weather, it is pretty much a requirement that your backup power system be able to run your heat. I've had multiday wintertime outages here (typically after ice storms), and the two main things I care about keeping running are the furnaces for heat and the well pump for water. Without heat, you're going to be in rough shape after 12-24 hours or so.


  12. walta100 | | #22

    Your post is the first to bring CERV to my attention. I like the idea of a heat pump ERV.

    I doubt you could count on keeping you pipes from freeing by add 5,000 BTUs to the incoming cold air in an effort to replace the 36,000 BTUs of the heat pump.

    I assume the CERV can and will need to defrost itself. Is their any data on how many minutes of run time one is likely to get between defrost cycles?


    1. boxfactory | | #27

      I agree that the CERV does not produce enough heat by itself in the winter. Our hope is to build a relatively small Pretty Good House, and with a little supplemental power from two electric vehicles (that we are likely to purchase at some point), to coast through outages more or less unscathed.

      We will have a far better idea of what we are doing after an upcoming meeting with the electrician.

      We will have to talk with our new neighbors, about frequently / duration of outages, and whether they have generators. Based upon our location, I believe outages will be short. The folks who work on the power lines in VT are truly amazing at what they do.

      Thanks again for the discussion

  13. AydanL | | #28

    If you live in a place with frequent power outages, I don't think it's safe to have only one source of power, after all, generators do break down, using a generator+solar does ensure that the equipment will function properly in the event of a power outage. It's worth mentioning that if you need a lot of power to support your home for hours or even days, which can only be done by paralleling a lot of Teslas, which is expensive and takes up a lot of space, you can also use a more flexible rack mounted battery similar to instead. In the end, I hope you find a satisfactory solution.

    1. Expert Member
      BILL WICHERS | | #29

      Spamity spam spam spam.

      Generators have the advantage over solar/battery systems for long term backup power, and batteries are not without their own problems.


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