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Community and Q&A

Provisions for Emergency Power

_Stephen_ | Posted in Mechanicals on

Hi guys,

I’d like in the future to be able to backup certain essential circuits in my home, so that things like hot water, the fireplace, some lights, and perhaps the wifi can stay up during a power outage.

I think my two options are either a battery backup attached to the solar array, or a stand alone backup generator running off of natural gas.

I’m not going to do this now, because we weren’t originally planning on the solar array, and that cut about $30k in the budget, but I’m wondering which route I should plan on going, and if there is anything I can have my builder do now, to save me some money later, when I do implement this.



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  1. GBA Editor
    Martin Holladay | | #1

    The short answer is that you need a gasoline-powered generator or a natural-gas powered generator rated at 5,000 to 8,000 watts. Expect to pay $2,000 to $6,000, depending on the size and the bells and whistles. Talk to your electrician.

    GBA recently published a long Q&A thread on the issue, with at least 24 comments. Here is the link: How do I figure out how much solar energy for emergency power?

  2. _Stephen_ | | #2

    So, there's no advantage to setting up anything during the build in preparation?

    I was wondering about installing an automatic transfer subpanel now, that can't transfer to anything, as there isn't a generator there yet?

    Or is adding something like that later no big deal?

  3. GBA Editor
    Martin Holladay | | #3

    When I advised you (in Comment #1) to "talk to your electrician," the idea is that your electrician will advise you on where a transfer switch might be located, the possibility of setting up some dedicated circuits for emergency use, and possible locations for a small concrete pad for your future generator.

  4. _Stephen_ | | #4

    Ah, OK.

    Thanks Martin.

  5. Expert Member
    Michael Maines | | #5

    Stephen, if your electrical meter, panel and future generator are all located near each other, there is probably very little penalty for putting off planning for the future. On a current project of mine, those three components are all apart from each other, so we ran conduit between them and will set up a manual transfer switch for now, which could be swapped out for an automatic transfer switch in the future.

    I have worked on large homes that needed a 14kW standby generator, and I've seen larger--a mansion my brother helped build had a generator the size of a bulldozer engine, no exaggeration--but for more conventional homes Martin's sizing matches my experience.

  6. _Stephen_ | | #6

    @Michael, Thanks for your answer.

    The generator will be in the backyard, but the meter, and panel are quite close to each other in the front corner of the house.

    I think Martin's sizing is spot on. 8000 watts would be big enough to run the AC, though that would be rather expensive cooling.

  7. Expert Member
    Michael Maines | | #7

    In my experience electricians will not wire just a few circuits, they will only do the whole house, though in the past and in other locations I have heard that it's common to not wire all of the circuits to the generator. Personally I have a 7500W emergency generator and extension cords for the fridge, freezer, coffee maker and computer/router--you know, the essentials.

  8. _Stephen_ | | #8

    I did the extension cord thing in the past. However, I can't put my water heater on an extension cord. Or the fireplace. Or the air handler. Or the heat pump. These are all hardwired.

    The thing I was thinking of having them installed is something like this:


  9. jberks | | #9


    I'm going through this right now with my build.

    If you want to do Solar PV panels, Tesla is launching some nice looking battery systems for backup power. I personally am not a fan of the price of PV panels just yet and don't feel its worth the cost. so I'm opting not to install them.

    I plan to add a NG backup generator on my roof.

    The big question to you right now is: is the electrical service, meter and panel that you want for your build already in? If yes, do you have a main switch in before your panel? if not, there then there isn't really much to do to future plan other than running/hiding conduits to you backyard from the panel. Putting in a transfer switch now would require the cost of disconnecting from the city grid to put it in.

    In my current build, I upgraded the electrical stack and panel to 200A. so when I called for my service disconnect, I took advantage of the situation and had my electrician install a main switch and transfer switch before the new panel for me. Now I'll run two wire conduits and a NG line to my roof through the walls while they're open prior to drywall. Then in the future when I'm ready to install the generator, I'll just drop it on my roof and I'm not tearing anything up.

    Some things to note. first, I didn't have to do a transfer switch now, I could have left it at the main switch and added the transfer witch after, but doing it now makes for a cleaner/easier install. Second, I opted for a KW rating high enough to power the entire house, becasue it ends up a wash in cost and requires less equipment/cleaner install. The extra $1500 for a higher capacity generator is better value than buying a lower capacity generator and paying a similar cost for the extra electrical labour to wire in a sub panel to transfer to specific circuits.

  10. _Stephen_ | | #10

    @Jamie, thanks for your answer.

    The problem with a whole house switch when combined with solar panels is that you can't put power to the grid-tie inverter, or it'll try and dump a full 10 kW of power into my little islanded backup power system, and wreck the place, so that would have to be a little more carefully thought out then I can do on the back of a napkin.

    As for solar panel value, I admit, it matters where you live. I live in an area with relatively high hydro rates that are expected to climb significantly over the next few years, so solar has about a 10% return on investment.

    So far, the house hasn't even broken ground yet, so I need to know if it makes sense for them to do something before that happens, so I can discuss it with my builder.

  11. GBA Editor
    Martin Holladay | | #11

    You are worried that your PV system "will try and dump a full 10 kW of power into my little islanded backup power system."

    That happens all the time, and doesn't cause problems. Your islanded backup power system will have an inverter and battery. The inverter will provide 120 VAC to your refrigerator and lights. Excess power will flow through the charge controller to your battery. As your battery fills up, charging will taper off. All 10 kW of PV output may or may not be used, but nothing gets hot or explodes.

  12. _Stephen_ | | #12


    My PV system is (will be??) a grid-tie only system. There is no battery bank, and there is no charger, unless I spend an additional $10k to upgrade it to a StorEdge style system.

    That $10k buys a lot of generator, and automatic transfer switch, and the generator can provide a lot more power over an extended power outage, however, I think in the design of such a system, you would have to make sure that the generator didn't back feed either the grid, or light up the grid-tie inverter. I believe once the grid tie inverter sensed phase, it'll try and dump any available power on the "grid", and hilarity and expense would ensue.

  13. jberks | | #13

    Interesting. You've gotten into electrical engineering territory.

    I wonder if you had your Generator transfer switch downstream off the PV transfer switch. So essentially the Gas generator would only kick in when there is both no grid or solar power.

    Best find a electrical consultant. I'm curious to know what solution you come up with.

  14. Expert Member
    MALCOLM TAYLOR | | #14

    Stephen G,

    When a nearby lighthouse was decommissioned a local preservation group took control of it and the surrounding park. They installed a grid-tied PV system. I noticed that they also installed four, six foot baseboard heaters on a wall. When I asked why they said it was to dump excess power when the grid was down.

  15. _Stephen_ | | #15

    @Jamie, the house hasn't been built yet. This remains a theoretical problem for the time being.

  16. GBA Editor
    Martin Holladay | | #16

    Wind turbines need to dump excess power. PV arrays don't. If you don't use the power, the PVs are happy. As I said, nothing gets hot or explodes.

  17. Expert Member
    MALCOLM TAYLOR | | #17


    I know next to nothing about PV and was surprised to find the heaters mounted in the lighthouse where they would provide no useful heat. I'll email the engineer who installed them and ask what function they perform.

  18. Jon_R | | #18

    Solar panels run slightly hotter when you don't use their output. But as Martin says, this causes no problems.

    There is some argument for adding load when running lightly loaded diesel generators (wet stacking).

  19. Expert Member
    MALCOLM TAYLOR | | #19


    The engineer, Steve Unger of Viridian Energy, kindly replied:

    "The baseboard heaters at the lighthouse are a safety mechanism only for when the grid is out, the solar panels are producing more energy than the lighthouse is using and the battery is full. Primarily the inverters will shut-down and stop the flow of surplus energy. However, as a redundancy or secondary diversion load, there is the baseboard heaters. The likelihood is that these baseboards will seldom if ever energize."

    I guess it is a cheap insurance policy.

  20. GBA Editor
    Martin Holladay | | #20

    Steve Unger is an engineer, and I am not. But as far as I know, this diversion load is unnecessary.

    During the summer, the condition he describes -- batteries full, and the sun shining brightly on the PV array -- happens frequently at my house. I don't have a diversion load. Some of my PV modules are now 38 years old, and they are all still working fine.

  21. Expert Member
    MALCOLM TAYLOR | | #21


    This discussion has has only a theoretic interest for me. I can't remember the last time I saw the sun.

    I suspect Steve installs the back-up in much the same spirit I leave my co-corker Luke behind me with a large stick when I work on a live electrical panel: It can't hurt.

  22. user-4053553 | | #22

    @Martin Perhaps they were better made 38 years ago :)
    Just kidding, its probably a designer/engineer who didn't know you don't need to disperse unused output

  23. Jon_R | | #23

    Strange - in such a case, the inverter doesn't shut down, it simply takes less wattage from the panels (ie, throttles back).

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