I have a standby generator with an automatic transfer switch. It is set up to power all the circuits in my house. It does not have high enough wattage to power an EV charger in addition to the current load. If I installed a level 2 EV charger is there a way to ensure it will not draw power while the standby generator is running?

  • Unplug your car?
    – Jon Custer
    Commented Dec 22, 2022 at 15:06
  • 15
    It's hard to unplug your car when your asleep and you don't know the generator is starting due to a power loss.
    – Jared
    Commented Dec 22, 2022 at 15:08
  • How long does your car charge for on an average night? If you are asleep your power usage is pretty low. But, I get it, you want to use the transfer switch to shut off the charger circuit? (Maybe I’m a light sleeper, but I wake up if the power goes out even momentarily.)
    – Jon Custer
    Commented Dec 22, 2022 at 15:12
  • I don't currently have an EV, but I may be getting a new car in the near future so am determining if an EV with fast charging is a reasonable option.
    – Jared
    Commented Dec 22, 2022 at 15:29
  • 14
    @JonCuster I don't think OP is looking for a workaround. It is reasonable to ask how this can be done without manual intervention. It is also mandatory in many jurisdictions that an ATS-equipped generator be correctly sized, so relying on "unplug things in anticipation" or "don't worry you'll wake up" makes the installation non-compliant. Finally, by the time you wake up and go unplug the car, the generator's breakers will already have popped. And if you're not fast enough, or not there, that means your sump pumps and fridges go dead.
    – jay613
    Commented Dec 22, 2022 at 16:45

7 Answers 7


You could split your electric service into two panels: a main and a sub.

The main panel would hold all the devices you don't wish to run on generator:

  • EV charger
  • Electric water heater
  • Electric dryer
  • Electric stove
  • Outdoor A/C unit
  • Etc...

The subpanel would be everything else:

  • Regular outlets
  • Lights
  • Gas furnace
  • Well pump
  • Sump pump
  • Refrigerator
  • Etc...

Now put the automatic transfer switch in the subpanel.

  • Except that if you expect widespread, possibly long-lasting, outages, you want your EV (and your phone and your laptop) to be charged up 100%. Commented Dec 22, 2022 at 17:14
  • 4
    @manassehkatz-Moving2Codidact: I'd put outlets for phone and laptop charger on the protected side in the first place, or physically move them if necessary.For the car...You could use a second transfer switch to let the car charger move between the two sources, if the generator can handle it; otherwise have a lower-amperage/slower charging option from the secondary panel.
    – keshlam
    Commented Dec 22, 2022 at 18:00
  • 3
    @manassehkatz-Moving2Codidact 's comment leads me to realize: If you only have an EV you better have a natural gas generator because if you can't charge the car you can't go get gas for the generator :(
    – jay613
    Commented Dec 22, 2022 at 19:38
  • 3
    @jay613 True. But in my area at least, standby whole-house generators are all connected to utility natural gas. I've got a small (manual transfer interlock, inlet, etc.) tri-fuel - though I've only used it so far with gasoline. Hooking it up to natural gas would eat up a good chunk of what I saved by just getting a small generator. Commented Dec 22, 2022 at 20:01
  • Most modern portable gas generators either run or can be easily modified to run on propane. Power is slightly less than gasoline but the ease of safe storage (and shelf life) make it worth the tradeoff for me. Most EV owners probably also have a BBQ so there's your emergency charge source. :)
    – Chris O
    Commented Dec 24, 2022 at 4:23

Large enough to carry the load to be served

This is the requirement on generator size for ANY installation with an automatic transfer switch.

"The Load to be Served" is decided by a NEC Article 220 Load Calculation on the loads served.

You are specifically not allowed to go "ah, well, my loads rarely exceed 10kW so I'll just use a 10kW generator and not worry about an unlikely case. I can just turn off circuits if it's a problem". That is not allowed. (not least, you might not be there!) That would be allowed with a manual transfer switch, since you are there.

However, Load Shed loads do not count

There are technologies which attach to a load and listen for the generator starting to bog down, and will drop out the load when that happens. These are called "Load Shed" - they work the same as a power company doing rolling blackouts to relieve stress on generating capacity. Load Sheds are appropriate for - well, any potential load, but commonly water heaters, dryers, A/C units and certainly electric vehicle supply equipment.

So you can get your house's loads (inclusive of EVSE) within spec simply by attaching Load Shed devices to them.

Otherwise, consider a Critical Loads Subpanel

You have your electric meter, main breaker, and main panel. Then, in the main panel, you have a breaker that feeds a subpanel.

This subpanel has a transfer switch on it. All the loads you want to be accessible to generator are placed in the critical-loads subpanel.

There is no limit to the size of the critical loads subpanel. You can use a 40-space subpanel and put most of your loads in there if you want to. However, the more loads you install (less any on Load Shed), the larger your generator must be in order to use an ATS.

If you don't want to move almost every circuit from your main panel to a critical loads sub, you could add a new main panel elsewhere and convert this panel to the critical loads subpanel.

Note that EV charge rate is variable

EV novices tend to be swayed into the belief that all level 2 EV charging is 50A. This is because every car comes with a free TRAVEL charger" emphasis travel! that is to be kept in the trunk and used for opportunity charging on the road. Naturally they provide the two sockets most often found on the road - the normal one and the RV socket NEMA 14-50. People get their cars home and take one look at that socket, and go "OH! Level 2 must be 50A!" No. Not at all.

Actually level 2 charging can happen at any speed from 15A (70-90 miles in 10 hours) to 100A (500-600 miles in 10 hours, WTH, or more realistic, 200 miles in an MRAP). The car can work with any of these; thanks to the miracle of 1980s microcontroller tech, the EVSE (charge unit) tells the car the safe amp rate. On the travel units, this is a microchip inside the plug itself, which also reduces amp rate in case of a hot plug. Note that you're not married to just the two speeds; 240V/15A, 240V/20A, and 240V/30A plug dongles are available.

Or EVSEs can be hardwired. This removes the NEC 2020 requirement for a $150 GFCI breaker.

Certain hardwired EVSEs can adjust charging amps dynamically and this can act as a Load Shed, keeping system power draw within limits by turning down the EV charge rate.

To talk more about that last option, the EVSE places a Current Transformer inside your main panel that clamps the utility supply wire(s). You then tell it "don't let service draw exceed 80A inclusive of your load". So if the water heater kicks on and suddenly it's 90A, the EVSE will cut the car from 24 to 14 amps. Now, if you want to do that on generator also, and that needs to happen at a lower ampacity from utility, you can cheat a bit. CT's operate on ampere-turns. Most wires go once through the CT and counts as accurate amps. But if you loop a wire through a CT twice, it will "see" twice as much current. So if you have 100A service and 25A generator, route the generator wire through the CT 4 times (3 loops) so the EVSE will read the generator current as 4x what it actually is.

  • I assume something like the following would work if I had an electrician install a 220 outlet for a charger then? pspproducts.com/load-management/sak-60-wireless-load-management
    – Jared
    Commented Dec 22, 2022 at 19:56
  • @Jared yeah, the load shed market is pretty big. Must be UL listed and installed according to instructions. Commented Dec 22, 2022 at 20:12
  • 2
    @Jared -- the Generac loadshed boxes are your best bet, actually (they work off mains frequency, so they don't care what brand/model generator you have) Commented Dec 23, 2022 at 3:48

You'll have to re route your EV charger feed from the main panel to a small sub panel/ fused disconnect fed from the utility side of the transfer switch. You could also add a normally closed contactor / relay on the existing feed that would open when the generator runs.

  • So, if I understand correctly, I would have an electrician install a subpanel. The subpanel would be wired directly to the utility side of the transfer switch while the main panel would keep its existing wiring? I have no intention of doing this myself but I want to be somewhat well informed when talking to an electrician.
    – Jared
    Commented Dec 22, 2022 at 16:36
  • Basically. The exact specifics will depend a bit on how your panel is set up (where/how the transfer is done, what type of feed, what type of interlock, etc.) and also on whether you have a meter main before your "main" panel. Pictures of whatever you've got would help. Commented Dec 22, 2022 at 16:39
  • 3
    The electrician would either install a new sub-panel, and most of the loads that are currently on the main panel would be moved to it, or the electrician would install a new main panel, and its only clients would be the EV charger, the existing (now sub) panel, and maybe one or two other things you move over like the clothes dryer. Which approach would be easier depends on the existing layout. Either way, the resultant subpanel containing most of your home loads would be the one fed by the generator.
    – jay613
    Commented Dec 22, 2022 at 16:49

Some great suggestions here to get the load off the generator by using a subpanel or even a contactor.

Another option: replace the EVSE breaker with a shunt trip circuit breaker. Connect the shunt coil to a 120v output from the generator. When the generator starts up and starts outputting voltage, it will trip the EVSE breaker and stop the charging. If they are available for your panel it could potentially save a bunch of rewiring work. One downside, if you go on generator and it trips charging won't start again until you reset the breaker. This isn't a big deal unless you both regularly have power outages and also drive many miles on a daily basis.

  • 3
    Actually they are available for most panels. And if they're not, in 1" panels you can use an Eaton CL, which aren't made in Shunt Trip but are made in Remote Control Breaker e.g. CLR230. The CLR line will behave like a Shunt Trip breaker if you connect the shunt wires to the "OFF" solenoid. (It is a plain breaker in series with a GE RR7 style latching relay, which can be hand-operated by a second handle.) Commented Dec 23, 2022 at 5:20
  • @Harper-ReinstateMonica nice. I was not seeing many options at Platt but I honestly didn't look very hard as shunt trip breakers are something I know about but have never used.
    – KMJ
    Commented Dec 23, 2022 at 6:12
  • Edited to include that they are more available than I thought.
    – KMJ
    Commented Dec 23, 2022 at 6:13
  • Yeah, directly available in CH, QO and BR. For all the rest, Eaton CL if the manufacturer doesn't have one (HOM won't). Of course if you're really backed into a corner, you could always have a relay pick up and connect a 20k ohm resistor between hot and ground. That would trip a GFCI. Though that would involve putting an EVSE on a GFCI, which is redundant. Commented Dec 23, 2022 at 23:01
  • Part of the reason I suggested this is everything is listed and being used as intended. I wanted to avoid home-brew solutions even though some are quite sensible and likely what I would do (such as the NC contactor suggestion).
    – KMJ
    Commented Dec 23, 2022 at 23:08

Does your charger have a remote shutdown or remote enable input? If so, that could be as simple as wiring a relay to an outlet wired from the generator side on the transfer switch.

Similarly, you could get a suitably sized normally closed contactor to turn power to the charger off. If you can get a contactor with 120/240 v coil inputs, that would be easiest from a wiring perspective. Just connect the generator output to the coil inputs. Contactors that take 24 v AC on the coil input are more common. In that case you would get a door bell transformer and connect that to the generator outputs, and the 24 v output of the transformer to the coil input of the contactor.

If you go with the contactor, make sure you get a normally closed contactor. That's less common, so make double sure you're getting the right one.


Install a Generac smart management module it will shed the EV load when running on back up power. It’s fully automatic and simple to install.

  • 1
    Yeah, the Generac SMMs are one of the things they managed to get dead-on right -- they base their shedding decisions on line frequency, which means they are self-contained and work with any generator under the sun. That said, this is only really second-best to having a critical loads subpanel Commented Jun 9 at 21:51

Get rid of the generator you have a car battery backing up your house. Take the gas car to work on days without power. The AC to DC transformer MIGHT run both ways. Tesla does if I remember right, most others probably don't.

  • 1
    GREAT point... an EV is an enormous battery that will provide days if not weeks of emergency critical-loads power to a house. Provided it has a 120V or 120/240V inverter output. Commented Dec 23, 2022 at 7:38
  • 2
    No, the car can't reliably provide backup power to the house. Unless you're willing to leave it home all the time, in which case there are better options that don't waste your money on wheels, seats and the rest. Commented Dec 23, 2022 at 15:30
  • 1
    Tesla specifically voids their warranty if you use the car as a stationary power source. Some other manufacturers are OK with it such as Ford.
    – KMJ
    Commented Dec 23, 2022 at 23:10

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