There seem to be devices to let you charge an EV on houses that wouldn't otherwise have the service capacity (Electric Vehicle Energy Management Systems). Some of them slow down EV charging, but it seems like others do a hard power cut to the EV, and that doesn't seem right.

But on the other hand, would the "hard power cut" option be a way to support simpler appliances that are not EVs?

  • With regard to chosing an EVEMS you might want to check out the patent infringment issues that have been coming up with regards to BlackBox EVEMS new patent that was issued in Jan 2024 with priority back to 2020. It looks like a lot of the devices discusssed in this chain will be infringing on that patent and may either disappear or have to licence the patented technology from them. Something to think about before you make a purchase as no one wants to buy a device from a company that disappears in 6 months.
    – Tan
    Mar 24 at 21:56
  • @Tan Can you link? FYI "BlackBox" is one of the dumb load shed devices. It does not talk to the car. I could see where some onboard smarts might benefit it, but such intelligence is irrelevant to the proper, dynamically-adjusting, talking-to-the-car EVEMS which is the bulk of the discussion here. That's safe from patent issues because it was invented by SAE in at least the J1772-2001 standard, with the capacity signal designed to adjust on the fly (for dynamic EVEMS!) and the rest is "obvious to any practitioner in the field". Mar 26 at 19:47

1 Answer 1


Yes, there is an exciting new collection of products designed to help people mitigate the need for a service upgrade and support a collection of loads their house otherwise could not. And they're not just for EVs, though EVs are making them popular. Some of them are pretty lousy for EVs, but great for simple appliances like hot tub, water heater, dryer and many other interruptible loads. And now, we have access to that toolkit!

But let's discuss it by way of EVs and we'll start with a look at the masterful design of EV charging, and why blunt methods are not necessary. If you really want a hot tub, skip to the end :)

The EV's ingenious secret

SAE (Society of Automotive Engineers) is an association of automakers, not electricians, and this is their fourth major attempt at an EV charging standard. As such - hear me on this - SAE is in this to sell cars, not service upgrades. They are well aware a service upgrade is a deal killer for auto sales, so it was a hot agenda item to make it easy to avoid them.

That "charger thing" is not a charger at all. It's an EVSE and it has 3 jobs: a) GFCI b) remote control switch c) Tell the car how much current can be safely taken right now. And that's it!

The car is simply hard connected to 120/240V AC, and onboard the car is a massive water-cooled battery charger about the size of a suitcase. The car's onboard real charger obeys the current signal from the EVSE.

That's new. This is not your father's battery charger! So let that one sink in.

Maybe watch that video I linked.

That signal is a 1000 Hz square wave on the CP signal pin. The duty cycle (in percent) of the square wave indicates allowed amps; generally 0.6A per percent, e.g. 53% is 32A. This is a simple signal that could be made "by a middle schooler with an Arduino".

But the design was fully approved by UL and CSA; they were "in the loop" and expected to be certifying smart EVSE's that use this signal.

On simpler wall-unit EVSEs, this amp signal is set via DIP switches (or sometimes a special WiFi network). On portable "travel unit" EVSEs, the square wave is generated by a microchip built into the dongle plug - a 20A plug signals 16A (20A x 80%), a 30A plug signals 24A, you get the idea.

The car responds in real time to this signal. And as you might guess, you can do some pretty slick things with it. For instance, several EVSEs bridged together could coordinate so they don't sum to more than X amps, but if 1 car is alone, it gets the full amount. Now we get into the various EVEMS systems.

The house has a secret, too

The secret is that even though you have a 60A, 100A or 200A service, that's there for unusual peaks. The vast majority of the time, your house uses next to nothing. EPA's data says the average house draws 29 kWH/day or 1200 watts or 5 amps. So if our peaks are over 100 amps, our valleys must be much lower than 5 amps for that to average out to 5 amps. And so it is - particularly all night. Your house has great gobs of extra capacity - the key is knowing WHEN.

Technology Connections covers this "when" problem nicely at 7:24 in their "home electrification part 1" video.

An actual EV Energy Management System

Mind you, this was all coordinated in advance among Tesla, SAE, and the standards organizations (UL, CSA, etc.)

The simplicity of the square-wave EVSE protocol makes it really easy to design energy management for EVs. Again, "a determined middle-schooler" could get an Arduino, and attach Current Transformer sensors to the AC mains wires (commonly seen on home energy montors such as the Sense or Curb)... and the CT clamps tell you the load the house is drawing from the grid right now. You know the service size, 100A, and the panel is rated for continuous 80% of that, so 80A.

So now you simply do a couple subtracts and set the square wave signal accordingly, and that's it. The car itself does the 'heavy lifting'. For instance if the house (other than EV) is drawing 63A right now, and has "100A service" 80A max, compute 80-63 = 17 amps. Send the "17A" square wave to the car and it immediately adjusts charge to draw 17 amps.

See how easy this is? Like I say, this is designed to be Code legal in North America - so if the equipment is UL/CSA/ETL listed per NEC 110.2, and installed according to instructions per NEC 110.3(B), the inspector and insurer cannot object.

Readily available COTS kit right now, just off the top of my head, are the Wallbox Pulsar Plus with a particular CT monitoring unit they use... and in Europe the Myenergi Zappi with proprietary wireless CT transformer. All this will be hard-wired "wall unit" types; having it "plug in" would be problematic. Someday they may have a way plug-in appliances can do energy management via power-line signaling; give that at least a decade.

Emporia, maker of an EVSE and a home power monitor, never safety-rated those products to be used in this safety role, but were able to retrofit the ability with a firmware update - so they say.

Or a "bang-bang" system that interrupts the signal

A simpler, dumber system could simply have a CT on the service wires as above, and when the EV load would create an overload, simply signal the EVSE to stop altogether. The EVSE then removes the square wave signal from the CP pin, and the EV stops charging.

Several EVSE's have ports which allows this, and the ClipperCreek family comes to mind, as they have ports nominally for Power Sharing but handy for this application as well.

Or it could be hacked into an EVSE not made for it, by simply adding a relay which interrupts the CP wire on the charging cord. The moment the EV sees the CP signal disappear, it will stop charging.

The EVSE will notice this change, and will be able to use its smart or networked features to let you know "oh hey, charging stopped".

The bluntest (and costliest) instrument: a Load Shed

Ridiculous for EVs, but great for hot tubs.

In this case, we're doing the above simple "bang-bang" logic, but instead of talking to the EVSE, use a rather large mechanical contactor to go "CLACK" and do a hard power cut to the EVSE. Compared to the above solutions, this is brutish and rather unnecessary. Since it's not doing an orderly shutdown of EV charging, I have no confidence in the long-term effect on the EV's onboard charger. I'm not a fan, though.

These dumb "Load Shed" devices are costly, because they are handling massive currents, with large terminals, large contactor and large wiring in/out of them. They are also interrupting a large inductive load, so it'll be hard on the contactor contacts and require an extra burly unit. It's also going to go "CLACK!" a lot, which may be annoying.

I'm very sorry that the manufuacters of these dumb Load Shed devices have taken to marketing them as EV Energy Management Systems", when this is misleading compared to the actual EVEMS kit I mentioned above.

It's useful to know about their existence, though, in case you ever get a hot tub or other storage load like a water heater. I once proposed these to be used between a tankless water heater and heat pump "emergency heat" resistor strips - use of water would knock out the heat strips.

But for EVs, they are stupid. They are especially stupid at $1200. That doesn't even include an EVSE and is almost double the cost of the Wallbox Pulsar Plus with recommended CT sensor module, the top shelf solution that gives you the best of all worlds.

The secretest of secrets: Slow down.

For understandable reasons, most novice EVers are mis-educated on a notion that level 2 charging must be 40-50 amps. This is balderdash; it's 6-7 times the speed of plain old level 1 charging, and that's bonkers overkill for daily commuting. It's like that, because cars are supplied with a "Travel unit EVSE" intended to be kept in the trunk and used "on the road" e.g. at RV parks - and in long distance travel, yeah, you need a total fill-up overnight.

The rest of the time, you just don't need that much. It's perfectly viable to do 240V level 2 charging on a

  • 15A circuit (over 2x Level 1; 75-130 miles a night)
  • 20A circuit (over 3x Level 1; 100-160 miles) or
  • 30A circuit (over 4x Level 1; 150-250 miles).

And often, the house which cannot support a 50A circuit in its Load Calculation can support a 15-20A circuit, and that's all you need if you're almost anybody.

For that "But Sometimes" moment, there's always DC fast charging.

You can use a wall unit set to a lower speed, or an alternate dongle for the travel unit to get a more achievable Level 2 speed.

By the way, the "Over 2x" etc. is because some power is needed simply for the overhead of charging, e.g. battery pre-heat, or water cooling. That comes right off the top, so e.g. if it's 200 watts, that's 15% of Level 1 charging but only 5% of 20A/240V charging.

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    This is one of the best concise explanations I think I’ve ever seen for all of this. Wish I could upvote it more than once. Jul 27, 2023 at 11:47
  • I've been using the trunkable EVSE that came with my 2012 Volt, nightly, for nearly a decade now. Perfectly adequate; pulls 12A from a 20A wall outlet in the garage. I have an equivalent unit at work (~30 miles away) and so I can do virtually all of my commuting on electric. I've used faster chargers when they're available for free but only for the convenience. Jul 27, 2023 at 15:16
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    Completely agree. There's no need I can discern for anything more than a 13A "granny charger" connection (I'm in the UK). I only use higher rate AC or super-fast DC chargers when I'm on long journeys on the road and need to top-up, for "destination" charging (e.g. at a hotel), or on the extremely rare occasion I did many miles one day and need to leave agan the next morning. My average trip is less than 30 miles a day. Jul 28, 2023 at 11:51
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    I missed this when asked in July. I'm bookmarking to add a [related] comment to darn near every EV charging question that comes up from now on. Added in the definition of SAE for completeness. (That's about the level of detail I feel qualified to add to a Harper answer).
    – FreeMan
    Oct 4, 2023 at 14:44
  • you know... i'm not sure what I just read here. how is it a masterful design of EV charging and how is it different than selecting 10A, 5A, or 2A on my car battery charger? and then you mention hot tub skip to the end but you make no mention of what context of hot tub... outdoor?
    – ron
    Oct 4, 2023 at 19:02

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