How to track and attribute multiple users of an EV charger

September 29, 2023 9:13 AM

I live in a 2-family house. We are thinking of installing a level-2 charger. We would like to let our neighbor use the same charger, if they get an EV. What is the best way to keep track of how much each of us is charging, so we can share the electricity cost?

Are there level-2 chargers that can keep track of multiple cars, and report how many watts each car has drawn? If not (or if those chargers are very expensive) what other approaches have people used to attribute electricity costs to multiple users of an EV charger? This has got to be a common need, right?

posted by Winnie the Proust to Travel & Transportation (6 answers total)

Not to un-ask the question, but depending on where you live and the general usage patterns, the split on the electricity used may be on the order of a few tens of dollars per bill, perhaps switching who pays more pretty regularly. So it might be better to just pick a split and stick with it. Getting a specialty system for tracking might cost more than it's worth (as it is probably intended for commercial or public use). If you want to, it is likely simpler to compare mileage and just say "OK this month you did 100 miles, I did 150, so we'll split it 60/40." But it also depends on whether you and your neighbor are more of type to err on the side of simplicity or whether exact accountability is important for whatever reason.

For comparison, we are having chargers like this installed in shared spots in our building, and we basically just looked at what the average costs are likely to be, plus the cost of the installation, and amortize that over a few years as a recurring cost. So it's more like establishing the value of the whole system in agreement and then splitting that equally (or according to some other agreed split) over time. We did this to avoid paperwork and granular tracking that would likely end up just being a few dollars here and there, basically.
posted by BlackLeotardFront at 10:46 AM on September 29, 2023

My spouse and I each have an electric car. We have a charger that is activated by transponder. We both have a transponder, and this way we are able to track how much each electric car charges. The charger even sends us an email after each charging stating how much energy was used and how much it cost. (The transponders also prevent other people from using the charger, which is outside, while we're not home.)
posted by amf at 11:04 AM on September 29, 2023

The Autel MaxiCharger has a sharing function in the app, and RFID cards for secure session start. I've seen on Reddit that you can break down kWh per user with the RFID cards, but at a glance I don't see any details about it in their manual or in the info about the app. The sharing function would seem to imply it does work this way though.

When I was looking at this about four years ago, I decided the easiest way to figure this out would be a simple mileage log for each car and a meter on the circuit, then just divide proportionally. It's not perfect especially if there's a lot of public charger use added in, but if both cars' charging habits are similar that is a simple way to do it.
posted by rabbitrabbit at 11:06 AM on September 29, 2023

As someone with both an EV and a PHEV for the family, and who has always found "swapping" them at a single charger quite annoying even without having to account for sharing between families, it might be worthwhile to think through some of the other implications of sharing an EVSE (ie, a charger) beyond power costs.

Do you and your housemate share the same electrical circuit? Do you share the same driveway? Some more information on how your power and driveway are shared would be helpful here. Tracking energy usage is likely the least of your concerns.

It's important to consider the placement of the EVSE, driveway access and preferred parking spaces. Even with faster Level 2 chargers, charging can take many, many hours. If the placement of the EVSE results in blocking your driveway for you or them to get in and out of while one of you are charging, this will be a natural problem. Similarly, if there will be "one space" to charge, and your car is finished charging but your car is still in the space (perhaps even because you've gone to bed) but they need to charge, do you want them to have to contact you to swap cars? Or vice versa? Are there enough spaces to keep all cars out of the charging space except when they actually need to charge? The issues with the parking and access logistics of sharing a charger will almost certainly outweigh the economics of sharing a charger.

EVSEs are not all that expensive compared with the cost of a car; in many areas, the cost of an EVSE and an install can be less than $1k. If you have separate driveways and parking spots currently, and if you have separate power connections, I can assure you that everyone will benefit from everyone just getting their own EVSE for their own spots unless something else is in play. What you're doing sounds neighborly, but with two daily-use EVs, it could end up being a massive inconvenience.

That said, with a shared power connection, you could still get two EVSEs as long as they're the same model with "load sharing" capabilities. This can allow two EVSEs sharing the same power circuit to charge two cars simultaneously at a lower power rate rather than having to swap spaces. My EVSE, an Enel Juicebox, will do this. In addition, each connected EVSE would generally track power consumption separately, so if you each use your own EVSE, that would result in being able to fairly easily check the use of each individual EVSE's power consumption levels and "firm up" with each power bill.
posted by eschatfische at 11:48 AM on September 29, 2023

Thanks to all who have answered so far. We're currently spending a hundred dollars a month on electricity to charge our car at home, so we would want to track usage if we shared a charger with a neighbor. Our driveway arrangement wouldn't cause any problems, though of course we'd have to deal with scheduling.

I appreciate the recommendations. If anyone can recommend specific chargers with key fobs or other features that would support tracking, that'd be great. Thanks.
posted by Winnie the Proust at 2:46 PM on September 29, 2023

Pretty much all EVs are capable of tracking how much charge they've taken, either with a phone app or in the car screen.

I guess it requires a bit of trust, but seems cheaper than buying a system to track it.
posted by chrispy108 at 2:18 AM on September 30, 2023