Electric Vehicles in Apartments

An introduction to plug-in electric vehicle chargers, and in particular the challenges of charging in apartment complexes. See Conclusions below for a summary

Electric Vehicle charger types

Standard charger

When you buy a plug-in electric vehicle, they come with what’s called a “mobile charger”: a cable that you can plug into any normal power point to charge the car.

Photo courtesy of Tesla Owners Club

To charge the car as quickly as possible with this cable, it is rated at the maximum power that a normal power point can deliver: 2,400 Watts, or 10 Amps – basically the same as plugging in a three-bar heater.

Generic three-bar heater

To avoid filling this article with numbers, I’ll use this as the unit of measure: one heater’s worth of load.

Charge time

So how fast can a mobile charger cable recharge a car? That depends on the model car, how large its battery is – and, of course, how empty that battery is! For example, the Tesla Model S, that has over 400 km range, would take over 18 hours to recharge from empty with this cable.

Wall charger

Thus, most new plug-in electric vehicles also come with a wall charger, often delivered weeks before the actual vehicle so that you can get a licensed electrician to install it. There are four kinds of these chargers:

A standard charger, fixed to the wall: no more powerful than the mobile charger
A three-phase charger, if the household has three-phase power, to triple the power available. This is fairly rare, since many plug-in vehicles can’t use three-phase power
A high-power charger (7.2kW), that uses a dedicated heavy duty circuit from the main switchboard; usually 32 Amps. This triples the power available, cutting recharge time by a third.
The best of both worlds: three-phase, high-power charger (22kW), for a total of nine times the power available – if the car can use all three phases.

Supercharger

Note that some electric vehicles can also use so-called “superchargers” or “DC fast charge” stations. These are installed in commercial environments, and require large transformers to generate the huge 100kW power involved (five times more than even the 22kW charger above): these cannot be installed in domestic situations.

Summary

So at-home wall chargers can be one, three or nine times faster than a normal cable – but of course that’s one, three or nine times the power requirements… The nine-times type uses over 90 Amps – that’s more than many houses!

Power Requirements

Circuit breaker

So how can a charger charge a car at those high rates, while competing with the house for electricity, to avoid tripping the main house circuit breaker? If you’re cooking dinner with the oven, stovetop, microwave and maybe air conditioner on full, there’s not much spare capacity to charge the car as well. However, in the middle of the night when the only thing running is probably the fridge and a couple of clocks, there’s plenty of spare capacity.

Charging schedule

So one technique is simply to schedule the charge for when the house power is at its lowest usage: the middle of the night, or the middle of the day if no-one’s home. Avoiding the morning and evening busy times will avoid competing with the house power.

Active Power Monitoring

A more sophisticated technique is for the charger to monitor the whole house power, and change what power it uses as the total house power changes. Unlike most electrical appliances, car chargers can vary their consumption to use as much or as little power as they like – another reason why the three-bar heater analogy works, since it’s like turning individual bars off to reduce the power load. Of course, that slows down how quickly the room is heated – or the car is charged.

Safety

The important point is that the whole house power system is designed and configured to be protected by circuit breakers rated at the maximum safe load. Only if the charger has active power management can this maximum be exceeded – on paper, at least. Of course, the circuit breakers will trip if the maximum is ever actually exceeded.

Apartments

Extrapolating houses

If a typical house has a 100 Amp main circuit (ten bar heaters), then a one hundred apartment complex would need 10,000 Amps (a thousand bar heaters), right? Not really.

Rule of Thumb

Here, the law of averages comes into play; when one apartment might be using a lot, another would probably be using very little. Of course, during peak periods like mornings and early evenings more power would be used across the complex. But studies have shown that when there’s more than ten apartments in a complex, a simple rule of thumb is to size the complex’s power load as: number of apartments, times 20 Amps:

A typical apartment usage

Other factors

There may be other large power draws, such as elevators, pool pumps, or ventilation systems: one common approach is to consider each of these as a single apartment load too.

Example calculation

So if the complex had:

  • Ninety apartments
  • Five elevators
  • A pool
  • An underground carpark requiring ventilation

then that could be considered a ninety-seven apartment complex:

97 “apartments” × 250 Volts × 20 Amps = 485,000 Volt Amps load.

So a dedicated 500kVA (kilo-Volt-Amp) substation would be barely adequate for this apartment complex.

Typical 500kVA apartment substation

Apartments and Electric Vehicles

Simple calculation

Considering that a typical wall-mounted charger is by itself more than a single apartment load, every car charger would be its own “apartment” at least. And if the car charger was the 22kW three-phase, high-power charger, then it would be more than four apartment loads while it was operating. So, while adding one or two chargers to an apartment complex could probably be absorbed with the existing capacity, if everyone wanted to fit a charger the power load calculation for the complex would double, at least. The substation and all of the power infrastructure would need to be upgraded heavily, at great cost.

Active Power Management

Unless active power management was used. If each charger was centrally controlled by a system that could measure how much other power was already being used in the rest of the complex, then each charger could be tuned to not exceed the power budget. That means that there needs to be a holistic approach to the installation of electric vehicle chargers; each apartment can’t just install their own charger. Also note that the chargers supplied with electric vehicles cannot be managed by a central system like this – a commercial charger is required.

Metering

Another obstacle to each apartment installing their own charger is getting the charger connected to the apartment’s electricity meter. Often the apartment car park is in the basement, where the electricity meters are – but the apartment’s main switchboard is in the apartment itself. It is against the rules to connect the charger directly to the electricity meter: the connection has to go through the main switchboard. Given that this could be many floors up, the cable run would not be easy.

Cabling

Worse, given that it is a high-power cable run, the cable used has to be thicker. Also, since the cable run could be very long, the cable has to again be thicker, compounding the previous increase.

This is typical apartment cabling. Note the four main power cores, and the smaller earth core, surrounded by white insulation and an orange jacket.
If the cabling has to go all the way up to the apartment, and then come back down again to the charger, then each core in the previous image would have to be the diameter of the single core in this image.

Alternative

To avoid the expensive and difficult cable runs, the alternative is to instead use an independent, parallel metering infrastructure. Instead of each charger being connected individually to its apartment’s meter, all chargers would be on one single power run – or perhaps a couple for better load sharing. The central power management system would thus be responsible for accounting for each charger’s power usage.

Misuse

Another difference between a charger installed at a home versus one installed in a semi-public space like an apartment complex is that a home charger will charge any car that is plugged in. To prevent this, one easy mechanism is to use RFID “swipe” cards which authorise the charger to start. These could even be tied back to the metering system, so the swipe rather than the actual meter identifies the bill to add this charge to.

Conclusions

In short, installing an electric vehicle charger in an apartment complex is nothing like installing one at a home. Sure, for specific individual apartments the installation process might be straightforward – but it is unlikely that most apartments in a complex will be easy, nor that every complex will have even one straightforward apartment.

In summary:

  • Power Draw: An electric vehicle charger would probably be the single appliance with the largest power draw installed in a home.
  • Capacity: An existing building (of any type) would probably only have a power feed adequate for the building. Adding one (or more!) chargers would stress that capacity to the point where the charging process needs to be managed.
  • Adaptability: But electric vehicle chargers are unique in that they can adjust their power draw to accommodate other loads on the system – if they can get the information to do so.
  • Multiples: Chargers supplied with electric vehicles are designed for home installations, not apartment complexes. They may come with power management, but are only designed to share with a couple of other chargers – if even that.
  • Metering: Installing a charger in a house is usually straightforward, since it can easily be connected to the house switchboard and electricity meter. Installing a charger in an apartment complex in the same way is much harder, if not actually impossible.
  • Authorisation: A charger installed in a home will charge any vehicle it is connected to. A charger in a semi-public place like an apartment carpark should have an authorisation system – a feature not provided on home chargers.
  • Summary: The differences described above between installing chargers in houses versus apartments require a completely different solution for installing chargers in an existing apartment complex. What is needed is:
    • A central power line (or lines), not connected to any apartment meter, supplying all of the chargers;
    • A central power management system that can control individual chargers to not exceed the building’s power capacity;
    • A central metering system to allow each charger’s usage to be billed to the appropriate owner;
    • Chargers that can interface to this central system, and offer user authorisation to avoid unauthorised use.

Example Systems

Both of the below systems use the Open Charge Port Protocol standard for integration with various management solutions. This allows different manufacturers’ chargers to be installed but the one management system can control all of them.

Note these are not recommendations, merely examples to give an idea of costing. Any system will of course need cabling infrastructure to be installed:

  • It would be cheaper overall, but with a high up-front cost, to get all of the cabling installed once.
  • Some apartments may have multiple car spots.
    • Does every apartment get wired for just one charger?
    • Does every car spot get wired for a charger?

Ocular IQ Commercial + Exploren

This is the preferred system of EVSE Australia. The management system has a maximum number of 80 charging points, on only one power feed (as of 1 Sep 2021).

Ocular 22kW

Charger

Single phase 7.2kW $2,100 – installation extra
Three phase 22kW $2,400 – installation extra

Manager

Power monitor $4,000 – installation extra
Exploren software + app $165 / charger / year

Easee

This is a Norwegian product that is so far only available in Europe and USA. The chargers talk to each other to share the available power, and can share up to 101 chargers per power feed. Multiple feeds are suggested to decrease how much sharing is required, maximising charging potential.

Easee Charger and Easy Ready wallplate

The charger comes in two halves:

  • The Ready wall plate is installed and cabled in up front, with a blanking plate for protection: $100 plus installation
  • The charger itself is “clicked on” later. A range of coloured faceplates are available: $2,000, no install required.
  • The Equalizer is the monitoring device, attached to the building power: $POA plus installation
Easee Equalizer