A Free-Market Energy Blog

Home Car Charging: A Primer

By Donn Dears -- May 16, 2017

“People buying EVs that cost $100,000 aren’t concerned about the cost of adding charging stations to their homes, but people in the market for EVs costing $35,000 may find the additional cost burdensome.”

A major public-policy issue is government subsidization of battery-powered or electric vehicles (EVs). But putting this aside, or given the existing situation, what are the practical issues of EVs for the home and, hypothetically, for wide implementation?

The two major issues are:

  • Is there sufficient electrical generation capacity to serve a mass market?
  • Is the distribution transformer serving the home large enough to charge one or more EVs?

Power Plant Capacity

The first question has been answered, in general terms, as yes, unless there is a large number of battery-powered vehicles (BEVs) concentrated in a specific geographic area. See Hidden Costs of EVs and PHEVs – Part I  for an explanation of this issue.

It would appear that there could be 87 million BEVs on the road if spread uniformly across the country, before new power plants would have to be built to accommodate the additional load caused by charging the batteries of BEVs.

Distribution Transformer Load

The load on the distribution transformer is determined by the voltage and current of the charging outlet or home charging station. The charging stations also determine how long it takes to recharge the BEV.

At 40 amps, it would take around 7 hours to fully charge the battery. At 72 amps, it would take around 3 hours. (This data is from the Tesla website. Other BEVs may have differing charging requirements.)

The first instance requires the equivalent of a 10 kVA transformer. In the second it’s around 17 kVA. Single-phase distribution transformer sizes are 5, 15, 25, 37 1/2, 50, 75, 100 and 167 kVA.

It’s not unusual to have four homes serviced by a single transformer, which is frequently a 50 kVA unit.

Pad mounted distribution transformer
Pad mounted distribution transformer

The fact is, no utility knows how fully their distribution transformers are loaded. With the advent of larger and larger TVs, the addition of more computers and other electronic gear, the load on distribution transformers has been slowly but constantly increasing.

Beware: It might only take one homeowner adding a EV using the simplest, lowest voltage and amperage charging system to overload an existing 50 kVA transformer. If all four homeowners add a EV and the existing 50 kVA transformer is 75% loaded, it could easily require that the 50 kVA unit be replaced with a 100 kVA transformer.

Consider what happens when a family has two EVs. It could require dedicated transformers for each home, or at least adding an additional distribution circuit.

These are costs the utility bears. They are hidden from the usual calculations of the impact that EVs have on the utility system.

Another consideration is that older homes may have service entrance boxes rated 100 amps, while new homes are likely to have a service entrance of 200 amps.

The owners of older homes are likely to have to pay for a new service entrance large enough to handle the current when charging their EV.

An earlier article on this subject, see Hidden Costs of EVs and PHEVs – Part II,  considers other aspects of charging EVs, such as charging during the day rather than at night. Since adding load to distribution transformers also adds load to substation transformers, there is the potential for having to replace substation units that cost a million dollars.

People buying EVs that cost $100,000 aren’t concerned about the cost of adding charging stations to their homes, but people in the market for EVs costing $35,000 may find the additional cost burdensome.

Whether this will affect the adoption of EVs is another question that only time will answer.

BEV Charging Basics


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