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Impact of Electric Vehicles on Grid

October 17, 2014

While EVs and PHEVs are still few in number, we should understand their effect on the grid if their numbers grow significantly.

Remember, hybrids, i.e., HEVs such as the Prius, do not affect the grid.

The following are some potential hidden costs of EVs and PHEVs.

1. Cost of building new power plants to supply the additional electricity needed for recharging batteries

2. Cost of replacing distribution and substation transformers as loads increase from charging batteries

From top left clockwise, transmission lines, sub-station, overhead distribution transformers and distribution lines, underground pad mounted transformers for underground distribution. Photos by D. Dears

From top left clockwise, transmission lines, sub-station, overhead distribution transformers and distribution lines, underground pad mounted transformers for underground distribution. Photos by D. Dears

Item 1:

The Pacific Northwest National Laboratory (PNNL) determined that 73% of the existing cars in the lower 48 states, or 142 million vehicles, not including trucks, could be EVs and PHEVs before it would be necessary to build new power plants.

While this would mean that EVs and PHEVs wouldn’t require new power plants until many years in the future, the PNNL study assumed that recharging would be spread throughout the 24-hour day, filling the valleys when the regular load was below the peak. This essentially assumed that the system could be operated at its peak available load throughout the day, which is unrealistic.

Restricting battery charging to off-peak hours reduces the number of vehicles before new power plants are needed to approximately 78 million vehicles.

Assuming the PNNL study is correct, it will be many years before EVs and PHEVs will require the building of new power plants.

There are two caveats to this conclusion:

  • EVs and PHEVs aren’t concentrated in a small number of metropolitan areas
  • Recharging is done at night and not primarily during the day, especially during periods where the grid is operating near its peak

Studies have not yet been done to ascertain the effect of EVs and PHEVs if they are concentrated in various metropolitan areas or where charging is done during the day.

Governments and environmental organizations encourage the building of charging stations in downtown areas so that vehicles can be charged away from home, during the day.

If 240-volt charging stations, costing $2,500 each, are used to charge batteries, with charging spread out over several hours from 8 am to 5 pm, charging will take place during peak periods. If people use rapid charging for only an hour, with charging stations that cost $25,000, the charging load will be greater, and, depending on when the daily peak occurs, could be done during peak periods.

Charging batteries downtown during the day will require building new power plants sooner rather than later.

While EVs and PHEVs are still small in number, the effect of charging during the day needs further study, especially in areas where these vehicles are concentrated.

The key is whether charging during the day increases the peak and encroaches on reserve margins.

Item 2:

When distribution and substation transformers become overloaded, they fail.

The distribution transformer is the green box sitting in the yard of a home or the blue or grey-can hanging from a utility pole down the street.

When a distribution transformer becomes overloaded and fails, it cuts the electricity to all the homes being served by the transformer. It can take several hours to replace failed units. Condos and apartments have significantly larger distribution transformers located on the property to supply all the residences.

Substation transformers supply distribution transformers and as the load on the distribution transformers increases, the load on substation transformers also increases.

Each residential distribution transformer supplies the electricity to four or five homes, and is usually a 25, 371/2 or 50 KVA unit, depending on the number of homes being served and the electrical load in those homes.

It’s difficult to determine how many EVs or PHEVs can be garaged (and recharged) in homes served by a distribution transformer before the distribution transformer becomes overloaded and causes an outage.

The existing load on distribution transformers is unknown to virtually every utility.

Homeowners have been adding appliances, such as flat panel TVs, so the load on the transformer has gradually increased to the point where the additional battery charging load could cause the transformer to become overloaded and fail.

If two or three EVs and PHEVs are charged simultaneously at night, between the hours of 10 pm and 6 am, when homes generally have very low loads, it’s unlikely the transformer serving the homes will fail.

Transformers typically fail without warning and the utility will have to rush to replace the failed unit to restore electrical service to the affected customers.

It’s a relatively simple matter to change out a smaller distribution transformer with a larger unit. There are few weight limitations for pole-type units, and the pads for pad-mounted residential units are usually large enough to accommodate up to a 100-KVA pad-mounted transformer.

Condominiums and commercial properties use larger distribution transformers where failures affect more people, and replacement transformers cost considerably more.

Transformers increase in size from small single phase units rated 15 KVA to 100 KVA, serving 4 to 6 homes, to transformers rated 167 KVA to around 4,000 KVA, single and three phase, serving commercial areas and condominiums. Next in the hierarchy are substation transformers that supply electricity to the distribution transformers located within a geographic area in a town or city. Next are the very large transmission transformers that are not likely to be affected by the use of EVs and PHEVs.

Substation transformers, which can easily cost as much as a million dollars, will require advance planning. If the transformer fails without notice, there could be several days, or even weeks, before the transformer can be replaced. An emergency spare, perhaps a mobile unit, might have to be installed temporarily at an additional cost.

When a transformer becomes overloaded and must be replaced with a larger unit, there is the cost of the new transformer, the cost of having a utility crew remove and replace the existing transformer, and the cost of re-inventorying or disposing of the old transformer.

The cost of changing out one 50 KVA distribution transformer, and replacing it with a 75 KVA unit is well over $3,000. The cost of replacing a substation transformer can exceed $1 million.


Eventually, as the population of EVs and PHEVs increases, these hidden costs will become apparent and affect the public and economy. These hidden costs could be substantial, especially for metropolitan areas where EVs and PHEVs are concentrated.

If it’s assumed one distribution transformer will have to be replaced, at a cost of $3,000 for every 10 EVs or PHEVs sold, and assuming 20 million EVs and PHEVs on the road in 2024, it would cost over $6 billion to replace the distribution transformers serving homes.

If it’s assumed that one substation transformer, with a cost of $1,000,000, will have to be replaced for every 5,000 EVs or PHEVs sold, it would cost $4 billion for substation transformer replacements.

While these assumptions may be more applicable to metropolitan areas than nationally, they provide a preview of the hidden costs and reliability issues associated with EVs and PHEVs.

EV and PHEV sales would have to be wildly more successful than they have been for hidden costs to become a major problem, but policy makers, and especially mayors and utility executives in major metropolitan areas, should be alert to how these vehicles can affect the grid.

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