People unfamiliar with electric cars often fear that a blossoming of EV ownership will bring chaos to the electric grid. For example, I recently spoke with the manager of a beach resort who was fearful of adding an EV charger, thinking it would create a huge additional power load. She didn’t realize that even a minor impact is largely avoidable.
How can this be so? First, electric cars require remarkably little electricity. They use about the same amount per year as an electric water heater, and we have about 60 million of them in the US today. Even the usage patterns are similar, with most usage in the early evening. The really good news is that EV charging — like electric water heater charging — can be controlled into low-cost, low-emission hours on the electric grid.
The table below compares the maximum demand, load shape, and annual usage of electric water heaters and EVs:
|Electric water heater||Electric car|
|Maximum demand||4.4 kW to 5.5 kW||3.3 kW to 11.2 kW|
|Typical annual usage||2,000 kWh to 4,000 kWh||2,000 kWh to 4,000 kWh|
|Load shape if not controlled||Morning and early evening peaking||Early to mid-evening peaking|
|Storage capacity||1 day of usage||2-5 days of normal usage|
|Hours of charging per day||3 hours = 1 day of hot water||2 hours at 6.6 kW = 50 miles of driving|
They don’t look alike, but they definitely act alike from the perspective of an electricity grid.
These are figures for a typical water heater (apartment or single-family home) and a typical electric car (Nissan Leaf, Chevy Bolt, or Tesla 3). An electric UPS or FedEx truck, or electric semi-truck is a different animal altogether for EV charging (as is a laundromat or municipal swimming pool from a hot water perspective).
So, the impact of millions of electric cars is likely to be very manageable, even without direct control of when the cars charge — about the same as water heaters. But it only takes two to three hours of charging a day to serve the typical commute and errands of the typical EV owner. While we do need high-voltage “fast” chargers along freeways to enable longer trips, 90% of the time cars stay within their home range, and can be charged overnight at home or during the day at work.
Most people with EVs come home from work, plug their car in, and by morning it’s fully recharged. They don’t really care whether the three hours’ charging they needed occurred from 6 to 9 p.m. or 2 to 5 a.m., as long as they are ready to go by morning. But from a grid perspective, it matters a lot: late afternoon and early evening are peak periods for electricity grids, and adding EV charging during those hours can be very expensive for the power system.
Both water heaters and EVs need to be charging two to three hours on a typical day. We can minimize their impacts — and costs — by concentrating those few hours into the lowest-cost, lowest-emission part of the day.
This is why time-of-use (TOU) rates for EV charging are so important. Nearly every electric car comes with a built in “charge controller” that can be programmed to charge only during specified hours. If the driver comes home at 6 p.m., but sets the charger to come on at midnight, he or she will still have a fully-charged car by morning.
Burbank Water and Power, a municipal electric utility near Los Angeles, offers an optional TOU “whole-house” rate for homes with electric cars. The table below compares this rate with Burbank’s standard rate to show the cost a customer would incur under each to charge an EV with 300 kWh during off-peak periods. (This assumes the customer uses at least 300 kWh per month for other household lights and appliances, and would thus be in the second block of Burbank’s standard rate.)
|Standard rate||Cents/kWh||EV rate||Cents/kWh|
|First 300 kWh||11.3¢||Off-peak (overnight)||8.2¢|
|Over 300 kWh||16.4¢||Mid-peak||16.3¢|
|4-7 pm summer weekdays||24.5¢|
|Cost for 300 kWh off-peak||$50.00||Cost for 300 kWh off-peak||$25.00|
|Cost/gallon @ 3 miles/kWh vs 30 MPG||$1.65||Cost/gallon @ 3 miles/kWh vs 30 MPG||$0.83|
Water heaters can also benefit from TOU rates; in fact, millions of homes in the rural Midwest already have electric water heaters on TOU rates. Instead of programming an EV charger, water heater customers on TOU rates can simply install a timer to ensure that the water heater charges when power is low-cost and low-emissions.
A Better Way: Smart Charging
While TOU rates are a good way to serve these loads, the next generation of EVs and electric water heaters will provide even greater opportunity. They will be able to use advanced electronic controls to turn charging on and off as grid conditions and wholesale market power prices vary through the day. This capability will become increasingly important to the grid as wind and solar become a bigger part of the power supply.
For example, in Hawaii, where wind is available overnight and solar during the daytime, a smart water heater charger might power the water heater from 3-5 a.m. and 11 a.m.-2 pm, while the actual hot water usage would be concentrated from 5-8 a.m. and 5-8 p.m. The tank holds enough hot water that the consumer never notices — even the smaller tanks often found in apartments and condos in the Aloha State work well with twice-a-day charging. Similarly, in California, where solar power is concentrated in the middle of the day, utilities are experimenting with workplace EV charging to take advantage of this low-cost, low-emission power. By contrast, Texas has lots of nighttime wind energy, the incentives there will be to charge overnight.
Smart charging can greatly ease the impact on the grid of existing electric water heaters, and of the growing number of EVs. By ensuring that both are not charging at the same time, utilities can avoid distribution transformer upgrade costs. (This cannot be achieved with TOU rates alone, since the same low-cost period would apply to both.) A smart rate design, included in RAP’s Smart Rate Design for a Smart Future, offers the incentives needed:
This rate design, by imposing a small $1/kW/month system infrastructure charge, provides an incentive to separate the charging of the water heater from that of the EV, while the TOU rate provides an incentive to get both jobs done during the off-peak window. But the system infrastructure charge is not so large as to punish the customer who has occasional high usage the way a more traditional $6/kW demand charge would do. The critical peak charge provides a powerful incentive not to charge either the EV or the water heater when the grid is under stress.
A smart charging app for an EV will let the owner request either an “economy” charge that makes sure that it’s done in a low-cost period, or an “urgent” charge for rare occasions when he or she really needs it. Similarly, a water heater could be set to use high-cost power only when an override button is pushed (e.g., when houseguests cause more hot water usage than normal).
Electric cars and electric water heaters certainly look different. But they offer similar opportunities to reduce fossil fuel dependence, similar opportunities to take advantage of low-cost, low-emission resources on the electric grid, and similar opportunities to save consumers money when charged in a smart manner.
Back to the beach resort: When the manager learned that an EV uses about the same amount of power as a water heater, she relaxed; the resort already has dozens of electric water heaters. We found that the resort’s maintenance shed had an existing 50 amp 240V circuit for a welding outlet inside that had not been used in its 12-year history. Moving that outlet outside at the cost of only a $200 electrician bill — and adding a $300 40A charger made charging available to EV owners visiting the resort. I used it recently for my Kia Niro, taking turns with another guest who charged their Tesla X. An honor box lets EV owners pay for the power we use and help pay for the charger over time. A few bucks at a time.