Energy Consumption: A Data Point

For the first time since last Spring, we were able to run only in EV mode for the month of August.



That works out to 105 kWh for 501 miles of of driving, or about 0.209 kWh per mile.

From an environmental (and sheep herding) perspective, we were delighted to end the month ranked 47th among the Prius Prime universe. And, of course, with sheep, clouds, sun, rainbow, pinetrees, oranges and turlips.

Interestingly, from an economic perspective, regular gas in our neighborhood is selling for $2.80/gal and our electric rate is about 21-cents per kWh.

That works out to $22.05 for 501 miles of driving. $22.05 would have purchased 7.875 gallons of gas. The would yield an equivalent fuel economy of 63.6 mpg.

During the couple of long trips which we made in May, June and July, calculated our gas-only mileage as about 56 mpg. So, 501 miles at $2.80 per gallon would have cost $25.06 -- which is not much different than our all-electric cost. On an annual basis, that would work out to a unimpressive fuel cost savings of about $36.

Fortunately, as I've written elsewhere on PriusChat, our motivation for buying the Prime was the feel and appeal of electric driving, environmental considerations, and creature comfort -- not economics. And, of course, if you live in a part of the continent with lower electric rates, your mileage will indeed vary.

 

Unfortunately, although our utility offers a Variable Peak Pricing option, I am not aware of anyone using it. The economics are not particularly compelling as explained in this post about Time-of-Use-Rates in one of the Tesla online discussion groups:

There is no charge to install a meter. The Eversource charge for a normal residential service is $6.43/month. The charge for a residential TOU rate is $9.99/month. That is what I have, a separate electrical service and meter for my car charging, but in my case I had to have it because my parking spot is not near my condo and its electrical supply.

If you are able to do all charging in off-peak periods, then you can potentially save money compared to normal residential rates. First, you'd need to drive enough and charge enough to offset the cost of the monthly service. Right now the standard residential rate (excluding energy) is $0.10774 per kWh. The monthly TOU fee of $9.99 is equal to the cost of 92.8 kWh at the normal rate. So you need to buy more energy than that just to break even on the monthly fee for a separate service on TOU rates. That is not very much driving.

Assuming you meet that test, the next question is whether the pattern of use will be more economical on TOUR. In winter, the peak rate is nearly 3 times the off-peak rate, so you need to use at least 3 times as much power in off-peak periods as in peak periods, to break even. In summer, it is worse: the summer peak rate is more than 4 times the off-peak rate, and the peak rate starts earlier in the evening, so you need to keep the peak period usage at less than 1/4 of the off-peak usage, to break even.

I estimated that I could save an average of $10/month on TOUR. Someone who drives more could save more. For me, the challenge will be to avoid pre-heating or pre-cooling the car in peak periods. It would not take too much of that to eat a big portion of my savings. In that sense, winter is more challenging, because the peak period starts at 8 AM. Since I am retired, I rarely leave the house before 8. So if I need to leave the house at, say, 9 AM, and want to preheat the car on a cold winter day, I will be using energy at peak rates.
. . .
The Time-of-Use rate is complex where you pay about 23.1 cents/kWh for 4 months June-Sept, 9am-6pm weekdays and only 4.9 cents overnight, weekends, and 12 holidays. During the other 8 months, October-May you pay about 15.4 cents, 8am-9pm weekday and only 3.2 cents overnight, weekends, and holidays. This compares to my standard rate of only 9.5 cents. The TOU plan also has a $3.50/mo extra account fee. While they give you a very attractive off-peak rate, the on-peak rate is dreadful.​

As you can see, the rate structure is particularly difficult if you need to use air-conditioning during the day in the summer.

Our bill provides separate charges for "generation" and "distribution" and it is possible to choose an alternate generation supplier. The state's Department of Public Utilities provides a very useful list of available suppliers, their rates, and their percentage of renewable energy. Energy Switch Massachusetts | The Massachusetts Department Of Public Utilities' Shopping for Electric Supply website

In our area, using the distribution utility's default provider, our rate looks like this:

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There is also a fixed connection charge of about $6.43 per month (as noted in the Tesla post above) regardless of the amount of electricity used.