Chevy Volt & Electric Car Off Peak Charging Cost Analysis
I will start by saying that I thought this would be a relatively simple article to write, but as I delved into the details of how time of use metering works with tiered electricity pricing I began to realize that in fact the analysis of electricity costs becomes quite complicated. As you will see in this complete electric car off peak charging analysis, there are numerous factors that need to be considered when you are actually calculating the marginal cost of charging on an electric car on your electricity bill. This complete analysis is broken down into several sections and a conclusion at the end, but the intent of this article is to give you an idea of how you might figure out exactly how much time of use metering will help reduce your Volt’s operating costs.
1.) Current Energy Usage Analysis
The first step in performing an analysis of the “marginal electricity cost” of operating the Chevy Volt or any other electric vehicle is to understand your current energy usage and layer on top of that the energy usage you expect from having the Volt. These numbers are important for two reasons.
First off, most electricity providers have a Tiered Pricing System where you pay a lower rate for a baseline amount of usage per month, and higher incremental rates for higher usage rates. Think – progressive federal income taxes. Where I live, the local utility, PG&E, charges 11.6 cents per KwH up to 300 KwH per month, 13.5 cents per Kwh between 300 and 390 KwH in a month, and 29 cents per KwH between 390 and 600 KwH per month. These are winter rates in the county I live in – summer rates are in fact higher, but for the sake of consistency this entire analysis will be based on winter rates.
Secondly, understanding your current usage by hour of day will be important to understand how your usage will be billed once you move into a Time of Use Metering schedule, which adds a second dimension of complexity to the cost analysis.
So basically, you need to understand your overall usage in a month (to pin down your price tiering) and when you use it (to figure out your time of use pricing).
To do this I downloaded and aggregated 4 weeks of data from PG&E’s smart meter system. (If you are interested in the nuts and bolts, here is a link to the Chevy Volt Electricity Usage Analysis spreadsheets I used to build my cost analysis). To build the analysis, I boiled down the data to an average weekday-day and average weekend-day to perform my analysis. The charts are below.
You will notice a trend of usage that differs in time of use and total amount of usage depending on whether it is a weekday or weekend. To give you some background, the reason my electricity usage profile looks like this is because I live in a house with one other person, who also works during the day. As such, most of the electricity usage happens in the evening hours. Your personal electricity usage profile will depend of course if you stay at home during the day, have kids, or run air conditioning. We are fortunate in the San Francisco area to have natural air condition most of the time.
2.) Expected Energy Usage with Volt charging
The second step in building your cost analysis is to figure out just how much electricity you will usage if you are plugging in your Volt every night. Fortunately you can control when your Volt charges, so you can plan to have it charging basically during off peak hours. In my analysis, I assume that I will put 20 miles of electric range into the battery on an average day. As such, given the EPA estimates of 36 KwH per 100 miles, you will need to put roughly 7.2 KwH into the Chevy Volt per day. I have spread out the charge time as if you are charging on a 110v outlet, but realistically since you can control when you charge it doesn’t really matter if you have it on 240v in this analysis, rather – what’s most important is your assumptions on total average electricity usage per day. Below are my charts on time of electricity use with a Volt on weekdays and weekends.
You will notice in the charts that charging an electric vehicle is actually a considerable amount of additional energy usage on a person’s electricity bill. Which makes the electric utility’s concern on mass electric vehicle adoption effect on the grid a realistic concern.
Lastly, below is a chart of the total electricity usage in my case on an average weekday vs an average weekend with and without a Chevy Volt Charging.
3.) Energy Cost Analysis based on tiered pricing without Time of Use Metering.
Before we enter the complications of Time of Use Metering, the first analysis you should do is to figure out how much more you would have to pay to charge your Volt without time of use metering to create a baseline. The reason this analysis is not as straightforward as one might think is because all that additional energy usage that you are adding by charging your electric car will actually push you into a higher tiered rate. Again, like US taxes, think of the your when you got pushed into the higher tax bracket (ouch right?). While in my case, because I am already well below the baseline usage rates, anyone who drives the Volt the full 40 miles per day or even worse, drives a full electric car like the Nissan Leaf will see their rates pushed into a much higher rate. I failed to mention before that PG&E actually has a tier 4 rate at 40 cents per KwH, but in my case I come nowhere close to that usage. But with a Leaf, or heavily used Volt, it is entirely possible. Below is a graph breaking down the amount of usage as it would fall into the tiered pricing rates, as well as the total costs of that usage.
You’ll notice that for my case, charging a Volt would push me into tier 3 rates, although not very much of it. Not a good thing if you want to keep your incremental costs for charging the car lower. Again in my case the effect is more muted because I have 100 KwH of usage under the baseload rate at tier 1 that I am not utilizing. Many other people would see significant tier 3 charges.
Finally let’s add it all up. Basically all things considered, in an average winter month – I would normally pay about $24 for electricity. With the Chevy Volt Charging, I would pay $56 – which amounts to a marginal electricity cost of $32 per month. This is assuming I put 20 miles of electricity into the car per day, or 600 miles a month (216 KwH). So $32 divided by 600 miles gets me 5.3 cents per mile or a marginal electricity cost or 14.8 cents per KwH. Not bad, but also not great. Let’s see what we can do with Time of Use Metering.
4.) Energy Cost Analysis with Time of Use Metering.
Here’s where the analysis gets complicated. Not only do you get charged more for overall usage in a month, but your charges vary on whether you use electricity on or off peak. Also if you use a lot of electricity your off peak rates will also rise. Also, your total usage on a weekday or weekend makes a difference because when in the month you start accruing higher tiered pricing charges depends on when you hit the tiered price threshold. In PG&E case, you basically have the following rate structure for winter usage.
Tier 1: 11.6 cents per KwH on Peak hours – 6.3 cents per KwH off Peak
Tier 2: Same as tier 1
Tier 3: 16.5 cents per KwH on Peak hours – 11.4 cents per KwH off Peak.
Weekday Off Peak Hours: 0:00 – 7:00
Weekend Off Peak Hours: 21:00 – 17:00
You’ll notice that the weekend Off Peak hours are actually quite long. You’ll also notice that PG&E actually has lower rates in all cases using this time of use metering. In the summer, this changes as the Peak electricity rates during the middle of the day actually rises dramatically, but it is not included in this analysis.
Below you will find charts that show Average On vs Off Peak Usage in a Day.
This analysis is pretty easy, just add up your usage by hour depending if it’s on or off peak. However, at a monthly level, understanding your electricity cost based on tiered pricing is a little more difficult. Basically you’ll need to figure out what day in the month you’ll hit the incremental pricing tiers and charge accrue the energy usage on vs off peak (Consider, also that your energy usage changes on weekends! so you’ll need to account for that as well). In my case, after 3 weeks I will hit tier 2 pricing. (15 weekday-days and 6 weekend-days), at 4 weeks I will hit tier 3 pricing (an incremental 5 weekday-days and 2 weekend days). This leaves 2 weekday-days in tier 3 pricing. The graph below shows exactly this breakdown.
You can guess already that there will be significant energy savings, but keep in mind in my case the savings are less pronounced because of my already low energy usage and the fact that I won’t be fully charging the Chevy Volt every night. Anyone who uses more energy to charge their car will actually realize more incremental savings!
Finally we get to the cost chart now that we understand usage by time of day and by pricing tiers.
You’ll notice that even though off peak usage is significantly higher than on peak usage, the total costs are much less – which is exactly what we wanted! Let’s add it all up and see how we did.
If you remember our baseline at a flat pricing structure without an electric car was $24 a month. With TOU metering, we see a total electricity charge of $39 a month. Incrementally, this is $15 more per month for the same 600 miles per month or 216 KwH per month put into the Volt. This equates to 2.5 cents per mile or 6.9 cents per KwH to charge the Volt. This is much better math for operating your electric car!
The way utilities have setup their off peak rates combined with the fact that plugging in an electric car will likely push you into a higher electricity rate bracket makes using Time of Use Metering a no brainer for your Chevrolet Volt or any electric car. With PG&E, you can only access these rates if you own an electric car, but as soon as you take delivery, make sure you sign up for these programs and use the Volt’s charge timing system.
Hope you found this analysis informative! Any questions about your case I can help answer feel free to leave a note!