Go Home

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.

weekdayswithoutvolt weekendswithoutvolt

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.

weekdayswithvolt weekendswithvolt

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.

energyusageperday

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.

flatrateelectricityusage flatrateelectricitycost

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.

onvsoffpeakusageperday

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.

onvsoffpeakusagepermonth

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.

tourateelectricitycost

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!

Conclusion

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!

Sources:  PG&E E-9 Electricity Rate Schedule, PG&E Smart Meter Program.

Download:  Chevy Volt Electricity Analysis Spreadsheet

27 Responses to “Chevy Volt & Electric Car Off Peak Charging Cost Analysis”

  1. Jeff N says:

    Excellent article!

    I’m also planning to just use 120V to charge my Volt. In my case, I have too many Tivos and others ghost charges running during the day and a big plasma TV at night so I’m running around 450 KWhrs a month pre-Volt. Otherwise, we have similar usage patterns and commute distances. I have a Watts Up? energy monitor and plan to go around the house auditing where the power is going to see if I can reduce my peak usages while patiently await my Volt delivery…..

  2. Fred says:

    Great analysis Patrick, I was planning on doing something similar myself but now I’ll cheat and use your spreadsheet. Do you know if you can actually control the total charge (kwHr) that the Volt charges each nite, or just the time (from which you can indirectly control the amount of charge).

    I’m a data geek so enjoy looking at the PG&E website showing my hourly usage. It seems like it would be a simple matter for them to provide a “what if” analysis of past months and show you what the cost would be for different pricing (TOU vs regular), but they don’t offer that. I may write a python script to calculate it.

  3. PatrickZWang says:

    Hey Fred, Glad you enjoyed. Yes I do this kind of stuff all the time at work and was thinking or writing a script, but figured it would be easier for other people to use if it was in excel. Sadly PG&E web interface to dump daily usage data is pretty cumbersome, so it took some slogging to aggregate the data together.

    I thought this would be dump and a 15 minute analysis, but the two dimensions of variability, total usage and TOU usage really threw a curveball. Summer rates would be even worse as there are 3 different time of use rates. (I avoided that for now since most people only have a 2 – on and off peak)

    I’m eagerly awaiting my Volt now!

    Patrick

  4. Fred says:

    Patrick, the link for the spreadsheet zip file has 4 w’s in the www, so the link doesn’t work. I manually typed it in and was able to get the zip file.

  5. PatrickZWang says:

    Oops, my bad – fixed now.

    Patrick

  6. Fred says:

    Did you get your TOU rates from this page:
    http://www.pge.com/includes/docs/pdfs/about/rates/rateinfo/rateoptions/res/e-9.pdf

    If so, I believe those are out of date (that page is dated 2002 I think). This page gives the current numbers:
    http://www.pge.com/nots/rates/tariffs/ResTOUCurrent.xls. It is linked to from this page: http://www.pge.com/nots/rates/tariffs/electric.shtml#RESELEC

    The tier 1/2 rates are a little lower than the ones you used for E9A ($0.06006 off peak and $0.10799 partial peak), but the the tier 3 rates are much higher ($0.14698 for off peak and $0.26385 for partial peak).

    I can’t figure out how to get the baseline KwH amounts for each time period for use with TOU — where did you get those?

  7. PatrickZWang says:

    Hey Fred,

    Thanks for that update on the rates. Same analysis, but different variables. As far as baseline rates, check out this page. You need to find your county, then multiply your daily baseline by number of days in the month.

    http://www.pge.com/myhome/customerservice/financialassistance/medicalbaseline/understand/

  8. Fred says:

    Ah, thanks. That page shows me (in San Carlos) being in territory Q, which has 9.8 KwH baseline for winter, yet my latest bill shows 12.6, which is the value for territory X. So that page doesnt’ seem to be correct. Looking into it further, I see this page has the correct values: http://www.pge.com/nots/rates/tariffs/ResElecBaselineCurrent.xls. It shows territories Q and X both getting 12.6 KwH baseline in winter.

    So for TOU billing, do you use that same baseline allotment for each time period (peak and off peak)? That doesn’t seem right, otherwise in summer when there are 3 time periods you would get 50% more baseline.

  9. PatrickZWang says:

    For TOU, my assumptions were that basically you would accrue usage through the month (regardless of when you used it). The day/hour you hit your baseline rate (or one of the other tiered thresholds, you move up to your next price tier, so on and so forth.

    I believe how it work is if you have a 10 KwH baseline, regardless of how you accrued your electricity usage (on or off peak) you would hit the next tier the second you accrued 10KwH of usage in a month.

    I have changed to the E9-A rate effective today, so I’ll be able to check for sure on my next electricity bill :)

    Thanks for catching these btw.

  10. Fred says:

    I posted a question over on the Leaf forums, where they had a long thread about PG&E rates, and the reply was that the Tier usage is pro-rated across the time periods, so you don’t need to keep track when during the month you crossed each tier. See the thread here: http://www.mynissanleaf.com/viewtopic.php?f=25&t=661&p=43371#p43371

  11. PatrickZWang says:

    Fortunately for my analysis, it averaged out weekday vs weekend usage – so it shouldn’t affect things, but good to know they do proration. For most people this shouldn’t be a material issue though since their usage patterns likely will be the same any particuliar day of the month. (Weather aside, but you can’t really control for that)

  12. Dave Myers says:

    My understanding from talking to a PG&E rep is that to get the benefit of the lower E-9 rates that you have to have a separate smart meter for your car charging requirements circuit than for your regular household electricity use. If you use a single smart meter for everything including charging the car, the rates actually go up for the various tiers regardless of what you are using the electricity for. Is this correct?

  13. PatrickZWang says:

    There is E9-A Rates for a single meter and E9-B for a separate meter.

    Installing a seperate meter is fairly expensive (I think it was over $1,000), so the marginal saving of having essentially a seperate tiered rate structure for your seperate meter only makes sense if you already use a lot of electricity (Already in a higher usage tier)

    Since I’m at about 50% of baseload, E9-A was just fine for me, but if you are already at 200% of baseload, your electric car would push you into tier 4 etc. It may be more cost effective to pay for the seperate meter install.

  14. Chris Abbott says:

    This is a fantastic article. Thank you very much. I wanted to do this same analysis, but I did not realize the complexity. I live in San Diego so all the details change slightly. I know SDGE is considering a TOU option where time of day is all that matters to the pricing tiers. I hope they do that. That will also make the return on solar panels more favorable. Put power in the grid at peak and take it out in off hours (charge car)

  15. Steve D says:

    Excellent article to explain such a complicated system. I am really glad that I don’t live in CA, because our system is WAY simpler – and cheaper. In Phoenix we have a rate plan that has peak hours of 3 pm to 6 pm. Everything else is off peak. Off peak is .079/Kw in the summer, .072/Kw in the other months. On peak is .35/Kw summer and .25/Kw in the winter. Needless to say, we work to not use much energy between 3 and 6 pm. It was easy to program the Volt to charge off peak and there is ample time for charging using 110. Also, I have solar, so I will be buying little, if any, electricity, anyway.

    By the way, Volt advisor told me today that the maximum consumption for a full charge is 9.6 Kw.

  16. jeffhre says:

    On second thought, is GM now allowing 10.3 kWh to be used from the total of 16kWh – maybe that’s what I last read, i’m not sure it’s been a while.

  17. Ken McClelland says:

    I am in an all electric house with no discounts for off peak form my Ohio supplier. Last month I used 2500 KW hrs at $.135 per KW hr. The most important part of your report was to say that it takes 7.2 KW hrs to go 20 miles. Is that correct? If so, that means it will cost less than a dollar to go 20 miles. I am presently spending $200 a month for gas at 16 miles per gallon. Sounds like I will be loking for a used electric car pretty soon.

  18. Jerry von Brethorst says:

    Two things that need to be factored is the orignal cost of Volt vehicle versus the price of a Toyota Corolla. Inlclude the fiance charges as well. The second is when everyone is buying electric, what will the demand for electricity do to pricing in the future.

  19. Dave says:

    I’m not one of those trolls hitting up comments, I consider myself a logical person and I just don’t understand the electric trend. People say the car companies are holding us back to support the oil companies but….

    What are your (anyone) opinions on the following scenario: What happens when electric cars become more common and everyone goes on metered plans (which appear to be based on demand), usage goes through the roof and electricity gets cranked up over all hours of the day? Inevitably will the prices then not go so high as to make it just as expensive as gas? You’ll be driving yourself and your community out of their home? No? Perhaps not that high, but certainly if you can still afford it, you’ll be getting hit with the rolling blackouts after supper when everyone jams their electric car into the outlet.. no? Takes a while to build those polluting coal power plants…

    …and … go…

  20. Ron jernigan says:

    I can’t believe all you frickin number crunching freaks that actually own an electric car. Don’t have a clue the simple math about your piece of crap car.
    I’m sure you’ve owned it long enough to figure out, without all the bullcrap KWs and peak or off-peak usage And charts BS jargon. The question that needs to be answered for a plain hick like me is not! Like all the lame commercials state about almost never having to get gas. How MUCH DOES IT COST Each. Of you to drive that electric gutless piece of crap, on average, down the road, say a couple hundred miles. TOTAL COST electricity, gas etc.
    Come on, you know that figure so give it up and save folks like me from havin ta read all that kw…. Peak / off peak crap that we don’t care about.
    IS IT CHEAPER TO DRIVE a NORMAL trip AFTER CHARGING WHEN iNEEDED IT, a couple hundred miles???????????????? Plain and simple. Electricity & gas.

    • nadine says:

      I agree with you Ron….. how the heck do i find out how much it costs….. Im not stupid….. i know it cost me 20buck more a month to run my house ac …. just by looking at the bill.,,,,, i dont think they want us to know……

  21. rlflucker says:

    This is the stupidist and most complex answer to a simple question I have ever heard. The author says that that 36kwh is required to charge the batter system of a Volt that has driven 100 miles. Then the author gets all tied up in electric pricin systems to show that he is “knowledgeable about the intrancies of residential electric supply pricing. Bull shit.

    Residential prices for electrical energy range from a low of about $.15/kwh to a high of about $20/kwh. So lets take them both. For 100 miles of driving, whether you do it in one day a week, 20 miles/day or in one day, 100 miles, the results are the same on a mp/kwh basis.

    This guy is merely confusing a simple problem to make himself look.

    At $.15/kwh, 36 kwh for 100 miles of driving the basic electricty will cost you $5.40 or 5.4 cents per mile or at $20/kwh it will be $7.20 or 7.2 cents permile. However the author does not tell about the other constraints. Where can you plug in? At your friends house when you are having a beer with him and you plug in to his outside three prong, and he does’t know what you are doing? Or will you only be able to plug in to a 240v spedcially adapted high amp system since to get 36kwh in one hour will require a high amp specially design circut which might cost you several thousand dollars.

    Too bad someone isn’t making this clear and simple, but as usual the girus are saying trust me.

    • PatrickZWang says:

      Hold on one second – you are asking to model the complexity of expected electric usability (e.g. where you can charge) which is highly dependent on your own driving habits, but you don’t want to consider when you are likely to use electricity and simplify the model to a fixed cost per Kw-h?

      If you want to real answer, I’m sorry there isn’t a one sized fits all solution. In some cases, you actually do not benefit from owning an electric car if you use an enormous amount of electricity, and you are paying high rates (e.g. $.30-$.40 per Kw-h). In other cases, like mine – I don’t use a lot of electricity, and I can benefit from really cheap rates off-hours, which reduces my cost substantially – or if you have a solar install which can average out your rates.

      My driving use case is also such that I don’t need to charge at a friend’s house most of the time. Your case will be different than mine – no question. But in asking for a simple solution to a complex problem, and then to introduce more complexity is asking to have your cake and eat it too.

      Patrick

  22. Tailfins says:

    I’m considering trading in my Malibu for a Volt, and I was pleased to read the careful and considerate analysis of most of the people who posted here.

    Then I saw the appeals to ignorance (a basic flaw of reasoning) committed by some of the writers, along with misspellings, poor grammar and cursing. Tsk tsk.

    After researching the cost to recharge the batteries on a Volt, using the marginal electric rates I pay to Southern California Edison, it sadly does not make economic sense for me to buy one. I pay a marginal rate of $0.28, when I average in my summer versus winter usage. At that rate, my calculations indicate that the cost to operate a Volt for 15,000 miles per year is not a whole lot lower than my 25 mpg four cylinder Malibu.

    On the other hand, if I install a second electric meter, the off peak rate for the charger will be $0.12 per kilowatt hour. Hmm. I think I’ll run the numbers again…

Leave a Reply