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Tracking PHEV Costs

If you are anything like me, you like meticulously tracking every dollar that goes into the tank of your car.  What do you do when you add a plug?  Attempting to track MPG becomes meaningless when you only gas up every 1000 miles and if you calculate total miles / gallons used it isn’t a representation of how efficient you are driving or the energy being used.  Take this screenshot from my Volt as an example:


It is like saying my 5 mile commute to work is only 20 steps.  Sure, I only take 20 steps or so to get to work, but I drive the car for 5 miles.  I need to state the energy used from the electricity beside the gas used to get an accurate picture.  250+ MPG (Clarity Picks 199) doesn’t tell me useful information.  Marketing number.

So what do we do instead?  We calculate MPGe.

In order to track your gas, write down the date, odometer, gallons, and cost per gallon (or total cost).  For the electricity record the odometer reading and the total kWh used for the month.  For assistance tracking electricity install a Charge Point Home or if you already have an L2 charger you can have an electrician install a hall effect sensor to measure power like this Power Meter.  If you use the L1 charger, one of these should work, but watch it that the plug doesn’t get warm: Kill-A-Watt. The last two options require manually recording the reading every month.

Once you get this information, how do you make use of it?

odo miles cpg gal cost
3/7/2018 4575 574 2.41 3.432 $8.27
3/13/2018 4884 309 2.36 2.433 $5.74
3/14/2018 4993 109 2.35 1.797 $4.22
3/21/2018 5447 454 2.58 2.887 $7.45
3/27/2018 5767 320 2.53 2.312 $5.85
3/28/2018 6034 267 2.59 6.343 $16.43
3/29/2018 6245 211 2.59 3.817 $9.89
3/30/2018 6467 222 2.49 6.37 $15.86
3/30/2018 6612 145 2.49 2.394 $5.96
March Total 2611 31.785 $79.67
kWh $/kWh Cost Cost/Mile MPGe
443.249 0.1074 $47.60 $0.0487 58.10

Steps to Calculate MPGe:

  • Figure total miles
  • Figure total Gallons
  • Figure total kWh
  • Figure MPGe
    • Total Miles Driven / (Total kWh Used/33.7 + Gas Gallons Burned)
    • 33.7 kWh/Gal of gasoline is a constant found on web and is energy in 1 Gal of gas

To make this more interesting, lets look at some graphs:


We put a lot of gas miles on our car so the MPGe is lower.  Combined rating is 117 MPGe from EPA.  January was cold and July was almost all highway trips (over 3000 miles of trips).   So looking at this graph, if we are near 40 MPGe we are running almost all gasoline, and if we are near 100 MPGe+ it is almost all electric.


The electric is pretty stable around $40 to $50 but the gas costs is dependent on trips.  It is obvious we drove a lot of highway miles in July, and still have a few days left.

We have driven 17,000 miles in 7 months with the Clarity.  How much have we saved vs the Nissan Quest that averaged about 17 MPG?  Well, that would be 1000 gallons of gas vs the 200 gallons used in the Clarity, plus 100 electric equivalent gallons, so in terms of raw gas we have saved 800 gallons in only 7 months, but added 100 gallons equivalent of electricity.  If this was from renewable energy it could be ignored as well.  Even if it was from coal we cut our fuel usage and emissions dramatically.  Even compared to a hypothetical Clarity running gas only we cut our energy usage dramatically.

Again, this requires just a little information tracked with each fill up and a reading once a month or use of an energy tracking charging station.  Not a lot of effort, but I would only do this if you like the data.  It can be useful to track loss of efficiency over time or other issues that might show slowly.



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