Home > Energy > C-Max Energi “Value” Charge uses more electricity

C-Max Energi “Value” Charge uses more electricity

December 28, 2014 Leave a comment Go to comments

One potential issue with widespread adoption of electric cars is increasing peak electricity use. Imagine driving home on a hot summer day and plugging your car in to charge at the same time you turn on your air conditioning, start cooking a meal in your electric oven, and watch TV while surfing the Internet on your computer. All that electric load has to be generated, in real time, by a power plant somewhere on the electrical grid. To meet that load, utilities employ a combination of base load, load following, and peaking power plants. The precise mix of electricity sources depend on the power plants available to your utility, the current weather, and customer consumption patterns. But, in general, utilities will use their least expensive and most efficient power plants first, followed by ever more expensive and dirtier plants as power demand increases. Armed with this knowledge, and the knowledge that my utility has a significant number of power plants that burn fossil fuels, I try to shift electricity consumption to off-peak times, typically by running my dishwasher or charging my PHEV at night. These are easy changes and help my utility burn less natural gas – the fuel typically used for peaking power plants.

After I installed an eGauge home energy monitor I noticed that my Ford C-Max Energi draws approximately 70W for the entire time it is plugged in waiting to “Value” charge at night. (It’s called “Value” charging since some utilities offer significantly reduced night time rates.) I then ran comparisons between energy consumed during a full “Value” charge cycle vs simply charging immediately when the car is plugged in and found a dramatic difference in charge efficiency. (The car always reports using less energy than the energy monitor shows the car consuming from the electric grid. Up to this point, I have assumed that this difference is the result of power lost during the charging process.)

The graph below is from a series of trips that consumed 3.5kWh according to the car vs. 4.99kWh according to the energy monitor for a resulting charge efficiency of just over 70%. I have seen charge efficiencies as low as 52% when the car was plugged in for a very long time before the “Value” charge started. I regularly see charge efficiencies of around 80% when not using “Value” charging.

Given what I have observed so far, it seems clear that “Value” charging on the C-Max Energi is only useful if you have steep discounts for off-peak energy use and will actually increase your overall electricity consumption. I’ll continue to gather data but for now, my “value” charging days are over. The best I’ll be willing to do is plug the car in before I go to bed or possibly just charge it when my solar panels are generating power.


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  1. aarond12
    December 29, 2014 at 12:28 pm

    Sheesh. You should be able to power the entire car’s computer systems with that level of power draw. Doesn’t Ford have a separate module to regulate and time charging? Do they really require the entire car’s computers to be powered on?

  2. December 31, 2014 at 12:35 pm

    Tesla Model S used to draw more than this 24/7/365 before they taught it how to sleep.

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