Measuring Power - II

As the previous post indicates, I am now measuring the kWh used to charge the car. I've been keeping track of per-day costs, and here is the chart for the first couple of weeks. As you can see, the cost per mile does not correlate well with the miles driven - which means that the theory I had about the rebalancing contributing a major portion of the cost for short trips seems to be incorrect (at least, for the length of trip that I've been taking).

The gap in the chart is for when I watered the batteries - no miles on that charge, so no wH / mi.

Over this period, I have driven 111.4 miles, and used 54.9 kWh to recharge. This nets out to an average of 493 Wh / mi (as shown by the green line on the chart). My electricity costs 7.5 cents per kWh, which means my average cost per mile is 3.7 cents, which, at $4.00 per gallon, is the equivalent of 108 MPG.

Measuring Power

I've had many people ask, "how much does it cost to run your car" . So far, I've been unable to answer very accurately. I have a decent understanding of the Wh/mile performance - but that does not account for charging loss, running the fans, etc. So, I picked up this little device over at ekmmetering.com:


I then made a 24-inch long extension cord with this inline to see how well it works. Seems to be working well so far. If I continue to be happy with it, I'll get it mounted in the wall next to the 240V outlet (this extension cord thing is just a temporary kludge). I'll update this post later with the actual *measured* wall-to-road Wh/mile performance of my car.

June 3 2008 edit: I used 6.70 kWh to charge last night - roughly 50 cents worth of electricity. I drove 13.1 miles, which gives 489 wH/m - twice the cost I calculated from watts used while driving. This is likely because of the "finish" / "balancing" stage of charge, and also likely because I bet my tires need filling and my batteries need watering. I'll chart out this over the summer, and include significant maintenance events on the chart.