How We Measure Electricity Consumption - 2011 Chevrolet Volt Long-Term Road Test

2011 Chevrolet Volt Long Term Road Test

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2011 Chevrolet Volt: How We Measure Electricity Consumption

January 26, 2011


By now you may be wondering how the heck we're measuring our 2011 Chevrolet Volt's gasoline and electricity consumption. It's complicated, but not difficult if you follow a few steps.

Step One is measuring gasoline consumption at the pump. That's easy. We already know how to do that.

Step Two is measuring electricity consumption. That's quite a bit trickier, but not too difficult with the right equipment.

Above is one of our two secret weapons: A Coulomb ChargePoint SAE J1772-compliant charger that we paid for with our own money. It can charge two ways: a) 240 volts by plugging that white SAE pistol grip charger directly into the car, or; b) 120 volts by plugging the car's own charge cord into a socket hidden beneath that ribbed hatch labeled with the numeral "1". Better still, it's possible to use both at once, as illustrated in our Volt vs. Prius plug-in comparo.


And it's dead-simple to use. The Volt's key ring has this little card on it, and you simply wave it in front of the ChargePoint to authorize a charge. You can plug the car in before or after you do this, but it's easier to plug the car in first. Once plugged and waved, you wait a few seconds to see the Volt illuminate a green status light on the dash and honk its horn in confirmation.

This charger is connected to the internet, which gives us several advantages. When the car is full, the card owner gets an e-mail. If some jackwagon of a passerby unplugs the car before it's full, the card owner gets an e-mail (the Volt's alarm goes off, too). In our case, with so many different people driving the Volt and sharing one card, Vehicle Testing Manager Mike Schmidt gets the e-mails.


A single card is associated with a single vehicle, not an individual, so we can visit our account on the Coulomb website to track exactly how many kWh of juice was dispensed at each charge event over the life of a given car. Careful day and date association of this data with entries in the vehicle logbook allow us to match each charge to a driven number of electric miles.

You'll notice that the last two charges amounted to about 12.4 or 12.5 kWh. GM engineers have told us that the usable capacity of the Volt's battery is 10-point-something kWh -- they won't tell us the number with any more precision than that.

Yes, I know the Volt's rated capacity is 16 kWh, but rated capacity and usable capacity are two different animals and the usable capacity is the one that matters to us. All hybrids and electric vehicles employ State of Charge (SoC) management strategies to ensure long battery life, which means their batteries are never allowed to be fully charged or fully discharged. So the central 10.x kWh "slice" of the Volt's battery is the usable portion owners can actually access. Usable capacity is the part of the battery you "empty" and "fill-up", even though SoC management means neither of these terms is literally true.

So if the usable portion is 10.x, and we dispensed 12.5, what happened to the other 2 kWh? Charging losses. The very act of charging generates raw heat in the battery, as any laptop user can attest. Additional heat is generated in the cord itself and the car's on-board charge monitoring system, and then there's the juice consumed on-the-spot to run the cooling systems for these components.

So while you may use 10.x kWh to drive a certain distance on the road, the Volt consumed 12.5 kWh to drive those miles as far as the electric company and your wallet are concerned. Charging losses are a fact of life with all plug-in vehicles, and they simply must be included in electricity consumption calculations.

Away from home our Volt's key card allows us to visit any other Coulomb chargers on the network. Such chargers can be set to "free vend" by the owners or they can charge a fee. The billing rate isn't necessarily the going rate for electricity, either -- that's a whole 'nuther story we'll get into later.

Either way, use of our Volt's card at other Coulomb stations will create the all-important line entry on our Coulomb webpage with all the data we need. It's pretty slick.

Those green arrows above are there to head off a potential question. You'll notice that the Volt has been on the charger for some 8 hours, but there's no "end" time. The Volt is full, because 8 hours is more than enough to fill it and 12.134 kWh is consistent with a full charge. The lack of an end time merely means the cord is still plugged in and the charger is still occupied. The end time will get filled in as soon as the cord is disconnected. In other words, the "Occupied" time is not the charge time. For that you have to look at the time stamp on Mike's e-mails.

"That's great," you say. "But what about charging at home?"


At least one of you knows the answer, because I saw the name in an earlier post. This, my friends, is the Kill-A-Watt, an aptly-named device that measures the kWh flowing through it to whatever you plug into it: stereos, a fridge, a big screen or a 2011 Chevrolet Volt. The "Menu" button is used to scroll through kWh, outlet voltage, and a couple of other measured parameters. But kWh is the only one we care about.

A Kill-A-Watt costs $19.95 at Fry's, and they usually have them on the shelf near the extension cords (which you should never use when charging a Volt or any other plug-in car). My purchase of the Kill-A-Watt EZ was a mistake, because this $39.95 version isn't worth the extra 20 bucks. Basically, it allows you to manually enter what you THINK you electricity rate is, and it converts kWh to dollars. Pass. I can do that on my calculator. Buy the cheaper Kill-A-Watt and save some dough.

Right away, you can see that last night's home charge required 13.01 kWh -- about 0.5 kWh more than most of our 240V charges at the office. It's too early to know for sure, but it could be that the charging losses associated with charging on 120V household current are greater than those generating during 240V charging. After all, it takes over twice as long to charge, so the cooling systems will run twice as long. We'll keep an eye peeled to see if this is an actual trend or a fluke.

That's it. That's how we're measuring the electricity consumption of our Volt. In the next installment I'll go through how we use this data to disentangle electricity consumption from gasoline consumption and ultimately calculate gasoline MPG and electricity consumption in kWh per 100 miles (kWh/100). We might even tackle cost.

Dan Edmunds, Director of Vehicle Testing

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