Simple power metering for PHEV charging at 120V

Where I live, in the Kansas City area, electricity to my home is rated at about $0.12 per kWh. This rate doesn't change based on the time of the day or the day of the week. Given the static electrical rate, I was looking at simple metering devices that I could use for not only monitoring how much electricity is consumed by my PHEV Prius when it's charging, but also as a quick & easy means to determine the actual cost of charging, as it relates to my per kWh rate from the local utility.

I've used the Kill-O-Watt meter from P3 International for measuring other devices around the house & at work & was thinking this might work well for monitoring the PHEV Prius too. It's a fairly simple device where you just program in the kWh rate & it keeps a running score of the total power & costs consumed. According to the manufacturer's page, it says the Kill-O-Watt meter supports up to 15 amps of max power at 115-125 VAC & the Prius owner's manual (pages 80 & 90) states that a full PHEV charge requires 12 amps continuous for about 2.5 to 3 hours.

The Prius owner's manual is very explicit about the importance of not using an extension cord (page 79) which makes sense given the potential for electrical shorts or overheating, but I don't think that advice applies to a device like the Kill-O-Watt meter. An electrician once told me that circuit breakers & extension cords are designed to be safely operated at up to 80% of their rated maximum. If I take 15 amps (the max rating for the Kill-O-Watt meter) and multiply that times 80%, I get 12 amps, which just happens to match what the PHEV draws while charging.

Am I overlooking anything here that would create a safety risk with my simple math on this, or does anyone have a suggestion for an even better metering device that is relatively inexpensive (< $100)?

 

From everyone's feedback so far, it sounds like the Kill-O-Watt (model P4400) should work in theory, based on the experience others have had with the Volt given the similar charging characteristics both cars have in common. I know it's common to plug the Kill-O-Watt meter into an extension cord, since the meter will otherwise cover 2 full outlets in a duplex power receptacle, given the meter's size, but since I'm going to have a dedicated duplex receptacle on a stand-alone 20 amp circuit just for charging my PHEV Prius, I'm thinking that plugging the Kill-O-Watt directly into the power receptacle and the charging cable directly into the Kill-O-Watt with no extension cords should be safe. I think that cwerdna's suggestion of checking how hot it's getting after at least an hour or two of charging is a good idea. And like seilerts pointed out, arcing from in-rush current is a problem that I've seen as well with another Kill-O-Watt meter I already own, although I've never managed to melt one yet. And as Jeff N suggested, I hadn't previously heard of the Watt's Up series of meters, but I'm going to check those out. I'm not opposed to paying a bit of a price premium if the meter is constructed of higher grade materials, especially if it could reduce the chances of part of the meter melting. But, I'd have to figure out a way to do an apples-to-apples comparison of Kill-O-Watt and Watt's Up meters to make an informed decision on it.