JGaerland, I use the same wattmeter. They're quite repeatable on the same unit, but a 10% variation between units is typical. They're also fragile. I have about three just quit working. Oh well, I buy them on ebay for $11-12.

As I wrote earlier, if I ride at a slower pace, about 8WH/mile at 10-11 mph. It goes up with speed.