How to use a wattmeter?

[crazyjr]

I have just bought a TME Xtrema Lipo charger with a watt meter. I know those who use a watt meter usually put it in the vehicle they want to test, but with the xtrema that's not possible(its as big as a Triton), so i thought if i could mimic the load on the motor to test, it would be ok to see what i got and not have to speculate and possibly be wrong. Does anybody have any idea what size props would best mimic a 4-5 pound stadium truck and a 9 pound revo conversion?
thanks for any info