If you've got an electrical meter handy that you can measure Ohms with it's a snap to do the math for Watts.
Measure the resistance (Ohms) of your new grip heaters across the leads that would attach to the battery. Check both Hi and Low settings if you want both wattages.
The math is Voltage squared, divided by Resistance (Ohms) equals Watts.
So just round off your voltage to 13 volts (a few tenths either way won't be that big of a deal) then square it (13x13=169) now divide that (169) by the Ohms reading on your grips and there ya have it.