Hi !
I got one "Watts Up " I had use it for my RC model and add it to my E Bike and it works very well on an E Bike also. On E bike is most important to see Ah, so I know how much power I have used.
If you got lot of money you could invest in a Speedict eBike. But you will need a android- or i-phone. It will keep track of your tour and watt usage. But you must always have the phone with you and it is no display on the device. I store my Ah usage in an excel-file so I know how much I have bike and Ah left. With RC models I use it mostly for max amp and voltage. More amp, more power, faster car. Races are mostly 5 to 8 minutes so no worries for empty batteries.