Based on a post over at electric-fatbike.com, I ordered a shiny wattmeter/power monitor/etc gizmo for my bike.

By name?  G.T.POWER RC 130A Watt Meter and Power Analyzer High Precision LCD 60V GT-Power

It measures everything you might be interested in!  Instant amperage.  Instant voltage.  Instant power (watts).  Peak watts.  Minimum voltage.  Peak amps.  Total power consumed.  It’s perfect!

**sigh** So the box says.  Of course, I bought one.  And built it up into a nice system to mount on my stem, running power up from my battery pack.

Excited, I wired this up with 10 gauge wire for minimum losses (Anderson PowerPoles at the end, nice crimp connectors at the wattmeter side), mounted it on my stem (above my blob of DC converter), verified it showed a sane voltage, and set off on a trip (to collect a baby toy)!

I took a 5.8 mile ride to collect some data.  And I got the following data:
1254 Watts Peak
32.50 Amps Peak
35.06 Volts Minimum
2.694 Amp-hours Used
35.2 Watt-Hours Used

Huh.  That’s odd.  For people who don’t have an EE background, I’d like you to focus on the Watt-Hours Used (35.2Wh), the Amp-Hours Used (2.694Ah), and the Volts Minimum (35.06v).

Watt-hours are easily calculated.  They’re watts.  Over time.

Watts are volts * amps.

Watt-hours are volts * amp-hours.

My battery is around 42V fully charged, and on this trip, dropped as low as 35.06v (as measured at the end of a fairly long run of 10 gauge).

Based on the data, my watt-hours used should have been between ~113Wh (42v) and 94.5Wh (35v).

35.2Wh is right out.  That’s not even close.  It’s not remotely right.  It’s impossible.

The display also isn’t very easy to read in most conditions.  It’s OK at night, but I could not find a good position for it with sunlight.

I took some other rides, and the watt-hours reading is consistently wrong.  I don’t know what’s wrong with it, but it’s just flat out wrong.  Period.  It’s flat out impossible, given the voltage and amp-hours measured by the unit, unless you redefine most of math.

I’ve found other reviews mentioning the same problem with some of these units, and people generally consider them to either be very nice, or somehow broken.  They’re not particularly reliable, either.

So, if you want a wattmeter for your ebike, or for anything else, I wouldn’t go with this one.  Spend a little bit more on something higher quality.

At least I was able to get my money back from the seller.