what do you use to measure the kJ at the hub. i think it's pretty cool stuff actually.

i'm not super convinced by strava and wonder what kind of power i really am putting down.

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Strava does a relatively good job on solo, long, moderate climbs with no wind. It's absolutely useless in a group ride on flat land.

cool ... interesting device.

yeah, my commute is roughly flat 25km RT and all it does is bounce up and down between 0 and 250 or so.

i'm just more curious from a nutritional standpoint what i'm burning. i guess 300±100 kcal.

At total load of 195 lbs and speed of 18.64 mph:
Assume on the easy end:
Crr = 0.0033
CdA = 0.33
Tailwind = 5mph
Grade = -0.3%
Power = 67.7 watts, energy = 1.1*67.7*60*40 = 179 kcal

Assume on the hard end:
Crr = 0.0038
CdA = 0.38
Headwind = 5mph
Grade = +0.3%
Power = 277.9 watts, energy = 1.1*277.9*60*40 = 734 kcal

So, calories burned = 457±278

(Also assuming 95% efficiency in bicycle)

Generalize much?
Since my commute is very short (10 miles total) I tend to cycle more vigorously than on training rides. I burn about ~800-900 calories a day commuting (800 foot elevation gain - 190 lbs - 20ish rolling average).

I can never get these calculations to match my actual experience. I seem to need fewer calories than the calculators say I should and burn fewer than they say I do. About 12 years ago I needed to lose some weight, about 25 pounds. The calculators were helpful for giving me a sense of what to aim for. When I realized I couldn't eat what they said I could, I adjusted downward until I found the sweet spot. I lost 20 pounds and maintained that for 12 years. Never could drop that last 5 pounds. Until now. I started bike commuting 12 miles r/t each day back in September. I haven't changed anything else and the five pounds is gone. Just like that.

not to needlessly complicate this any more. But does age factor into all of this?

Yes but only to make us wiser.