I rode my XM-2000 down to red today - although it wasn't cutting out yet. According to my Kill A Watt power meter it just took about 8 hours and 2.05 kWh to bring it back to full charge (as indicated by the charger). This seems believable.
The max capacity of the XM is 38 Ah * 60 V = 2.28 kWh However, if I understand things correctly
(a) you'll never get all 2.28 kWh out as usable motor torque
(b) the charger will take more than 2.28 kWh to do a full charge
(c) you're only supposed to discharge batteries down to 80% DOD (1.824 kWh)
So it's nigh on impossible to figure this out on paper - that's why it's more effective to measure the power in and the range achieved. Now that I have a full charge I'm at a known starting point. I'll ride 12 miles to work, plug in the Kill A Watt and charger, then I'll ride home and plug in the Kill A Watt and charger. When back to fully charged I should have a reasonably accurate way of calculating my cost per mile. I'll post my results here and also on PJD's thread (didn't want to make that thread too much longer).