All your Watts will be propelling you and the massive battery pack.

A Dinotte XML-3 tail with 4 cell battery pack (240g) has a run time of 5hrs at 50% power. So realistically to get say 30 hrs of runtime, you'll need 6-8 battery packs, and a total weight of ~2kg for batteries. Given the hassle of recharging multiple packs along with swapping mid-ride.

I'm normally a big advocate of battery lights, this is one case where hub power makes a lot more sense.