I'm assuming they're neoprene lined? If so, what's the back of the heel (inside) like? Of the 3 pairs of different neoprene lined wellies I've had, they all rub through the neoprene in the heel cup (causing blisters) well before anything else gives out.

You're still using the energy though, just in one chunk rather than several. Water takes a fixed amount of energy to heat up (per volume), so all you're getting out of this arrangement is rubbish tea.

The only reason boiling it all at once would make sense would be if your energy was considerably cheaper at certain times of day, but even then, you only need 24h.