You should have had the agreement down on paper from the beginning.

If things got nasty, there's no telling of how it would turn out.

Does your ownership agreement say _anything_ about this? Is it
listed as an easement?

I think it's _way_ too late to consider recouping the cost of the
initial installation. Chalk that up to experience, and don't do
it again.

You could purchase a Killawatt meter and monitor the power consumption
for a month or more, split the power consumption in half, and bill him
for that at the current going rate. This only works if the pump is
120V (and preferably plug-connected). Killawatts are only about $40.
At 240V, I dunno of any solutions that cheap.

As for maintenance - the common plumbing elements a well, pump,
pressure tank, and anything else his water goes through before it
goes off to his house. Like a water softener. He should pay half
the repair bills of any equipment in your place it goes through before
it gets to his. If he objects to that, you will have to consider
whether it's in your best interest to eat the costs, or go the legal
route.

Without some pre-agreement on maintenance, paper or otherwise, you
may well be screwed. A court may consider what little agreement
that there was to be a binding "contract", in which case you're stuck
with it. Legal advice is advised, but don't threaten/start proceedings
until you're ready to make a long term enemy.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.