Pre-determine the entry point of power into the HPWC, mine had a rear or side choice. The power line is very thick and stiff and can only bend in an arc of maybe a 2 foot diameter circle. Walk the path and mentally peg where that sucker is going to get routed from panel to unit.
The cable is tacked along the wall on studs. Clear the path. Mark the studs.
You need a specific diameter hole saw to cut a hole in side of HPWC to bring heavy power line into the side of the unit. You can only go in one side (left) of the unit and not the other. You could drill it out in advance for the electrician. He would appreciate that. If you don't drill into the guts an cables inside the unit. So do it right.
You need a stud to mount HPWC on the wall, or if on concrete, then use concrete anchors. If you have no stud, you need to mount a piece of plywood. Use 3/4" thick plywood. Prepare the site. Measure, mark... etc. Put your own anchors in, that's a pain for electrician.
These things trim time, if you're being billed by the hour and you value your time less (or want a certain quality / care put into those things).
Let the electrician source the electrical materials, they get it cheaper than you.
Definitely let them torque the nuts down on wire lugs, they'll use the appropriate lube at all needed connection points, and don't mess with those after the job is done. It's a lot power flowing (a whole house worth) and you don't want these connections heating up if disturbed. Call an electrician back if you need to do anything with the power supply line, or it's connections in the future.