40 kW is a big electric use, but with a large family it isn't unreasonable.
This link:
Solar power system sizing for your solar electric project.
Explains more about sizing.
Give me a specification page, and I'll look over it.
Solar panels would be basically rated as the power at full sun. i.e. watts.
In order to get an idea of what the cells will produce you need the average insolation. Per day when the entire year is taken into account would give you the best possibility of reaching all of your goal.
The angle of the mounting and the roof area available will put constraints on your selection. Total cost has to be factored into payback to chose.
Check out
Home Power Magazine: Solar | Wind | Water | Design | Build.
In recent months there has been articles in the magazine Circuit Cellar on sizing.
I know your not ready for this, but tracking systems and pole mountings are available.
One thing that was discussed in one of the Circuit Cellar articles is the mounting of the panels on the roof. The method chosen was basically the same method used for plumbing vent stacks. They provided the mounting points and a solar mounting frame was attached to these. This was definitely clever.
Remember that the panels will likely have to be removed when the shingles are replaced.