The calculations are OK, but the assumptions are probably
unreasonable.

First of all, I doubt that you would need all 15 lights for 15 hours a
day, the year round. You can divide the lights into groups and
switch some on at a time. For example, I have some in the ceiling
away from the walls and some closer to the walls to give reflected
light. I probably have about a third of them on for many purposes
and more when needed.

I guess careful lighting design is important. I don't want to end up in the
situation where I am switching ligts on and off every 5 minutes.

Don't you like sex? ;-)

It doesn't usually cost this much ;-)

Being able to mix and select different types of light at different
times is quite appealing.

Is there a certain configuration that you use 95% of the time?

Colin