re: Efficiency of cloud computing is a giant myth.
Efficiency of private servers is even worse! You've really got to expect private servers to be used less than 10% of the time at most and even that is very hopeful when you consider they are only really used a third of the day and their access is very bursty. At least cloud servers might get more use from around the world.
The thing that is important with servers is to stop spending money on powering them when they're not doing anything useful. That's the really worthwhile saving rather than cutting down on maximum capacity.
I'm using iCloud for Notes and Reminders, Google Drive for email, calendar, documents. It seems that there isn't anyone out there that can do a reasonably good job on everything.
Servers are bought to handle essential peak time-critical loads and total cumulative work required over each day and each week not meaningless averages. Anyone who thinks otherwise has not worked in a real server environment having to do real work for a company.