The cycle for servers is 3 years, the cycle for the building is 30 years. What I'm trying to say is that if you can deliver more widgets per watt per sq ft by

The cycle for servers is 3 years, the cycle for the building is 30 years. What I'm trying to say is that if you can deliver more widgets per watt per sq ft by replacing the server than you go for that. If you can't why not keep the already depreciated server operating for another year. The incremental costs are mostly the power (and ac) plus the opportunity cost of putting a more efficient server in a year later. If it fails you just put in the newer server. If it doesn't you may save a bit.