How do I work out the cost benefit of optimisation?

I want to figure out how much money I'd save if I optimise some part of my web app. If I 开发者_开发问答save 100 cpu milliseconds over 50K calls to the app, how much electricity is that not using in a day? How about over a year?

I've tried to find some figures thru google, but my googling mojo is failing me at present.

You can't calculate something that specific. You can only conduct an experiment and see what happens.

But honestly I would rather spend time refactoring code for better maintainability and adding new features the customers will like and pay for, so that I won't have to think about electricity.

When "optimizing" it is always important to focus on what you want to "optimize" - in this case, your electricity bill. I would not even bother looking at changing code in an attempt to affect your electricity bill. I would look at the computer's power supply, cooling fans, heat sink, etc. and optimize those things for energy efficiency (buy new, more efficient components). More than likely it will cost less than several hours of a software engineer "optimizing" code for energy efficiency.