If I have some servers set to static high performance mode, is there a way to generate a report on what power savings mode would have done, if it were enabled? I am looking to generate reports of the form, on this day we used this many kWh that we didn't really need to, summed across the estate. Or an alternate method to do this?
['https://serverfault.com/questions/654064', 'https://serverfault.com', 'https://serverfault.com/users/58037/']
I'm unaware of any current server that holds this data and provides estimates based on it's history, nor any upcoming servers that do so. The only way to get close to a real answer would be to test it yourself with a power meter over a period of time with both settings. That said I buy HP servers and their 'balanced' power profile is expected to save at least 15% of power draw over their 'maximum performance' option.
Measure it on typical servers and calculate that on this base. And/or ask your hardware manufactorer.

Recent servers should be equipped with online power consumption measurement means and even statistics. (e.g. Dell PowerEdge x9xx or Rxxx models)