Easy start would be to have any of the desktop / laptops go into sleep / hibernation mode in the evening. Or have the turned off at night. Depending on how many computers you have that will save a TON. I mean aside from the file servers you don't need all of those computers running at night. Who is in the office between 7 PM and 6 AM? Probably no one but IT people (you
Secondly would be you could run some type of switch system that would turn off unneeded routers and switches at night. There are controllers out there that can stop / start power going to through objects (like a surge protector or just one device). Plug these into the unneeded network devices that are dumb and can't power themseleves off. Then set the timer for no power between a selected time frame. Problem being if someone comes in at 9 PM to run some reports off the server it wouldn't be accessible (maybe run a wirless option for evening so you are just running one wireless router that would only be on at night?).
Just a few ideas - I too had to do something similar for a small company I worked for.