Agree with all of gee4's points - and turning the lights off will help. Although I once remember working at an office where the exec office lights were timer switched so if boss sat there concentrating still for too long the lights went off and he had to wave his arms about ....

Not just from a cost saving, but from an environmental and positive company image/CSR standpoint energy conservation should be encouraged (switching off pcs properly, paperless, working from home etc).  If you go with the energy saving as well as cost saving, there are some great posters etc that you can get and put up - or ideally publicise it on the company intranet if you have one.

I'm coming to end of my MA in Environmental Policy (whilst working full time, same as I did with my BA), and it usually takes an admin person to change the 'little things don't matter' attitude.