I don't know if you're looking for a pure-IC solution (no idea if so), but if you're using or can use Redpower, you could set up a tube link between two batboxes(or MFEs, or even MFUs) carrying 1+ batteries(or crystals). By varying the timings and/or numbers of energy carriers, this could give you a lot of control over how much power gets to the system; in this way it would technically be on a separate circuit, but drawing power automatically from the main one. You could set up a similar system with Buildcraft too, but you'd have less control. With either, avoid building the system across chunk lines; rarely tubes and pipes will drop items on edges as a chunk loads or unloads.
If you want your machine to draw a little power when checking for work, but then a lot when it actually has work, is there a way you can monitor its output to see when it's actually working or not? If you could do that, you could mix the above system with an EU-Splitter cable; trickle-feed it power from batteries when idle, then open a direct connection if it detects work is actually being done. If there's no way to detect when it's out of work to do, you could hook it up to a redstone or (preferably) redpower timing system.