I already pay to have my computer on all day, 2 U2's drawing another 4W off of the already running 600W PSU is a net zero change.
I'll just pretend it's my electric company giving me back 0.1% of my bill per month. In BTC.
600W is the upper output limit of the PSU. It doesn't mean you are drawing that much - if you are, it's too close to the limit for comfort.
I have a 750W PSU, but even with with the GPU, CPU, two block eruptors, and a Jalapeño all running flat-out, it draws only about 135W.