How much a desktop computer uses when left on all the time? I take care to switch off lights and do not leave tvs on standby in order to save on electricity but as my desktop takes so long to boot up, I tend to leave it on but don't know how much this is costing me.
There's no harm in turning-off your screen, which will save you some power. Also don't have any screen-savers running as they keep the CPU active.
To really minimise the power usage put the machine into "sleep" mode.