A PSU only draws from the wall what is required of it, plus 15 - 30% for inefficiencies (which is wasted in the form of heat). So if your motherboard, RAM, graphics cards, and drives (everything your PSU supplies power to) only need 250 watts, then power drawn from the wall will only be 250 watts (plus another 50 or so for waste). This is true regardless if your PSU is a 400W PSU, or 1000W PSU.
So if you can find the data sheets for all your hardware, you could get an idea just how much power you are using by simply adding the power specs.
CPUID and other HW monitors won't work as they only tell your voltages. You need the current value too to determine the wattage.
shame
oh well
Need to understand this is not really a computer issue - but a power issue. Also note that even if there was a program that monitored the current on the motherboard, drives and other devices are direct connected to the PSU, so would not be factored in with the motherboard. So the only solution is with a device such as suggested by stormy. But even still, monitors, speakers, and many external drives and readers have their own power connections, so they too would not be factored in a program running on the machine, and again, you would need a measuring device, as suggested by stormy, and connect all devices to an extension cord, then plug the cord into the meter.
Finally, to be accurate, the software or meter needs to be used with the system under load as the power requirements change drastically from idle to maxed out.
The alternative is for the PSU makers to include a meter in the PSU - and I think that is a good idea and you should start a letter campaign to Antec, Corsair, TT, PC P&C and some of the other big PSU makers.