Sure, if you in the USA this gadget seems to be popular there: http://www.p3international.com/products/p4400.html
You plug gadget to wall, comp to gadget, set display to 'Watts', then wonder on how little PIII draws in fact even at full steam.
This side of the pond I use: http://www.velleman.eu/products/view/?country=nl〈=en&id=374522
Notice that these gadgets measure Watts AC 'from the wall'. As PSU efficiency is never 100%, and in oldie SMPSs can be as low as 70% or even less, the real Watts DC that the comp draws are always less than what the gadget shows.
AC Watts from the wall * PSU efficiency = Real comp DC draw
So when my PIII relic draws 93W AC from the wall as shown by the gadget, with the low ~70% efficiency of the ancient (but restored) Oh Deer PSU the real DC draw is:
93 * .70 = 65W DC
Pay no attention, that's just FUD spread by GPU makers to cover their asses, they recommend overkill PSUs to minimize computer barbecues (because most cheapo PSUs can't really deliver the watts advertised on the label).
Many of those online 'PSU calculators' do the same heavy overclaiming.
Total system draw with a 6200 onboard (on a Pentium 4 520 2.8GHz system, a CPU which draws quite more than any PIII), Watts AC from the wall:
http://techreport.com/review/7447/nvidia-geforce-6200-graphics-processor/14