Watts consumed by digital multimeter = less than 1 watt 

I take it you mean use the digital multimeter to check PC wattage use. Yes, its possible, set to AC current measurement, then measure the current taken by the PSU and multiply it by 230 (or if you want to be so damn accurate, measure the voltage and use that instead). Perfect accuracy can be achieved if you're using a active PFC psu and a sine wave digital multimeter, but the accuracy of a normal digital multimeter is still pretty ok with an active PFC psu. With non active PFC psu, readings might not be accurate because you have to account for the phase shift in the current, which is pretty hard to measure using easily available equipment.

PS : to measure current you'll have to cut one of the AC power wires and put the multimeter in connected to both cut ends of the cable. DO NOT ever connect the multimeter directly to Live and Neutral on AC while in Ampere (current) measurement mode, if you're lucky you'll just blow the multimeter's fuse, if not you'll see smoke coming from the multimeter.

sorry for the misleading sentences. i mean calculate pc wattage use using digital multimeter.

that's sound hard. anyway, thanks for sharing the info. btw, how much the special watt meter like mr sniper's?
spam?

Thanks for pointing that out, though I'll have to clarify a bit.

Power factor should have an effect on current readings taken with a digital multimeter. If the power factor is poor, the current (based on the apparent power) which is measured by the DMM should be higher for any given power intake by the PSU-AFAIK the PSU will only use the real power component. So for non-PFC psu, the watt reading will be higher, which will not reflect the power consumption of the PSU. Its bad enough that we don't have a decent method of measuring the PSU's real life efficiency, having the power factor further compound the problem of getting accurate power consumption readings is just    (theoretically, its possible for the measurement to be as much as 2x off the actual value, assuming a power factor of 0.7 and efficiency of 75%, which are typical values for cheap PSUs)

Most affordable clampjaw meters don't have enough accuracy to measure the AC current used by a typical PC. Most PCs take up 1-2A only (even a fully loaded 1000W PSU will only take 5A at most), so a clamp meter must be able to read up to about 0.01A to be reasonably accurate to a few percent, and those are pretty expensive. If accuracy is any worse, might as well use an affordable DMM (which would have similar accuracy).

And yeah, a shunt works too, like you mentioned. But you still have to cut a cable to put it in.

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Added on June 24, 2007, 3:40 pmJust remembered the key reason why I said using an active PFC PSU is important-active PFC PSUs typically have power factors of 0.99 or better, which will mean the current waveform will be sinusoidal (assuming the AC voltage is sinusoidal-not likely to be perfect sine wave, but should be ok unless your AC supply is outright shit). A sinusoidal current waveform will allow even non-RMS DMMs to give decently accurate readings (my estimate is it would be less than 30% off, which is better than most of the affordable clamp meters)