Are you energy hungry? Want to
monitor your power consumption? Monitor your kilowatt/hour appetite using
the PC sound card as a dual channel analog acquisition board: it is a cheap
but effective way.
A sound card with stereo recording capability is needed, with 8 bits converters or better.
Tip: If measurement-quality voltage transformers are unavailable, then use two ordinary power supply transformers with series connected primaries to reduce distortion. Standard power supply transformers work near to saturation, un unwanted situation in this case. For the same reason, don't use them for supplying power to your circuitry. And don't forget to protect your PC from overvoltages, applying surge soppressors like VDRs.
Everything else is a matter of software. Sample 20 or more cycles, then compute the instantaneous power multiplyng the samples, W = V x A. Integrate over time to get the enenergy in watt-hours.
To get accurate results, you should linearize the two transformers using calibration tables. Then you have to null the phase lag introduced by the transformers (using a pure resistive load as reference). A typical transformer exhibits a phase lag of 3 - 7 degrees over its current range.
A lot of math is involved,
however external components are limited to a minimum.
But don't take it easy. The main difficulty arises from power factor, because a phase angle differences of a degree or two are difficult to measure. Power meters design is a task hiding many pitfalls. A well known load is a must for accurarcies better than 5 - 10%. Better to have a reference meter at hand (what about using the one provided by your electric company?), otherwise calibration can prove difficult.
DISCLAIMER: The information above is provided as an idea only for experienced designers. Use it at your own risk. Components involved can damage your equipements, cause injury to people and even be lethal. Always keep safety first.