If you can borrow a clip on ammeter from an electrician you can compare the them. You can also just integrate the readings over a day and then compare that to the electricity meter. The Efergy display units also do this using a running average. This of course assumes that you have a reasonably stable voltage. In most countries this should not be a problem. The Efergy meters measure current and multiply by the voltage you set to get the power, and the sound card solution will need to do the same, while the electricity meter measures power directly.