In which case one of the home energy monitors given away by utilities should
do fine - there are plenty on ebay, with and without USB ability. You just
have to break out live and neutral and fit their current sensor around only
one of them.
That will probably be easier/neater that measuring voltage or time - they
already come with a control panel gadget where you can see your 'usage' over
time. Many of them have wireless current sensors so you don't need to
locate the control panel near the sensor.
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