I am trying to develop a non-contact whole-house power usage monitor.
I have reviewed jasonT's project, which uses AC clamps to measure current usage.
However, from reading this post on how meters measure power, and this post on apparent vs. real power, it seems I need to be able to measure the instantaneous voltage as well as the instantaneous current, in order to calculate an accurate "real" power usage. From the first referenced post:
Real power is calculated by multiplying the voltage and current at any instant (one pair of samples). Apparent power can be calculated by measuring the amplitudes of the voltage and current over the course of one or more cycles and multiplying the two results together.
(I'm wondering if they got it backward! Real power = requires measurements over multiple cycles, but Apparent power = one instantaneous VA measurement?)
My question is this: how can I measure the instantaneous voltage in a non-contact manner? I understand that current can be measured inductively with a clamp, which is just a simple transformer which outputs a voltage proportional to the current flowing through the conductor.
If I'm not mistaken, it seems that using a capacitive measurement technique would be correct (like in an electrician's voltage detector probe), but I can't fully wrap my head around what such a circuit would look like. Would it be just a simple wire placed near the mains conductor, at a fixed distance, with only one end connected to an amplifier (for example, similar to this circuit)? If so, how would such a circuit be tuned to the correct proportional output?
If anyone could point me to any resources or suggestions I would be grateful.