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Hello Ian, unfortunately its not that easy to transmit the voltage signal independently, the way the more accurate real power calculation works is that its sampling at high frequency (52 samples per 50Hz sinus) and multiplying the instantaneous voltage and current reading together for every sample. Some ideas have been discussed for transmitting the phase timing information between two emontx's but it gets quite complicated.

You do get both readings, the first is realpower and the second apparent power, there's more on this in the AC power theory section of building blocks here: http://openenergymonitor.org/emon/buildingblocks

What Trystan is saying is that you can measure voltage in one emonTx and current separately in another, do the rms calculation on each and multiply the two rms values together to get apparent power (VA).

In order to measure real power, you need to measure both voltage and current at the same instant (or very nearly), multiply the two together and then take the average. There's no practical method to synchronise two emonTx's and to transmit the instantaneous readings from one to the other in order to do the real power calculation. Note that real power = V x I x cos(Φ) only works for pure sine waves,  the average of instantaneous V x instantaneous I works for a distorted wave too.

Thanks for the explanations - that makes sense!

Sounds like drilling through the wall might be the best option!

Thanks,

Ian