I am going to use an emontxV3 with an RFM69Pi connected to a Raspberry Pi. I will power the emontx from a 9V AC adapter. I have the capacity to power the emontx 9V AC adapter from a UPS (the Raspberry Pi will be connected to the UPS). My question is whether that is a good or bad thing to do. I know the emontx uses the AC power to provide a voltage signal for Real Power calculations, so if I power it from the UPS, does this affect/defeat the real power calculations? Thanks.
Re: emontxV3 running off a UPS - good or bad?
I think it depends whether your UPS is an offline UPS or an online UPS. Offline UPSs pass the mains straight through to the load and only intervene when the mains is out of spec. In that case you should be fine, and when you have a blackout you'll have the joy of seeing 0W consumption being reported, rather than silence.
Online UPSs always generate their own output V sine wave and feed that to the protected loads. In that case I don't think it would work at all well, assuming your CT is out at the fusebox. It will be measuring a current that is relative to the grid V, while the V your monitor is measuring will be a completely unrelated signal (certainly in amplitude and probably in phase too).
Online UPSs are pretty expensive. Most typical, affordable residential UPSs are offline. A rough test would be to measure the voltage of your UPS output (with your OEM monitor even). If it bounces around quite a bit when your loads are relatively steady, it's likely to be the grid you're measuring, i.e. it's an offline UPS. If it's rock solid at 230.0V (or whatever your online UPS is set to) it's an online.
Another test would be what sort of noises does it make when you kill the power to it? On offline UPS will let out clicks as it switches across to its inverter. A power failure to an online UPS is a bit of a non-event, just like unplugging your laptop from the mains. It basically just keeps trucking, but stops charging the batteries and probably raises an alarm.
Re: emontxV3 running off a UPS - good or bad?
A cheaper alternative to a UPS for emonTx is to use a simply USB battery pack like this http://amzn.to/1STE5rr
Just leave the battery plugged in charging whilst you also power the emonTX from it, if the power goes the battery powers the emonTx for a few hours.