I have the normal 5v DC power input for my emonPi, along with the optional 9V AC sampler plug. I am only using the emonPi for measuring temperatures and the pulse counting meter. Is the 9V AC plug needed?
No. It’s there to sample the voltage waveform, current transformers sample the current waveform, and that’s how instantaneous power is calculated. If you don’t want that, and you have the 5 V USB power, there’s no need for a v.t. or c.t’s.