If the load will always be resistive, like light bulbs or heater elements then a "precision rectifier" and a simple linear equation will do the job.

If there are inductive loads, the resulting waveform will not be a sine wave, and you'll need to do an R-M-S conversion. Almost impossible with a PIC, unless that's the only thing the PIC is doing.

There are several RMS converter chips available that give a DC output so you can use a linear equation again. I like the LTC1966, but that's just because it's the only one I've figured out how to use. I'm not good with hardware.
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