Jeez, you gents like to sukkel.
Set the ADC to convert at 1kSPS, then make two buffers of 32 values at 16 bits.
Now get the ADC interrupt (since you don't have DMA) to push a sample into the buffer at each int.
When the buffer fills, you simply switch to buffer 2, and start processing buffer 1.
Run through and check for zero-crosses, isolate the points in the buffer, and you should have 20 points for a 50Hz signal.
Run RMS algorithm on the buffer.
Tune the code above, and its good for <1% accuracy. I've built a power monitoring system that does 17 channels at once, 16 RMS currents, and one RMS voltage. Total accuracy was at 0.3%