How to offset AC voltage with varying input?
Thanks to vofa's comments I found the below Positive Unbiased Clamp.



As per the referenced web page:


  In the negative cycle of the input AC signal, the diode is forward biased and conducts, charging the capacitor to the peak negative value of VIN. During the positive cycle, the diode is reverse biased and thus does not conduct. The output voltage is therefore equal to the voltage stored in the capacitor plus the input voltage, so VOUT = VIN + VINpeak.


This circuit will do the job.