Please consider that the impedance is expressed as series equivalent circuit. The real chip can be more realistically modelled as C parallel R circuit.
Considering that the source impedance is outside the circuit of interest, I vote for V=sqrt(P*R).
Anyway, the series equivalent circuit isn't real. Inside the chip you have load capacitance in parallel to the real "energy harvesting" load. So if you want to get an idea of expectable voltage levels, you'll refer to parallel equivalent circuit.
If -20 dBm is the input level at the antenna of impedance 50 ohms, and if its matched, then you only get -23 dBm at your load, and then its only 15.8 mV.