Voltage proportional comparison (division?)
You can do this with an IC multiplier, however it will not be cheap. Also, note the bipolar supplies. 



I suggest reducing the input resistor (to get gain on \$E\$) and using another op-amp (maybe half of a dual) to amplify \$E_X\$ so the input signals are reasonably high. 

Alternatively, you could use an external ADC that has multiple simultaneously sampled channels. If memory serves, there are even a few microcontrollers that have that built in, for such applications as 3-\$\phi\$ energy metering.