Power consumption of iMac
It depends on which model of iMac it is as the various models have differents types of RAM - for example DDR2, DDR3, LPDDR3 and LPDDR4. In particular make note that some models use LPDDR RAM, where the "LP" stands for Low-Power.

The latest mid-2017 iMac uses PC4-19200 DDR4 SO-DIMM RAM modules. Here the difference in power usage between 8 GB and 16 GB is approximately 3 watts.

The difference between in power usage in having 1 x 16 GB, vs 2 x 8 GB, vs 4 x 4 GB modules is negligible.

Note that the mid-2017 27" iMac consumes about 217 watts when the CPU is maxed out, so the RAM power usage is very small compared to the rest of the system.