Wok and electric hob - which ring to use?
The larger ring will transmit more energy, but spread over the size of the ring; so the density is probably roughly the same.

So using an element bigger than your pot/wok bottom is basically just a waste of heat/energy.



Some supporting evidence added by DMA57361:

I've recently gotten a free energy monitor, which tells me the current power consumption of my entire home. So, with a bit of experimentation, I have worked out the approximate power output of each ring on my hob, and this backs up that they have approximately the same output density.

Position     |  Diameter  |  Area   |  Power  |  Density
             |   cm       |   cm²   |   kW    |   kW/cm²
-------------+------------+---------+---------+-----------
Back-Left    |   18       |  254.5  |  1.90   |  7.46
Back-Right   |   16       |  201.0  |  1.45   |  7.19
Front-Left   |   14       |  154.0  |  1.25   |  8.11
Front-Right  |   18       |  254.5  |  1.84   |  7.24