Can anyone explain why the i7 seems to need much more wattage than the i5 in the following review even though they have the same TDW?
The i7 utilises hyper-threading, is running 8 threads to the i5's 4, and runs at 500Mhz faster (4Ghz to the i5's 3.5Ghz).
TDW - Not sure what that is. If you mean TDP, then that is just guidance for heatsink designers, but will take into account case temps, airflow, altitude and anything that might affect the temps.
Thermal Design Power (TDP) represents the near maximum power a product can draw for a thermally significant period while running commercially available software.
TDP represents real-time usage over a given period of time, and not peak usage. It is the measure of what a cooling system will have to dissipate to ensure continued running of the system (under real-time conditions). It is not power draw.
If a less than adequate cooling solution is used, modern CPUs will engage the throttling feature (if available and enabled), in this case the 'Adaptive Thermal Monitor' if the CPU runs beyond the TDP for a given period of time.
I imagine that over-clocking is not factored into TDP values, in that stock cooling is not sufficient to keep CPUs cool enough under load for long periods of time.
The cooler used for the review is also used in CPU reviews where the TDP is 95W, so the cooler will probably be up to the task of the 84W TDP.
I'll try to sum it up. It doesn't represent power draw. It represents the amount of heat that needs to be dissipated by a cooler for continued running at a given stock level. Both CPUs in the article require the same dissipation of heat to run at their own levels, but they will draw different levels of power as they are different processors.
Notice that the more powerful, more power-hungry 4790k was running hotter at same clock speeds. The hyper-threading was probably the difference in temps and power draw.