Resource comparing power consumption of different water heaters?
You have qualified your question to just Electric On-Demand water heater.  Most energy efficient is not necessary the same as lowest cost when evaluating different energy source.  
I do not believe you will find a source that meets the “&quot;real-world&quot; energy consumption of different electric tankless water heaters under different conditions.  Any ideas?”.  
Mainly because it is costing the manufacturers enough just to test under the specific guidelines required by the DOE.  Using the DOE energy consumption guide for all appliances sold in the US will provide you the relative ranking of all the  tankless water heater.  This
list of electric tankless water heaters provides a list of 115 models sold or previously sold in the US.
From this listing you can make a list of what features you considered most important, i.e., operating cost, recovery, on-demand rate.