I am looking for a website the compares the "real-world" energy consumption of different electric tankless water heaters under different conditions. Any ideas?
The "real world" energy consumption of any electric tankless (lots of big fat wires and breakers) will almost directly match the amount of hot water you use, at what temperature, and what the water temperature of the supply is - they are nearly 100% "efficient" in that power going in is used almost entirely to heat water going out. Differences between units will be miniscule. Differences between the (US-Market) "Energy Guide" label and YOUR costs will depend on your actual use of hot water, your actual cost of electricity .vs. the amount assumed on that label (which is stated, at least) and your incoming/outgoing water temperatures .vs. what they assume. "Your real world cost" for any one of them will be pretty much identical, and may be higher or lower than the Energy Guide number (which is an attempt to come up with a "real world" number for comparison purposes.)
A plain electric resistance tank-type heater will (also) be almost indistinguishable in "real world operating costs" if you use hot water on a more or less daily basis, since modern insulation standards are quite high, and it will do so without requiring the typical 2 or 3 40A breakers to run it that an electric tankless does. "Standby losses" in current market electric tank heaters are very small.
That does not mean cost efficient in most markets, where a fuel based heater will usually cost much less to operate, due to comparative fuel/electricity prices, despite various real or perceived inefficiencies. Only where electricity is comparatively inexpensive does it win. Likewise, where your wintertime heat is not electric resistance (and perhaps even where it is given any cooling season to speak of) a tank-type heat pump water heater can be 200-300% efficient (in electric power terms - there is no magic, just borrowing heat from the air, and in wintertime your heat will work a bit harder as a result; but if your heat costs less to run than electric resistance, you'll still save money on heating water.)
Do be sure to insulate all your hot water lines and the cold water lines near the water heater if you are concerned with efficiency and operating costs.
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 “"real-world" 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.
When a tankless is operating, essentially 100% of the energy must go into the water. If even 1% went anywhere else, the unit would self-immolate! And why would it, when it has flowing water to carry heat away.
So your question seems silly.
But it’s not. The gotcha is standby aka vampire load. The 8 kWh question is, “What is the unit drawing the rest of the time?” That standby power is what you need to research.
It is inconsequential compared to active load, but it is on 24x7, so it adds up.