I don't think you can compare a propane burner under a kettle to a water heater - the water heater is designed for efficiency. Most of your heat from the burner under your kettle floats up the outside of the kettle and out into space. An electric element is completely submerged and 100% of it's energy is transferred into the water or wort.
There are other threads that have gone over this topic, and anyone who has used propane and have gone to gas I think will generally agree that electric is quicker and cheaper. This is from Kal's electricbrewery.com
What are the benefits of brewing with electricity vs. gas?
There are many benefits to brewing with electricity vs. gas:
Easier precise control of temperature
Safer for indoor brewing (no poisonous gases, no emissions)
Absolutely silent (the bigger gas burners required for brewing sound like jet engines)
Much more efficient use of energy (our 5500W heating elements produce ~20,000 BTUs of heat and will outperform an 80,000 BTU propane burner)
No tanks to refill
Cheaper to run (use $1-2 in electricity instead of approximately half a tank of propane per batch)
We don't know of one brewer who, after switching to electric, wished they were still using gas.
Here's another thread discussing costs and efficiencies of electric.
If you search the forum for 'gas vs electric' or 'cost per electric brew', you'll find loads of threads on the topic.