A BTU (British Thermal Unit) is a measure of heat energy defined as the amount of energy needed to heat one pound of water one degree Fahrenheit. That is, to heat a pint of water from room temperature (72 degrees F) to the boiling point would require 140 BTUs. Another way to think of it is that a BTU is approximately the heat generated by burning one wooden kitchen match.
One gallon of propane delivers about 91,000 BTUs (that is a lot of kitchen matches...) So at 75% efficiency, a gallon of propane will heat about 60 gallons of water from room temperature to the boiling point. One kWh of electrical energy delivers 3,412 BTUs, so to bring 60 gallons of water to a boil (even at 100% efficiency) would require 19.7 kWh of electrical energy.