Just to give you a feeling about BTUs:
The majority of homes which have electric water heaters use heaters that are 40 and 50 gallon size and 4500 watts. 4500 watts adds just over 15,000 BTUs/hour. A gas water heater with an input of 40,000 BTUs and an efficiency of 80% adds 32,000 BTUs/hour. So even the smaller BTU gas water heaters recover much quicker than most electric water heaters.