You want either BTUs or kilowatt hours. 1 kilowatt hour is equal to 3,412 BTUs.
There are approximately 125,000 BTUs in a gallon of gasoline.
You could also use joules, but that unit is not commonly used for the purposes of expressing electricity use (after all, your electric bill is in kilowatt hours, not joules).
The Wikipedia page gave the equivalents for all fuels and that’s what I went with. I think equivalents is better for the purpose of discussions on comparing fuel and their efficiencies.