At full load a 15 kW generator needs about 170,000 BTU/hour of heat energy assuming a 30% efficiency ratio.

Gasoline is about 115,000 BTU/gallon
Diesel is about 130,000 BTU/gallon
Natural gas is 100,000 BTU/therm
Propane is about 92,000 BTU/gallon

depending on what fuel you use and what the fuel cost is in your area should tell you the answer to your question, at least pretty close.

To put folks in the ballpark then, according to my last bill, I'm paying about .562 dollars per therm. If I multiply by 1.7 (to get to the 170,000 btu) I get .9554 dollars an hour to run a 15KW generator for an hour. But mine is 8kw, so call it about 40 percent less energy required, and my per hour cost is around .58 dollars an hour to run. Or $13.92 a day. Five gallons of gas in my old portaable generator (5kw) lasted about 12 hours. At local gas prices, that would be about 40 dollars a day. I have slightly over 110 hours on the stand-by generator so far.

My one concern about a standby generator (besides the one about not being able to afford one ) would be dependence on the gas company to reliably deliver fuel. Is it feasible to store fuel for a natural gas standby generator?

The problem with storing fuel is that it takes up a lot of room, and storing gasoline in quantity adds risk to your life, especially if done improperly. A bunch of 5 gallon cans of gas in the garage is not a safe thing to do. Diesel is much safer to store as it is not flammable.

NG is also much cheaper to run on than any other commonly available fuel. Many gas generators can be adapted to NG so they could run both ways. In fact many can be adapted to run propane as well so you can have 3 fuels available.

NG is the most reliable of all the utilities. It is not 100%, but it would be fairly easy to envision an event that would destroy your stockpile of gasoline or diesel that would not affect your NG supply.

I do not think there is a perfect answer to the fuel problem with generators.