Fat in food (or beer-bellies) contains 9 food calories per gram, which is 9000 heat calories.

Taking 300,000,000 Americans, if we assume they are an average 15 kg overweight, and assume all excess weight is fat and assume perfect conversion of fat to energy...

3x10^8 x 15x10^3 x 9x10^3 = 405x10^14 cal = 4x10^16 calories would be burned to shed all that excess weight.

1000 calories = approximately 1 watt hour

The fat burning therefore = 4x10^13 watt hours or 4X10^7 megawatt hours or 40 terawatt hours

US total power generation = 300 - 400 terawatt hours per month.

So you could theoretically supply 10% of the US energy needs by the exercycle/generator method.

But what happens when everyone has lost their excess weight?

Of course the savings to the health care system would be enormous, after the initial wave of heart attacks were dealt with.