The FDA generally bases nutritional labels off a 2,000 calorie a day intake.

Doing the conversion from calories to watt hours I get: 2.326 Wh or 8373.6 joules (watt seconds)

If you were to compare that to a cell phone battery (3.3v) the battery would have a storage size of 0.705 Ah or 705 mAh. Most smart phone batteries are larger. If you were to divide that power over 24 hours thats an average usage of 29 mA.

Given such a small amount of power it hardly seems possible the human body needs so little energy. Consider a flight of stairs offers a height difference of 3 meters (approx 9ft). Google says the average American weighs around 81.9kg (180lbs). Gravity is 9.78m/s^2.
http://www.arachnoid.com/gravitation_equations/power_energy_gravity.html
81.9kg * 9.78m/s^2 * 3m = 2402.946 joules of energy to go up the stairs.

8373.6 joules / 2402.946 joules = 3.84

Four trips up the stairs would be enough to consume 2000 calories.

We haven't factored in walking around, and the energy required to keep your body temperature up over the lower ambient temperature, or anything else.

Are my calculations off?

To simplify things, when you see "calories" in relation to diet, it's actually kilocalories.

I figured it was something simple like that. That would account for the seemingly huge gap in my calculations.

Thanks

That is why the food calorie is correctly spelled with a capitol 'C.' 'Calorie' vs. 'calorie.'

your cells also have a source of energy known as autophagy, which is essentially the cell recycling parts of itself. Pretty hard to calculate daily energy contribution from that though

A barely related note- FDA allows nutrition labels to be off by up to 20%.  

On a barely related note, a sort of sanity check, a human athlete (i.e., a bicyclist) can put out about 0.25 HP (186 W) and do it for extended periods of time.  Put out more and the duration drops.  Put out less and you can go longer.  

Cycling is one of if not the single most energy efficient means of transportation we know.

I believe our basal metabolism burns more calories than anything we do.

Since the standard lab method (actual burning in a flame) does NOT take into account how our body extracts energy from the food the numbers have significant inherent errors.

And basal to maintain body temp is the big energy hog.

Basic and organic chemistry.

kCal for food.  In fact my chem prof threw the exact same type problem at us.  I got it because I have an athletic background and have used a power meter when racing and training as a cyclist.