An average person uses approx. 1500-2500kcal/day. Since one kcal equals 4148J in SI units, that's between 6.2-10.4MJ per day. A day has 86400 seconds, which brings us to an average power consumption of 72-120W... about as much as a light bulb. :-)
Physical exercise varies between light (300kcal/h) at an additional 350W to very strenuous at probably six times as much, i.e. almost 2kW in chemical energy being consumed. As you can see, even light exercise will increase the metabolic rate of the human body enormously, which is good for ones health.
Since the efficiency of our muscle tissue is relatively low (18-26%), workout equipment that tells us our "power output" is usually calibrated around 15-20%, i.e. for a mechanical power output of 100W, the equipment will estimate the number of calories burned in the range of 500-600kcal/h.
This is only a rough estimate, of course. A precise measurement requires wearing a mask which can capture the CO2 that we breath out, which is a reasonably reliable chemical indicator for how much chemical energy we actually consume. The ratio between mechanical power output and chemical power consumption will vary from person to person and between different activities.
Another important way of looking at the power consumption of the human body is for environmental purposes. If we could power ourselves by sunlight directly and if the process was thermodynamically efficient, we could just about support our base metabolism by sunbathing all day long. If we could electrically recharge, we would require approx. half a dozen solar panels to get by. However, since we need chemical food sources and the synthesis of sugars, starches, protein and fatty acids in plants is very inefficient, a human being requires at least about a third of an acre of land to support his food needs by growing crops and a lot more if we want to have a diet rich in animal protein.