It's all in a day's work (as measured in Joules)

The title is from the "Physics Man" sketch on The Now Show. Work is another word for energy and there are two different units for it.

The calorie (cal) is the amount of energy required to heat 1g of water by 1°C. This is a tiny amount of energy. The dietary Calorie (Cal) = 1,000cal = 1kcal.

The Joule (J) is the SI unit of energy. 1J = 1kg*m^2/s^2.
1Joule/sec = 1Watt (W).

1kcal = 4.186kJ.

At rest, an average human body uses ~1kcal/min = ~4,186J/min = ~69.8J/sec = ~69.8W.

The brain uses ~5g of glucose/hour = 18.75kcal/hour (1g of carb = 3.75kcals, usually rounded-up to 4) = 78487.5J/hour = ~21.8W.

The heart uses ~10W. The liver, kidneys, gut and lungs run continuously so they use energy all of the time.

Skeletal muscle uses a variable amount of energy using a variable proportion of fuels, depending on what you're doing with it. A chap called Steve sent me a spreadsheet of results in 2004 when he underwent a metabolic test on a stationary bike while breathing through a respiratory gas analyser, which calculated kcals burned and fuel usage by measuring Respiratory Exchange Ratio (RER).

At~1kcal/min (resting), he burned ~95% from fat (~0.11g/min), ~5% from carbs (~0.01g/min).
At 2kcal/min (12% max), he burned 100% from fat (0.22g/min), 0% from carbs (0.00g/min).
At 3kcal/min (18% max), he burned 100% from fat (0.33g/min), 0% from carbs (0.00g/min).
At 4kcal/min (24% max), he burned 99% from fat (0.44g/min), 1% from carbs (0.01g/min).
At 5kcal/min (29% max), he burned 48% from fat (0.27g/min), 52% from carbs (0.69g/min).
At 6kcal/min (35% max), he burned 62% from fat (0.41g/min), 38% from carbs (0.61g/min).
At 7kcal/min (41% max), he burned 58% from fat (0.45g/min), 42% from carbs (0.78g/min).
At 8kcal/min (47% max), he burned 46% from fat (0.41g/min), 54% from carbs (1.15g/min).
At 9kcal/min (53% max), he burned 42% from fat (0.53g/min), 58% from carbs (1.39g/min).
At 10kcal/min (59% max), he burned 44% from fat (0.49g/min), 56% from carbs (1.49g/min).
At 11kcal/min (65% max), he burned 38% from fat (0.46g/min), 62% from carbs (1.82g/min).
At 12kcal/min (71% max), he burned 41% from fat (0.55g/min), 59% from carbs (1.89g/min).
At 13kcal/min (76% max), he burned 37% from fat (0.53g/min), 63% from carbs (2.18g/min).
At 14kcal/min (82% max), he burned 30% from fat (0.47g/min), 70% from carbs (2.61g/min).
At 15kcal/min (88% max), he burned 14% from fat (0.23g/min), 86% from carbs (3.44g/min).
At 16kcal/min (94% max), he burned 0% from fat (0.00g/min), 100% from carbs (4.27g/min).
At 17kcal/min (100% max), he burned 0% from fat (0.00g/min), 100% from carbs (4.53g/min).

Over a wide range of intensities, the number of grams of fat Steve burned/min was fairly constant.

Note that 17kcals/min = 1186.6W, or 1.19kW. This level can be maintained for only a few seconds, as carbs are burned both aerobically and anaerobically, which exhausts PhosphoCreatine stores in muscles and also causes an accumulation of lactate in muscles. See Why do muscles hurt after exercise?

Muscle mass is very metabolically-active compared to fat mass, as one pound of fat mass burns only about 2kcal a day. See also Dissecting the Energy Needs of the Body – Research Review

Here's another Physics Man.