How do you calculate a human's output of power?

well...depends on what the person is doing. The average person burns about 2000 calories a day. That can be calculated out into any number of things. If you want raw power output data about a specific task you would simply use a horsepower formula (550 ft·lbf/s) because:

1 hp ≡ 33,000 ft·lbf/min by definition

= 550 ft·lbf/s since 1 min = 60 s

= 550 × 0.3048 × 0.45359237 m·kgf/s since 1 ft = 0.3048 m and

= 76.0402249068 kgf·m/s

1 lb = 0.45359237 kg

= 76.0402249068 × 9.80665 kg·m²/s³

g = 9.80665 m/s²

= 745.69987158227022 W since 1 W ≡ 1 J/s = 1 N·m/s = 1 (kg·m/s²)·(m/s)

Since horsepower has nothing to do with a horse, and a horse is typically rated at about 18 horsepower, and a human is roughly 1/10th the size of a horse, your cheap answer is that a typical human is about 2 horsepower.