If he uses 9.8 × 4186 = 41022.8 J a minute, and spends 180 minutes, he used 7384104 J throughout the entire climb
Watts is a unit which translates to Joules per second. So, to calculate power, all you have to do is divide the energy used by the time taken
So power consumed = 7384104/10800 = 683.7 W (4 sig. fig.)
