Question

My little brother eats about 2500 kcal/day and doesn’t gain weight. Estimate his energy consumption per time, or power, in the following units. Please SHOW YOUR WORK. (a) W (b) kWh/day (c) erg/min (d) hp (e) BTU/s

Answer 1

a) W = J/s

2500 kcal = 2500*4.184 = 10460 kJ = 10460 *1000 J

per day so

1 day = 24*3600 = 86400 s

then

10460000/ 86400 J/s = 121.06 W

b)

kWh/day

change kcal to kWh

1 kcal = 860.421 kWh

then

2600 * 860.421 = 2237094.6 kWh ; since basis was 1 day

2237094.6 kWh/day

c)

erg/min

121.06 J/s * 60s / min =

7263.6 J/min

1 J = 10^7 erg

7263.6 *10^7 erg/min = 72636000000 erg/min

d)

hp = 745.6 W

121.06 W / 745.6 w/hp = 0.16241 HP

1 Hp = 0.7070715 Btu/s

0.16241 Hp = 0.7070715*0.16241 = 0.1148354 BTU/s