I.S. question

Source: Interactive Science Workbook 2A Enrichment Edition p.57 1(c)

The Question:
Ronald wants to reduce the cost of electricity,so he stops using the three air conditioners in his home.The power of each air conditioner is 1200W and
the average operating time for each of them is 120 hours per month.
Theoretically,he can save 380 dollars. However,the actual amount being is
less than the caculated value. Explain your answer briefly.(Hint:think about the cooling operation of the air conditioner.)