Chapter 6.1, Problem 8EE

Practical Applications

8. Electrical Trades As explained in Example 7, excess energy generated by solar panels can often be sold back to the power company. Over a 6-month period, a household consumed 212, 150, –160, –178, –148, and 135 kWh (kilowatt-hours) of electricity from the power company. What was the net amount of electricity consumed by the household? (Express your answer as a signed number.)

Electrical Trades Excess electricity generated by solar panels can often be sold back to a power company. Over the course of a year, one particular homeowner using solar energy consumed the following monthly amounts of electricity according to her electric bill: –112, –42, 86, 108, 123, –65, 144, –122, –186, –114, 22, and 65 kWh (kilowatt-hours). A positive number means she purchased electricity, while a negative number means she sold electricity back to the power company. We can calculate the net amount of electricity bought or sold during this period by adding the 12 monthly amounts. Rearranging the numbers into a positive group and a negative group, we have:

Net amount = (86 + 108 + 123 + 144 + 22 + 65) + [(—112) + (–42) + (–65) + (–122) + (–186) + (–114)]

= (+548) + (–641) = –93 kWh

Because the net amount was a negative number, the electric company had to pay the homeowner for 93 kWh of electricity for the year.