Union University
Union University Department of Physics

The Science Guys



Science Guys > April 2002

April 2002

Can I estimate how much it costs to run an appliance each month?

We often refer to our electric bill as our "power" bill, though power is certainly not what we are buying; we are buying energy. Scientifically there is a distinct difference between energy and power. Simply put you can think of energy as the product of the force on an object times the distance the object moves. Energy is calculated in units called Joules. Power on the other hand is the rate at which energy is used or the energy used divided by time to use that energy. Power is calculated in units called Watts (1 watt = 1 joule of energy used each second).

Appliances generally come with either power in watts or the voltage and amp rating written somewhere on them. To get the power multiply the voltage times the current in amps. Consider a 60 Watt light bulb. A 60 Watt bulb uses 60 Joules of energy each second it is on. To find the amount of energy actually used you must multiply the power in Watts by the amount of time the bulb has been on.

You are billed for the amount of energy that you use during the course of a month. But Jackson Energy Authority (JEA) doesn’t use units of Joules! Joules is a unit for physicists, but it isn’t practical for energy companies. It is more convenient for utility companies to use a unit called the kilowatt-hour, abbreviated kWh. To calculate the cost of operating any item we should then use kWh, so we can relate it to our utility bill.

Currently in Jackson electrical energy costs $0.05552 per kWh (about 5.5 cents per kWh). It’s just like buying gas at a cost of $1.15 per gallon- you pay so much per unit used. First, to get kW from watts divide by 1000. For the 60 W bulb, 60W/1000 = 0.06 kW. Assume the light is on 8 hours a day for 30 days, yielding 240 hours of burning time. The amount of energy used is 0.06 kW times 240h which equals 14.4 kWh of electrical energy consumed.

Now take the amount of energy used and multiply it by the cost per unit of energy. Thus, 14.4 kWh times $0.05552 per kWh equals $0.799, or 80 cents. This calculation is easily done for any appliance - just change the power rating to kW and multiply by how many hours you use the item each month. Next multiply the kWh by the cost per kWh, which can be obtained from your local utility. The actual cost of electricity varies from distributor to distributor. There you have how to calculate the cost to operate any device for specific period of time.

In symbolic form: total cost = power in kW x hours appliance is on x cost per kWh.