 # How to calculate electric energy cost of common household items

Electricity from the utility company is delivered to your house and connects to a meter that measures the amount of electricity used. The electricity is sent through the meter and routed to the service panel which split the electricity into the circuits throughout the house to power the appliances, lights and wall outlets.

Each month the utility company collects this usage information from the meter to determine how much electricity to charge you for. Electrical power is measured in watts and is charged by the kilowatt-hour. Kilowatt-hour is simply how many 1000′s of watts are consumed per hour.

### Let’s review all the terms you need to know to understand electrical cost.

Kilowatt is watts in units of 1000 (1 kilowatt = 1000 watts).

Watts (wattage) are calculated by multiplying the voltage by the current.  (Voltage x Current = Watts)

Kilowatt-hour (kWh) is how many 1000 watts are consumed in a one hour period.

Utility Rate is the cost per unit of electricity, measured per kWh (kilowatt-hour).

## Calculating Electrical Energy Cost

To calculate energy cost the “kilowatts” are multiplied by “time in hours” x “utility rate”. The average utility rate for electricity in southern Iowa seems to range between 11 -14 cents per kWh. I’ll use 12 cents for the calculations.

Just a quick note on time, since the rate is based on hours, converting minutes to hours may be required for calculation purposes (Total usage time in minutes/60 = hours). For example: 45 minutes would equal .75 hours.

Let’s look at two examples for calculating energy cost:

### Example 1 – 60 watt light bulb

The power consumed by the light bulb is 60 watts, or .06 kilowatts. Using the utility rate of 12 cents per kwh, each hour of use will cost slightly under 1 cent (\$.0072) If the light bulb is on for 10 hours per day then the cost per day is 7 cents for that single bulb (.06kw x \$.12 x 10 hours = \$.07).  Considering how many light bulbs operate in a household it’s easy to see how the lighting cost per month can add up to a few dollars.

For example: I have 6 recessed 65-watt light bulbs in the kitchen that were on today for 16 hours – I like to hang out in the kitchen. The total wattage for all the light bulbs is 390 watts, or .39 kW. At 12 cents per kilowatt-hour, my cost for lighting the kitchen today is \$.75. If that were a typical day, then my projected monthly cost would be \$22.50.  Time to rethink leaving the lights on!

### Example 2 – Space heater

The typical wattage of a space heater is 1500 watts. In kilowatts that equals 1.5 kW. Using the 12 cent kWh energy rate, the cost per hour for the heater is 18 cents. That doesn’t seem like much until you consider how that adds up over time.

 Hours per day Cost per day Cost per Month 5 hours \$.90 \$27 10 hours \$1.80 \$54 15 hours \$2.70 \$81 20 hours \$3.60 \$108

## Calculating Amperage

Sometimes you need to know the ampere draw of a device, especially for determining the correct circuit size. If you know the wattage then you can determine ampere draw by dividing the wattage by the voltage.  For example, the 1500 watt heater plugged into a 120 volt circuit will draw 12.5 amps (1500/120 = 12.5).