### Calculating Energy Use

How is energy use of Home Appliances calculated?

You just learned in a previous discussion on power that:

or

$$\text{Energy}=\text{Power}\times \text{DurationofUsage}\left(\text{Time}\right)$$By modifying this formula slightly, we can determine Energy Consumption per Day:

Where:

**Energy Consumption**will be measured in Kilowatt hours (kWh) - like on your utility bills.**Power Consumption**will be measured in Watts**Hours used per Day**will be the actual time you use the appliance.

Since we want to measure Energy Consumption in Kilowatt hours, we must change the way Power Consumption is measured from Watts to Kilowatts (kWh). We know that 1 kilowatt hour (kWh) = 1,000 Watts hours, so we can adjust the formula above to:

**Energy Consumption / Day ( KWh ) = Power Consumption ( Watts / 1000 ) × Hours Used / Day**

#### Example 1: Calculating Energy Use of a Ceiling Fan

**If you use a ceiling fan (200 watts) for four hours per day, and for 120 days per year, what would be the annual energy consumption?**

Use this formula:

**Energy Consumption / Day ( KWh ) = Power Consumption ( Watts / 1000 ) × Hours Used / Day **

**Energy Consumption per Day ( kWh ) = ( 200 / 1000 ) × 4 ( hours used per day ) **

**Energy Consumption per Day ( kWh ) = ( 1/5 ) × 4 Energy Consumption per Day ( kWh ) =4/5 or 0.8**

So the Energy Consumption * per Day* is 0.8 kWh To find out energy for 120 days, do simple multiplication: 0.8 x 120 = 96 kWh

#### Example 2: Calculating Annual Cost of a Ceiling Fan

**If the price per kWh for electricity is $0.0845, what is the annual cost to operate the ceiling fan?**

#### Want Another Example?

If you use a personal computer (120 Watts) and monitor (150 Watts) for four hours per day, and for 365 days per year, what would be the annual energy consumption?

$$\begin{array}{l}\text{EnergyConsumption/Day}\left(\text{kWh}\right)\text{}=\text{}\left(270/1000\right)\text{}\times \text{}4\text{}\left(\text{hoursused/day}\right)\text{}\\ \text{EnergyConsumptionperDay}\left(\text{kWh}\right)\text{}=\text{}1.08\end{array}$$So the Energy Consumption per Day is 1.08 kWh. To find out energy for 365 days, do simple multiplication:

$$1.08\text{kWh}\times \text{}365\text{days}=\text{}394.2\text{kWh}$$The annual cost if electricity is $0.0845 per kWh would be:

$$\text{Cost}=\text{}394.2\text{}kWh\text{}\times \mathrm{\$0}.0845/kWh\text{}=\$33.30$$