3. Cost of Electricity & Energy Usage

4. Energy Use Recall: Measured in Watts (W)
Electrical power is the rate at which electrical energy is produced or consumed in a given time
Measured in Watts (W)
SI Unit for measuring electrical energy usage is kW•h

5. Calculating Energy Use
Energy Use = Power × Time
Power: kW
Time: hours (h)
∴ Energy: kW/h
1000 W= 1 kW

6. Example
A laptop uses a 60 W adapter when plugged in to charge. Calculate how much energy the laptop uses in a year if it is used for 10 hours a day.
Given:
Power= 60 W/1000= kW
Time= 10 h × 365= 3650 h
Energy Use= ?
∴ the laptop uses 219 kW•h in a year.
Energy Use = Power × Time
= (0.060 kW) × (3650 h)
= 219 kW•h

7. Cost of Electricity
The price of electricity changes through the day:
- Weekdays from 9 AM to 5 PM is the most expensive because the demand for electricity is the greatest
- The cost of electricity during peak times can be 3x more expensive
- Using electrical energy outside of peak times (ie. running the dishwasher at night) helps you save money and reduce pollution
Cost to Operate = Power Used × Time × Cost of Electricity

8. Example
A laptop computer uses a 75 W adapter when it is plugged in. Electricity costs 5.6 ¢/ kW•h. Calculate how much it would cost to operate the laptop for 1 year for 24 hours per day.
Given:
Power= 75 W = kW
Time= 24 hours/day × 365 days = hours
Cost of Electricity= 5.6 ¢/ kW•h
Cost to Operate= ?
Analysis:
Cost to Operate = Power Used × Time × Cost of Electricity
= kW × 8760 h × 5.6 ¢/ kW•h
= 3679 ¢
∴ it would cost 3679 ¢ (or $36.79) to operate a laptop computer for 24 hours per day for 365 days.