Electric Power and Energy

Electric Power is the rate at which work is done. (See also: What is Work, Energy and Power?) Electric Power is the rate at which electricity does work or provides energy. The SI unit of power is the watt, one joule per second.

Electric power is usually produced by electric generators, but can also be supplied by electric batteries. Electric power is usually sold by electric companies as the kilowatt hour (3.6 MJ) which is the product of power in kilowatts multiplied by running time in hours. Electric utilities measure power using an electricity meter, which keeps a running total of the electric energy delivered to a customer.

Definition and Equations for Power

Electric power, is the rate of doing work, measured in watts, and represented by the letter P. The term wattage is used to mean "electric power in watts." The electric power in watts produced by an electric current I consisting of a charge of Q coulombs every t seconds passing through an electric potential (voltage) difference of V is:

P = work done per unit of time = VQ/t = (V)(I) or Power = Voltage x Current or Volts x Amperes

where: Q is electric charge in coulombs, t is time in seconds, I is electric current in amperes and V is electric potential or voltage in volts

Electric Energy

Electrical Energy = Power x Time. The total amount of electrical energy used depends on the total power used by all your electrical devices and the total time they are used in your home.

Electical Energy is measured in kilowatt-hours

Energy =Power x Time or Kilowatt-hours = Kilowatts x Hours

One kilowatt hour is equal to 1000 watts of power used for one hour of time.

How to calcuate cost of electricity

From Con Ed Bill --"We measure yur electricty by how many kilowatt hours ((kWh) you use. One kWh will light a 100 watt bulb for 10 hours". "In 2015, the average annual electricity consumption for a U.S. residential utility customer was 10,812 kilowatthours (kWh), an average of 901 kWh per month. Louisiana had the highest annual electricity consumption at 15,435 kWh per residential customer, and Hawaii had the lowest at 6,166 kWh per residential customer."


How much energy and power will it take to run a 900 watt air-conditioner for 10 hours straight?

Solution: Energy = Power x Time = 900 watts x 10 hrs = 9000 watt-hours = 9 kWh.




A lot goes into understading what you are paying for. There is not just a cost for fuel but also delivery charges and fees for various services and taxes.

To help explain this we use an Actual Con Ed Bill for a small apartment in New York City Using Con Edison.


From Con Edison:

Electricity you used during this 30 day billing period fro Jan03 2013 to feb 02,2017

We measure your electricity by how many kilowatt hours (kWh) you use.

One kWh will light a 100 watt bulb for 10 hours.

February 02,17 actual reading 95175 kWh

Jan 03 , 17 actual reading 94838 kWh

Your electricity use was therefore 337 kWh

YOUR SUPPLY /CHARGES -- were 337 kWh @0.5282c/kwh (this is the charge for electricity supplied to you by Con Ed = $18.83

Merchant function charge - charge associated with procuring electricity credit and collection related activities = $1.41

GRT and other taxes = $0.48

Total Supply charges = $20.52 which comes to 6.1 cents/per kWh.


Basic service charge $16.38

This is the change for basic system infrastructure and customer related services, including customer accounting, meter reading and meter maintenance.

Delivery 337 kWh @11.0208 c/kWh = $37.14

This is the charge for maintaining the system through which Con ed delivers electricity to you.

System Benefit Change at 0.6706 c/kWh = $2.26

This recovers costs associated with clean energy activities by New York STate Energy Research Associate

Temporary NY State Surcharge 0.1246c/kWh =0.42

Covers new fees imposed by the state

GRT and other surcharges $2.87

Total delivery charges $69.5

YOUR SALES TAX @4.5000% collected on behalf of NY State = $3.58



What is the supply charge to run a 600 watt refrigerator for 24 hours (of ON usage) at a cost of 0.06 cents/kWh? Note: Refrigerators do not run continuously.

Solution: Energy = Power x Time = 600 watts x 24 hours = 14,4 kWh x 0.06 cents/kWh = 864 cents = $8.64

Test your Understanding:

1. Which of these statements is not correct
a) Power can be expressed as work done per unit of time
b) Energy can be expressed as Power x Time
c) The units for Power are Kilowatts/hr
d) Power can be described in terms of Volts x Amps = Watts

2. In 2015 the average electric usage was about
a) 1901 kWh per month
b) 10,081 kWh per month
c) 337 kWh per month
d) 901 kWh per month

3. What is the energy usage of a 1000 watt hair dryer used for 10 minutes straight?
a) 1000 watt x 10 minutes = 10 kWh
b) 1000 watts/10 min = 100 kW/h
c) 1000 watts x 10 minutes = 100,000 kWh
d) none of the above

4. What is the supply cost of using a 1000 watt hair-blower for 10 minutes straight assuming a cost 6 cents/kWh

a) 6 cents
b) 60 cents
c) $6
d) none of the above

5. What would be the delivery cost for the same hair blower at a delivery cost of 11 cents/kWh
a) $1.10
b) 11 cents
c) $1.10