Power vs Energy

At this point it is necessary to discuss two terms we will use quit a bit.  It is important to understand the difference between them for many aspects of the design.

Energy (aka Work)

Energy is just a measurement of the task performed.  

For instance, if you move a pile of rocks, by hand, you do a certain amount of work.  It does not matter of you moved the pile in an hour or a day.  The work is the same. 

Energy is measured in joules.

Power

Power is a measure of how fast the work was performed.

If you moved the same pile of rocks in less time, it takes more power, but the same amount of work.

Power is measured in watts.

How the Electric Bill is Calculated

Electrical power is simply the voltage multiplied by the current.  If I am drawing 10 amps at 220 VAC, I am using 2200 watts.  That is not the whole story, though.  The electric company bills you for the ENERGY you use, not (necessarily) the POWER you use.

They calculate the energy you use by determining not only how much power you are drawing, but also the length of time you are drawing that power.  In the above example, If you drew 2200 watts for an hour, the electric company would charge you for 2200 Wh.  If you drew it for 10 hours, they would charge you for 22000 Wh.

Finally, since energy usage is typically in the thousands of watt-hours, the electric company bills in kWh (thousands of watt-hours)

Ultimately, the electric company is calculating the number of joules of energy you are using.  

`1 J = 1 A * 1 V * 1s`

`1 W = 1 A * 1 V`

`1 Wh = 1 A * 1 V * 1 h`

`1 Wh =1 A * 1 V * 60 m`

`1 Wh =1A * 1 V * 3600 s`

`1 kWh = 1000 Wh = 1000 * 1 A * 1 V * 3600 s`

Therefore 

`1 kWh  = 1000 * 3600 * (1A * 1V * 1s)`

`1 kWh = 3, 600, 000 J`

As a result, the kWh unit is just a more compact way to express energy.

Another way to look at it

Suppose a drop of water represented a unit of work (joule.)  If I could measure how fast you were using water at your house (amperes,)  I could calculate the number of barrels (kWh) of water you were using.

One last Note

Energy is not only a measure of work, but it is also a measure of the propensity to do work.  For instance, a battery may not be supplying energy to anything, but it stores energy.  That can also be calculated.