Why is electricity sold in kWh and not in joules?

Answer 1

I can imagine two reasons. The first is that a Ws (watt x second, equal to joule) is

a fairly small unit. However, considering only this, a MWs = MJ (million Ws = million

joule)) could be used instead.

The other reason is that you will most likely calculate the time you turn on a

light-bulb, a computer, etc. by the hour, rather than by the second. If a certain

device - say, a computer - uses 200 Watts (= 0.2 kW), and you keep it on for 3

hours, you can multiply that directly, to get the number of kWh - without a lot of

conversions. In this example, 0.2 kW x 3 hours = 0.6 kWh.

However, if there were a tremendous grass-roots movement in favor of Joules,

a change-over would be easy enough for the public utilities. Since the kWh and

the Joule are both units of energy, it would be a relatively simple matter to just

re-scale the numbers on the meter face and on your monthly bill. When the man

comes around to read your meter, instead of writing down, "243 kWh" in his note-

book, he could just as well write "874,800,000 J" and bill you accordingly.

Similarly, the astronomer, instead of writing down "4.2 LY" in his notebook for the

distance to Proxima Centauri, could just as well write "11,451,620,000,000 miles".

It's purely a matter of personal preference and convenience. Most astronomers

have agreed on the "LY" for interstellar distances. Joules are still most convenient

in the Physics lab, but most power utility providers have agreed on kWh for the

electrical energy shipped to your house.