A question came up at the solar talk on Tuesday…there is confusion on standard power conversions. What does it mean when a power plant is 15 MW? How about my PG&E bill that rates everything in kw h?

In science class, energy is generally labeled in Joules (sometime kilojoules – KJ or megajoules – MJ). Energy is time independent…it’s just energy.

But we never hear about Joules in normal life. We always hear about Watts (sometimes kW – kilowatt, MW – megawatt).

1 Watts = 1 Joule/second.

So when you hear about a 100 W bulb, that means it ‘s using 100 Joules of energy per second. A 15 MW plant (that number represents peak output) can produce 15 million joules per second.

Now to your electric bill. You get billed on a kw*h (kilowatt hour) basis.

1 kw* h= 1000 watts hour= 1000 (Joules/second) * hour = 1000J/sec * 60 sec/1 min * 60 min/hour = 3,600,000 Joules.

So your electric bill actual does state how much energy is being used. Just with a bastardized unit of measure.