# Joule-hour is Watts * 3600?

Maybe this is a dumb question, but I am trying to get my units straight.

I have an electrical system which has its consumption metered every hour, so I need units that are graduated in hours.

So, given SI units, should I base it in "Joule hours" which would be Watts * 3600?

In other words, let's say I have a 100 Watt light bulb and it runs for 3 hours. Then energy consumption would be 100 * 3600 * 3 = 1,080,000 Joule-hours. Is that right?

• uh, a Watt-hour is 3600 Joules – Jasen Dec 31 '16 at 10:11

In other words, let's say I have a 100 Watt light bulb and it runs for 3 hours. Then energy consumption would be 100 * 3600 * 3 = 1,080,000 Joule-hours. Is that right?

No, there is no need to say anything other than joules.

Your lightbulb has consumed, over the 3 hour period, 1,080,000 joules of energy. It's as simple as that.

If you had a tap running to fill up a 1000 gallon container and it took one hour to fill the 1000 gallon container you wouldn't say you have used 1000 gallon-hours of water. That would be stupid.

Watts are units of power.

Watts multiplied by time gives energy.

Joules are energy units.

1 Joule is 1 Watt multiplied by one second.

If you multiply Watts by a time period in seconds, then you have the energy in Joules.

Your example is 100 Watts for three hours. That is 100 * 3600 * 3= 1080000 Joules.

It is also 300 Watt hours. That may be where the confusion is coming from.

You are skipping the conversion: 1 W = 1 J/s

$Energy = P\ t = 100W \times 3 hr \times \frac {3600 s} {hr} = 1,080,000\ W\cdot s$ $Energy = 1,080,000\ W\cdot s = 1,080,000\ \frac{J}{s}\cdot s = 1.08\ MJ$