I would like to preface this by saying; thank you
for helping me understand electricity better. I have a bad habit of needing to understand something
and not just have surface knowledge.
Ummm...MrAI this may have blown my mind:
I guess i'm stuck on conventional rates. With a conventional rate there is always a 'per': my car
goes 60 miles per hour. Without the 'per hour' the 60 miles becomes meaningless. But what i
understand you saying is that with electricity the 1000 watt has meaning outside of a per hour or per
time definition.
To go back to the lightbulb explanation that MikeMI used. He went directly from the lightbulb uses
100 watts to the lightbulb uses 800Wh in 8 hours. That sounds like a per should be in there. The
lightbulb uses 100 watts per hour.
Can i assume that everything that has a wattage would use that amount of watts in one hour? So my tv
that uses 300 watts, if i watched tv for 1 hour it has used 300Wh. Or is that an incorrect way of
thinking?
And furthermore you guys must have the most generous electric providers in the country because
electricity here is $.35/kWh and going up again in January!
Hello again,
That statement you quoted was meant to show that the 'watts' and 'time' are independent of one another. You can have watts without any time period. If you have a 100 watt light bulb the rating never changes regardless how long it runs for...it's always 100 watts. Another example of a rating is the voltage that light bulb requires. If it requires 120v then it requires 120v, it's not 120v per second. Even though the voltage is there over every second, it's not 120v per second because that would mean the voltage itself was changing, which it does not.
Lets see what happens when we try to do that anyway...
If it was really 120v per second, then after the first second it would be 120v, then after the second second it would be 240v, then after the third second it would be 360v, etc. So the voltage would be increasing. But it is constant, so we just say "120 volts" and that's it.
Same with the wattage. If it is 120 watts now, it is still 120 watts 1 second from now, 2 seconds from now, 3 seconds from now, etc. It NEVER changes (under the assumed conditions of constant voltage which is usually the case for appliances).
Going back to the basic units, 1 Watt (W) is defined as 1 Joule per second (J/sec). So the Watt is a measure of the rate of energy consumption of an appliance for this example. The Joule is a measure of energy, and the second a measure of time, so we have in units alone the equality:
1 Watt=1 Joule/second, or
1 W=1 J/sec, or more simple written in simple units as:
W=J/s
So when we have 1000 watts that means we are using energy at the rate of 1000 Joules per second, and since the Joule is a measure of energy we can also think of it as:
W=energy/time
In other words, the Watt is a measure of energy per unit time.
So how do we find the energy used then, over a certain period of time so we know what the electric bill will be?
Well, since the watt is energy/time, and the electric company bills in units of energy, we need to multiply that by time NOT divide by time. This is the way units work.
So if we have a 1000 watt appliance and we want to know how much energy it uses, we multiply that rating of 1000 watts by the time it is being run for and we get the energy used over that time period.
Leaving that 1000 watt appliance on for 1 hour means that we have used 1000 Joules of energy per second over a 1 hour period but we have to work in time units of seconds so we multiply 1000 times 3600 seconds and we get:
3600000 Joules of energy used over a one hour period, and converting this to watt hours we get:
1000 watt hours, and converting that to kilowatt hours we get:
1 kwh.
Following this, if the appliance is left on for various times we can make a little table:
1000 watts left on for 1 hour means we've used 1 kwh
1000 watts left on for 2 hours means we've used 2 kwh
1000 watts left on for 5 hours means we've used 5 kwh.
So you can see how simple this is. Watts itself is not energy, it is the energy per unit time.
As a final example, say we dont want to use that 1 kilowatt hour of energy for that 1000 watt appliance. We want to use that for a 100 watt light bulb instead.
Since the energy for running a 1000 watt appliance over a 1 hour period is 1 kwh, if we instead run a 100 watt light bulb we can actually run this for 10 hours and use the same energy of 1 kilowatt hour:
100 watts*10 hours = 1000 watt hours = 1kwh
The cost of energy is usually quoted in dollars per kilowatt hour, or cents per kilowatt hour, or other currency. The cost i quote in this thread is chosen to simplify the math only and is not meant to represent any real life cost in any country. So for example if i quote 10 cents per kwh and in your area it is 30 cents per kwh, then simply multiply all my cost totals by 3.