Basic Watt Question

[Chapstone]

Im new to electronics but have been enjoying my introduction to it via Arduino. Electricity may be the most confusing thing in the world... I have a simple question that i can seem to find a simple answer to:

If a watt is per second and a fridge uses about 600 watts how come my electric bill isn't off the charts?
600*60seconds*60minutes*24hours*30days = 1.5million kWh
Clearly I'm missing something...

Secondly if both amps and watts are units of how much power something is using why isn't my electric bill in kilo amp hours?

Any thoughts would be much appreciated.

 

[MikeMl]

You pay for kWh, not watts.

If a 100W light bulb burns for 8 hours, the amount of energy used is 800Wh or 0.8kWh. In my area, where the rate is $0.12/kWh, that would cost about $0.096 or 10cents.

If a 600W fridge ran continuously for 30days, that would be 600W*30days * 24h/day =432000Wh = 432kWh

432kW*$0.12/kWh = $51.84 for 30 days of continuous operation. Fortunately, the duty cycle of a fridge is less than 25%, so about $13 per month.

 

[crutschow]

Watts are a measure of power and are not per second. A Watt is one Joule per second with a Joule being a measure of energy. You pay for the energy you use. This could be given as the number of Joules you use but it is commonly given as the number of kWh (kilowatt-hours) you use (equal to 1000W * 3600s = 3.6M Joules).

Amps are a measure of current, not power. You multiple amps times volts to get power (times the power factor for AC circuits).

Does that help?

 

[MrAl]

Hi,

Just to add a little to the other already very good posts here...

Imagine you have a refrigerator that uses exactly 1000 watts when it runs normally to cool the inside chambers (we'll talk about heating the inside chambers next though).

1000 watts is exactly equal to 1 kilowatt. That's it, no time units involved here.

Now if that refrigerator runs for 1 hour, that means it has used up 1 kilowatt hour (1 kwh) of energy.
If it ran for 2 hours, that means it used 2 kilowatt hours.
If it ran for 3 hours, 3 kilowatt hours, etc., etc.

So if it ran for 24 hours it would have used 24 kilowatt hours. If the door was left open, it would probably run for that 24 hours a day, and over 30 days it would use up 30*24=720 kilowatt hours of energy. That's a significant amount of energy usage.

But luckily the door is always closed after use, and with the insulation inside the door, walls, top and bottom, the refrigerator does not have to run that long every day. It could run as low as 3 hours a day depending on how good the insulation is. That brings the total down to 3*30=90 kilowatt hours per month. At 10 cents per kwh that means the total cost over the month is 9 dollars.

But then there is the defrost cycle, which might run for 5 minutes twice a day. The energy usage then depends on the heater element wattage, not the refrigerator motor. Now if the heater element uses 500 watts and stays on for only 5 minutes two times per day, that's 10 minutes per day, and 10 minutes is one sixth of an hour. So we have another 500 watts for 1/6 hour times 30 so that's 2.5 kwh which isnt that much additional energy really. This part varies quite a bit though depending on the size and ratings, but you could measure it and monitor your refrigerator over one or two days and see how long it runs in the defrost mode.

The main point though is that 'watts' itself does not involve time. If the refrigerator unit uses 1000 watts now, it uses 1000 watts tomorrow, and 1000 watts next year, etc. That never changes. What changes is the time it runs for not the wattage itself. The time it runs for is what ends up costing money.

So although the units uses 1000 watts every second, it does not mean that it uses 1000 watts per second, because it uses 1000 watts for any time period: every second, every microsecond, every millisecond, every hour, every week, every month, every year, etc., because it never changes.

Note that 1000 watts is simple written as:
1000 watts,

and that 1000 watts per second is written as:
1000 watts/second,

while 1000 watt seconds (energy) is written as:
1000 watts*seconds,

and it is the energy that you pay for not the watts itself.

It is true however that a unit that uses more wattage runs up the bill higher, but that is because the watts goes up and so the watts*seconds follows that.

 

[Chapstone]

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:

So although the units uses 1000 watts every second, it does not mean that it uses 1000 watts per second, because it uses 1000 watts for any time period: every second, every microsecond, every millisecond, every hour, every week, every month, every year, etc., because it never changes.

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![/QUOTE]

 

[Ian Rogers]

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!

I thought that!! Mine is @ 0.12p which made the UK really expensive.... @0.35c makes much more sense..

 

[crutschow]

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.

That sounds like a good habit to me.

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?
...................................

You are still trying to mix up the units. As I previously noted, watts already has time in its definition (Joules per second) so to add time again would be redundant and actually be incorrect. Thus 100W per second technically means that the power is changing by 100W (either up or down) each second. A 100W lightbulb uses 100W for every second, minute, hour, or millennium that it is on. Certainly if it's on for one hour then it uses 100Wh of energy (not power). So to say it uses 100 watts for each hour its on is correct but conveys redundant (no new) information.

Look at another example of power, a horsepower (where 1HP equals 746W). A motor puts out say 100HP. You don't say it puts out 100HP per hour. You just say it puts out 100HP.

 

[audioguru]

I think it is very sunny in Hawaii so electricity can be made with solar panels. It is probably windy so windmills can also be used.
Here in Canada we make cheap electricity with waterfalls and nuclear energy.

 

[Nigel Goodwin]

Here in Canada we make cheap electricity with waterfalls and nuclear energy.

I'd agree with waterfalls - but there's nothing 'cheap' about nuclear energy

Interestingly I delivered a washing machine last week to a house, the address was 'Mill Farm Close' - 'mill' being the operative word.

He had a small water wheel, shared between him and next door (the boundary line runs straight down the middle of the wheel pit), which feeds a 4KW generator.

He's lived there 20 years, but only recently fitted the generator, the wheel pit was covered by a concrete slab, but when they removed it the wheel pit was in perfect condition.

They simply fitted a metal frame in the pit and the wheel fits to that, the angle of the feed can be adjusted for different water conditions (it's a metal overshot wheel).

He had three little wireless LCD meters stacked together (presumably his neighbour has the same), one was showing 2.8KW from the generator, the second 0.6KW (his consumption) and the third 0.3KW (his neighbours consumption). He said it was too expensive to feed the excess to the grid, for what they pay, so the excess power was sent to convector heaters upstairs in both houses.

As you can perhaps tell from the detail, I was quite intrigued

 

[MrAl]

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.

 

[NorthGuy]

Fridge doesn't run at 600W all the time. It cycles on an off. When the motor is on, it may consume 600W. When the motor is off, it consumes very little.

Therefore, it doesn't consume 15kWh per day. Depending on the fridge, it may be from 1 to 5kWh per day.

 

[MrAl]

Hi NorthGuy,

Yes, but it appears that the main problem here is the interpretation of the word "Watt".

If we have to travel 1 mile, we have to travel 1 mile. If we do it in 1 minute, 2 minutes, 3 minutes or 100 minutes it's still that same 1 mile. The mile itself does not change.
If we do it faster we can say we traveled at a faster RATE, but that doesnt change the fact that it was always a constant 1 mile that we had to travel.

 

[audioguru]

The problem is that NorthGuy measure distance in kilometers, not miles.

 

[Chapstone]

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.

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.

Okay. I think I have it now. So Watt is a rate and not an amount. Similar to speed. By saying I'm going 60 miles per hour, I know only how fast I'm going not how much distance i've traveled. Not until I say 60 miles per hour for 2 hours do i have an amount. So Watts is a rate and not until I say 60 watts for 2 hours do i have an amount of energy i've used. So any time i see a wattage rating i can interpret that as merely a rate but then would have to think about how much time that item would actually be on. The fridge thing makes sense know. I didn't realize how little the fridge actually 'ran'.

So tell me if I have this one right: It seems like the energy company bills me for (kWh) kilo watt hours for the sake of keeping everyone on the same page. Because watt's are a rate they could potentially charge me for kilo watt days (kWd) or mega watt weeks (mWw). But similar to speed we could just as correctly say that my car goes 88 feet per second as we could say that it goes 60 miles per hour. They are the same rate but for convenience sake we talk in terms of miles per hour since thats how most people think in terms of cars and speed.

What do you think?

 

[NorthGuy]

They are the same rate but for convenience sake we talk in terms of miles per hour since thats how most people think in terms of cars and speed.

What do you think?

I'm with audioguru. Most people think in kiolmeters per hour rather than miles per hour

Your first post was actually almost correct. The only mistake you made is you calculated Watt-seconds, then called them Watt-hours. The correct calculatuon would be:

600*24hours*30days = 432 kWh

Except your fridge consumes less than this, because it often "travels" slower than 600W.

 

[MrAl]

Okay. I think I have it now. So Watt is a rate and not an amount. Similar to speed. By saying I'm going 60 miles per hour, I know only how fast I'm going not how much distance i've traveled. Not until I say 60 miles per hour for 2 hours do i have an amount. So Watts is a rate and not until I say 60 watts for 2 hours do i have an amount of energy i've used. So any time i see a wattage rating i can interpret that as merely a rate but then would have to think about how much time that item would actually be on. The fridge thing makes sense know. I didn't realize how little the fridge actually 'ran'.

So tell me if I have this one right: It seems like the energy company bills me for (kWh) kilo watt hours for the sake of keeping everyone on the same page. Because watt's are a rate they could potentially charge me for kilo watt days (kWd) or mega watt weeks (mWw). But similar to speed we could just as correctly say that my car goes 88 feet per second as we could say that it goes 60 miles per hour. They are the same rate but for convenience sake we talk in terms of miles per hour since thats how most people think in terms of cars and speed.

What do you think?

Hello again,

I agree with that really, and you'll note that the units sometimes change as you can see from some of the other posts where different units of distance are used. But regardless of the units, as you noted, the measurement we are talking about is a rate and requires another measurement in order to calculate anything concrete.

I have seen this question come up about maybe 20 times since around the year 1998. Even though that doesnt seem like much it's much more than other questions, and that's only what i myself have seen personally, not the entire internet of forums.
I think the reason it gets a little confusing at first is because time is considered to be always passing, and it seems like a separate thing from the physical reality we experience around us in daily life. It seems like everything is happening at a per second rate more or less. But really the physical reality is intrinsically linked to time in classical physics and that's closer to what we experience than anything else. Anything that moves through space also moves through time, which is not hard to understand, but even things that do not appear to move through space still move through time, so it sort of appears that everything should have a rate. When we break it down in physics though we try to separate those things that are not moving vs those things that really are moving, in our frame of reference. It's an interesting thing to think about.
Otherwise we might end up with statistical data such as the humorous:
"The big rock is sitting there at the rate of 1 sit per second" <chuckle>