Watt Hour

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chris kennedy

chris kennedy

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Miami Fla.
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60 yr old tool twisting electrician
I'm searching this site for a recent thread about converting watts to watt hours. Any one remember that one?
 
1 watt/hour is 1 watt consumed continuously for a period of 1 hour


For instance, a 10 watt load on for a period of 30 minutes would be 5 watt/hours
 
mdshunk said:
1 watt/hour is 1 watt consumed continuously for a period of 1 hour
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That sounds to easy. I'm looking to see how much 7200 watts of HID lighting burning for 11 hours a night costs.
 
power factor and line losses excluded, and assuming 9 cents a kilowatt hour:

((7200 * 11) / 1000) * .09 = $7.28

Don't forget about the power the ballast consumes above and beyone the wattage rating of the lamp. It's more accurate to snap an amp clamp around the circuits as installed and see where you're at in that regard.
 
iwire said:
7200 watts per hr x 11 hours =79,200 watt hrs or 79.2 KWH
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So I am to understand that a 1000 watt lamp consumes 1000 watts an hour?

Put an amprobe on that and at 120V I see it at 8 Amps at that instant. Call me kookie but I'm having a brain far_ here.
 
Yeah, 8 amps at 120 volts is 960 watts. Close enough to 1000. Keep it on for an hour, and you'll have consumed 1000 watt/hours or 1 kilowatt hour.
 
mdshunk said:
Yeah, 8 amps at 120 volts is 960 watts. Close enough to 1000. Keep it on for an hour, and you'll have consumed 1000 watt/hours or 1 kilowatt hour.
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OK, but if I'm consuming 960 watts at the instant my amprobe is on it, for the rest of the hour I'm not consuming anything?

I know thats not true, I just can't picture how this works.
 
chris kennedy said:
OK, but if I'm consuming 960 watts at the instant my amprobe is on it, for the rest of the hour I'm not consuming anything?

I know thats not true, I just can't picture how this works.
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The only thing "watts" and "watt hours" have in common is watts in the name.

A 1000 watt load is consuming 1000 watts at every small "slice" of time that you grab a measurement. It's only 1000 watt/hours when it's on for a whole hour.

Maybe that's clear as mud, but maybe there's a teacher on here that has a clearer way to say it.
 
Careful now:

Careful now:

The expression, "watts/hour" or "watts per hour" is meaningless. It is simply "watts". The watt is a unit for electrical power, and power is the RATE at which energy is converted to some other form, e.g., heat.

To be exact, 1 joule of energy converted in 1 second is defined as 1 watt of power.

The watt-hour is another unit of electrical energy, e.g., a 1000-watt heater operating for 1 hour converts 1000 watt-hours or one kilowatt-hour of electrical energy to heat--3713 BTU to be exact.

Just remember that,

energy converted = power x time

Also remember that you must keep the units straight!!
 
chris kennedy said:
So I am to understand that a 1000 watt lamp consumes 1000 watts an hour?
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No, it consumes 1000 watts. When it's been on for an hour, it has consumed 1000 watts for an hour. We call that one kilo-watt/hour, or 1 KWH.

If you have a load of 500 watts, it consumes 1/2 of a kilowatt. If it's been on for one hour, it has consumed 1/2 KWH; in two hours, 1 KWH.

Watts, like volts and amps, are instantaneous values. We use power to perform work, which requires both a level of power and a duration; aka energy.

The electric meter is fundamently a clock whose speed is determined by the amount of power (KW) passing through it in any given period of time.

The power company sells us energy (KWH), in the form of electrical power (KW), over a period of time (H). It calculates power using voltage and current.
 
rattus said:
Also remember that you must keep the units straight!!
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I don't know if I'm adding anything here, but the way I was taught to keep things straight is to include the units in the equation.

So if you have a car running 60 MPH it looks like:

60 Miles
1 Hour​

That's all that this particular fraction can do, because the units are different. So, looking at the kilowatthour problem, it's the same but different, since it's multiplication instead of division:
​

1000 Watts x 1 Hour = 1000 Watt*Hours = 1 KWH​
 
He's got it!

He's got it!

georgestolz said:
I don't know if I'm adding anything here, but the way I was taught to keep things straight is to include the units in the equation.

So if you have a car running 60 MPH it looks like:

60 Miles
1 Hour​

That's all that this particular fraction can do, because the units are different. So, looking at the kilowatthour problem, it's the same but different, since it's multiplication instead of division:
​

1000 Watts x 1 Hour = 1000 Watt*Hours = 1 KWH​
Click to expand...

You've got it George!

And,

60 miles/1 hour = 88 feet/second.

Loosely speaking, we say we use power, but strictly speaking we use energy, and power is the rate of energy usage. Power may be expressed either as an intantaneious value or as an average value, but NEVER as an RMS value.
 
Actually yesterday we had to work late, till 9:00 PM I told one of my men he needed to be back at 3:00 AM for replacing a defective Phase Failure relay on the Main Switchboard and he said "WATT HOUR"
 
Wait a minute. A Watt-hour is not the same as 1 W/h

A Watt-hour is one Watt times one Hour, which is a unit of energy

Watts are units of power. Power is energy/time.
A Watt-hour is (energy/time)*time = energy
 
Twodollar said:
Wait a minute. A Watt-hour is not the same as 1 W/h

A Watt-hour is one Watt times one Hour, which is a unit of energy

Watts are units of power. Power is energy/time.
A Watt-hour is (energy/time)*time = energy
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They are the same. While one watt per hour is a rate, it's also an amount of energy: a watt/hour.
 
Nuh-uh Larry:

Nuh-uh Larry:

LarryFine said:
They are the same. While one watt per hour is a rate, it's also an amount of energy: a watt/hour.
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The watt is the rate of energy transfer or conversion. It is defined as one joule per second.

One "watt/hour" indicates an increase in power of one watt in one hour, but I have never seen the expression so used. So for the electrician, it is meaningless. I doubt that any engineers use it either!

In other words, "watts/hour" indicates an acceleration of power

"watts/hour x hours" = watts not "watt-hours'

Gotta keep those units straight! The Hubble telescope malfunctioned because the engineers didn't pay enough attention to the units.
 
A Joule is a unit of Energy
A Watt is a unit of Power (rate of energy)

1 Watt =
1 Joule
-------
Sec

So, a Watt*Second =
1 Joule
------- * Sec, or, 1 Joule
Sec

A Watt*Hour, then =
1 Joule
------- * 3600 Sec = 3600 Joules
Sec

1 kWh =
1000 Joules
---------- * 3600 Sec = 3 600 000 Joules
Sec
 
chris kennedy said:
I'm searching this site for a recent thread about converting watts to watt hours. Any one remember that one?
Click to expand...

I suppose Chris has his answer by now... but for Pete's sake people -- it ain't that complicated.

Take the watts. Take the hours. Multiply them together. Voila -- "watt hours"
 
tallguy said:
I suppose Chris has his answer by now... but for Pete's sake people -- it ain't that complicated.

Take the watts. Take the hours. Multiply them together. Voila -- "watt hours"
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But we love this stuff. What else would we do? :D
 
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