Why are Watt Hours not Watts per Hour? Are they the same thing?
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They are not the same thing.
Watt Hours is power over time.
Using 10 Watts for 5 hours is 50 Watt Hours of energy. 10 Watts for 100 hours is 1,000 Watt Hours.
Either way, it’s 10 Watts of power, but using it longer requires more Watt Hours of energy.
For example, a light bulb is rated in Watts, but a battery is rated in Watt Hours.
A 100 Watt Hour battery could power a 1 W bulb for 100 hours, or 20 W bulb for 5 hours.
Think about a big tank of water.
Now imagine I hook a hose up to it and let water drain out for an hour.
The hose is “a thing that lets water out at a specific rate”. The water that drained is “how much water that hose let out in an hour”.
Watts are like the hose. They’re how much energy gets spent per unit time. But if I asked you “how many hoses per hour does this bigger pipe drain?” that question is weird, because we want “amount of water” not “amount of hoses”.
Same thing with Watts. It says “this much energy moves per unit time”. There’s already a “per time” in it. So if you divide by time again, things get weird.
What makes more sense is “If this pipe drains as fast as 2 hoses, how much water do I get if it is open for 3 hours?” Each hose let out “1 hose-hour” per hour, so this pipe (2 hoses) multiplied by 3 hours drains 6 hose-hours of water in 3 hours. (The answer will be even weirder due to gravity and some other Physics but that’s another story.)
That’s the real trick. The phrase “per hour” means DIVIDE by time. But a Watt-hour is what you get if you MULTIPLY Watts by time.
“Watts” are a name we gave to a unit of “energy per second”, it’s not an amount of energy, it’s an expression of how fast you’re using it. So saying “energy per second per hour” doesn’t really mean anything useful. That would be like saying “miles per hour per hour” for driving a car.
But if you drive 60 miles per hour for 1 hour, you will have gone 60 miles. In the same way, if you use 1 Watt (1 energy per second) for 1 hour, you will have used 1 Watt-hour (or 3,600 energy, which we call “Joules”).
To put it another way, you’re multiplying a *rate* (Watts) by a *time* (hours) to get an *amount* (Watt-hours, or Joules). That’s not the same as dividing a rate by a time, which doesn’t really give you anything meaningful.
“Watts” are a name we gave to a unit of “energy per second”, it’s not an amount of energy, it’s an expression of how fast you’re using it. So saying “energy per second per hour” doesn’t really mean anything useful. That would be like saying “miles per hour per hour” for driving a car.
But if you drive 60 miles per hour for 1 hour, you will have gone 60 miles. In the same way, if you use 1 Watt (1 energy per second) for 1 hour, you will have used 1 Watt-hour (or 3,600 energy, which we call “Joules”).
To put it another way, you’re multiplying a *rate* (Watts) by a *time* (hours) to get an *amount* (Watt-hours, or Joules). That’s not the same as dividing a rate by a time, which doesn’t really give you anything meaningful.
“Watts” are a name we gave to a unit of “energy per second”, it’s not an amount of energy, it’s an expression of how fast you’re using it. So saying “energy per second per hour” doesn’t really mean anything useful. That would be like saying “miles per hour per hour” for driving a car.
But if you drive 60 miles per hour for 1 hour, you will have gone 60 miles. In the same way, if you use 1 Watt (1 energy per second) for 1 hour, you will have used 1 Watt-hour (or 3,600 energy, which we call “Joules”).
To put it another way, you’re multiplying a *rate* (Watts) by a *time* (hours) to get an *amount* (Watt-hours, or Joules). That’s not the same as dividing a rate by a time, which doesn’t really give you anything meaningful.
Imagine machines that pump water. You might want to compare these to each other by the amount of water they can pump. So you invent a new unit for pumped gallons per minute, you call it a galmin. A machine with two galmins is twice as powerful as a machine with one galmin. But later you might want to know how much water the machine pumped in an hour, so you calculate 2 galmins * 60 minutes = 120 galminminutes. This is redundant, but all manuals are written with galmins in mind, laws already exist and whole industries only work with galmins, so instead of using gallons, you use galminminutes, even though those mean the same.
This might seem stupid, but it is what happened when steam engines became popular. People wanted to compare engines and so invented a unit to measure their power: the Watt. Only later did they want to calculate the energy, that “flows” through the engine. It took mankind some time to understand, that this “energy” was a real thing. Nowadays we think of energy as a real thing, we understand that for example a full battery is full of energy and an empty one is not. But we already build everything around Watt as the unit for power, now we hat to define energy as Watt * hours an engine is running = Watt Hours.
Imagine machines that pump water. You might want to compare these to each other by the amount of water they can pump. So you invent a new unit for pumped gallons per minute, you call it a galmin. A machine with two galmins is twice as powerful as a machine with one galmin. But later you might want to know how much water the machine pumped in an hour, so you calculate 2 galmins * 60 minutes = 120 galminminutes. This is redundant, but all manuals are written with galmins in mind, laws already exist and whole industries only work with galmins, so instead of using gallons, you use galminminutes, even though those mean the same.
This might seem stupid, but it is what happened when steam engines became popular. People wanted to compare engines and so invented a unit to measure their power: the Watt. Only later did they want to calculate the energy, that “flows” through the engine. It took mankind some time to understand, that this “energy” was a real thing. Nowadays we think of energy as a real thing, we understand that for example a full battery is full of energy and an empty one is not. But we already build everything around Watt as the unit for power, now we hat to define energy as Watt * hours an engine is running = Watt Hours.
No, watts per hour and watt hours are not the same thing. Watts per hour would be W/h and watt hours is W*h.
Top explain what both mean, I’ll first explain what both components mean:
Hours are a measure of time (not so hard I guess)
A Watt is a measure of power. It says something about the rate at which energy is transferred.
A Wh (watt hour) is energy (just like calories and joules). Energy is the ability to change things. Like heating up stuff or move an object.
Here is an example to show this.
Heating up a pot of water takes energy. You transfer energie from a heatsource into the water. It takes a 1000 calories (or 1.16222222 watt hours) to heat up 1 kilogram of water with 1 Kelvin. This transfer of doesn’t happen instantaneously, it takes time. If it takes 1 hour to change the temperature of this pot of water, then the power with which this water is heated would be 1.16/1= 1.16 watt. If you want to do it faster, you need to increase the power.
So energy is power applied for some time (Wh=W x h, or in your school textbook: E=P x t) and consequently; power is energy over time (Wh/h=W or in your school books it would be P=E/t)
So what would W/h be?
This would mean that power would change over time in a linear way. To stay with the same example as above. If you would gradually turn the knob of the stove to get the water to heat up faster than you would change the power over time which is weird to do in a linear fashion.
Imagine machines that pump water. You might want to compare these to each other by the amount of water they can pump. So you invent a new unit for pumped gallons per minute, you call it a galmin. A machine with two galmins is twice as powerful as a machine with one galmin. But later you might want to know how much water the machine pumped in an hour, so you calculate 2 galmins * 60 minutes = 120 galminminutes. This is redundant, but all manuals are written with galmins in mind, laws already exist and whole industries only work with galmins, so instead of using gallons, you use galminminutes, even though those mean the same.
This might seem stupid, but it is what happened when steam engines became popular. People wanted to compare engines and so invented a unit to measure their power: the Watt. Only later did they want to calculate the energy, that “flows” through the engine. It took mankind some time to understand, that this “energy” was a real thing. Nowadays we think of energy as a real thing, we understand that for example a full battery is full of energy and an empty one is not. But we already build everything around Watt as the unit for power, now we hat to define energy as Watt * hours an engine is running = Watt Hours.
Watt hours are a unit of energy or work. A Watt is a unit of power which is the rate of doing work. Watt hours or actually kilowatt hours are used to measure a quantity of electrical energy used. The cost of electricity is based on measuring this. A Watt is measure of how fast work (force times distance) can be done. Car engines could be rated in Watts rather than horsepower. Watt hours is like how much gasoline you burn over a period of time.
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