Why are Watt Hours not Watts per Hour? Are they the same thing?
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No, they’re the opposite.
“Watts per hour” means how many Watts in an hour, “Watt Hours” are the number of watts times the number of hours.
As Power is energy per unit time, that makes Watt Hours a unit of energy. Which is useful in a world where we are used to the cost per KiloWatt of energy and see power ratings on appliances. If a heater is 2KW and we use it for one hour, we know we’ve used 2 Kilowatt hours of energy.
Interestingly this would make Watts per hour a unit per time…. Per time! Which is most commonly seen in acceleration: meters per second, per second. It would imply a rate of change of power, which could be useful in some scenarios.
No, they’re the opposite.
“Watts per hour” means how many Watts in an hour, “Watt Hours” are the number of watts times the number of hours.
As Power is energy per unit time, that makes Watt Hours a unit of energy. Which is useful in a world where we are used to the cost per KiloWatt of energy and see power ratings on appliances. If a heater is 2KW and we use it for one hour, we know we’ve used 2 Kilowatt hours of energy.
Interestingly this would make Watts per hour a unit per time…. Per time! Which is most commonly seen in acceleration: meters per second, per second. It would imply a rate of change of power, which could be useful in some scenarios.
No, they’re the opposite.
“Watts per hour” means how many Watts in an hour, “Watt Hours” are the number of watts times the number of hours.
As Power is energy per unit time, that makes Watt Hours a unit of energy. Which is useful in a world where we are used to the cost per KiloWatt of energy and see power ratings on appliances. If a heater is 2KW and we use it for one hour, we know we’ve used 2 Kilowatt hours of energy.
Interestingly this would make Watts per hour a unit per time…. Per time! Which is most commonly seen in acceleration: meters per second, per second. It would imply a rate of change of power, which could be useful in some scenarios.
Imagine a “water hour”. It’s how much water you have poured into a bucket, in total, over the course of an hour. 24 water hours would be how much water you have poured in a day, for example. A “water hour” would therefore be a VOLUME of water. How much in total.
Water per hour would be how much water your tap can push out in an hour. It’s a rate. A “speed” if you like.
One is a measure of how much you’ve done IN TOTAL (no matter whether the tap was dripping or gushing).
One is a measure of how fast you can do something at any given time, i.e. how fast the tap was running at a given moment. (But don’t forget the water “rate” can change all the way down to zero by you turning the tap on and off… but the water “volume” will always increase).
They are “related” in a very loose way but only because in your head you wouldn’t try to fill a swimming pool with a dripping tap, or a thimble with a firehose.
Watt hours are what you did in total.
Watts per hour were how fast you were doing it at a particular moment (a rate)
Imagine a “water hour”. It’s how much water you have poured into a bucket, in total, over the course of an hour. 24 water hours would be how much water you have poured in a day, for example. A “water hour” would therefore be a VOLUME of water. How much in total.
Water per hour would be how much water your tap can push out in an hour. It’s a rate. A “speed” if you like.
One is a measure of how much you’ve done IN TOTAL (no matter whether the tap was dripping or gushing).
One is a measure of how fast you can do something at any given time, i.e. how fast the tap was running at a given moment. (But don’t forget the water “rate” can change all the way down to zero by you turning the tap on and off… but the water “volume” will always increase).
They are “related” in a very loose way but only because in your head you wouldn’t try to fill a swimming pool with a dripping tap, or a thimble with a firehose.
Watt hours are what you did in total.
Watts per hour were how fast you were doing it at a particular moment (a rate)
Imagine a “water hour”. It’s how much water you have poured into a bucket, in total, over the course of an hour. 24 water hours would be how much water you have poured in a day, for example. A “water hour” would therefore be a VOLUME of water. How much in total.
Water per hour would be how much water your tap can push out in an hour. It’s a rate. A “speed” if you like.
One is a measure of how much you’ve done IN TOTAL (no matter whether the tap was dripping or gushing).
One is a measure of how fast you can do something at any given time, i.e. how fast the tap was running at a given moment. (But don’t forget the water “rate” can change all the way down to zero by you turning the tap on and off… but the water “volume” will always increase).
They are “related” in a very loose way but only because in your head you wouldn’t try to fill a swimming pool with a dripping tap, or a thimble with a firehose.
Watt hours are what you did in total.
Watts per hour were how fast you were doing it at a particular moment (a rate)
Let’s say you have 20 amp hours of energy storage in a battery.
That could be 20 amps for an hour or 10 amps for 2 hours or 1 amp for 20 hours. Amps over time.
Amp hours is more like ‘how much fuel you have in the tank’.
Actual energy flow measured this second would just be amps.
Or watts. (Volts * amps = watts). Watt hours is kinda similar but factors in voltage.
Let’s say you have 20 amp hours of energy storage in a battery.
That could be 20 amps for an hour or 10 amps for 2 hours or 1 amp for 20 hours. Amps over time.
Amp hours is more like ‘how much fuel you have in the tank’.
Actual energy flow measured this second would just be amps.
Or watts. (Volts * amps = watts). Watt hours is kinda similar but factors in voltage.
Let’s say you have 20 amp hours of energy storage in a battery.
That could be 20 amps for an hour or 10 amps for 2 hours or 1 amp for 20 hours. Amps over time.
Amp hours is more like ‘how much fuel you have in the tank’.
Actual energy flow measured this second would just be amps.
Or watts. (Volts * amps = watts). Watt hours is kinda similar but factors in voltage.
It seems like some of the top comments are missing a piece of low hanging fruit:
Joule is a measure of energy.
Watt is a measure of power.
Power = energy/time (Watts or Joules/second, same thing)
So a Watt hour is (energy/time) x time. As you can see this returns back something that is just a unit of energy.
The reason we talk about energy in terms of Watt hours is because it’s something we’re familiar with. Notice how all of our electronics and appliances are rated for power consumption in terms of Watts (or kiloWatts [kW]).
My microwave draws 1000 Watts when it’s operating, so if I run it for the course of an hour I’ve used 1000 Watt hours or 1 kWh. This is relatable and I have context when I read my utility bill. Even though Joule is the proper unit of energy, I have very little conceptual idea how much a Joule is in terms of everyday life.
Last thing I’ll say is that there are some other instances when we talk about a quantity in terms of the product of two units. For example, you may have a car that can produce a certain amount of torque. We typically measure torque in terms of foot pounds. This one is a little bit quirkier, but it’s done for the same reason.
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