Why are Watt Hours not Watts per Hour? Are they the same thing?
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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.
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.
Watt-hours are units of Watt times hour.
A Watt is a unit of “power” which is energy per time, commonly used to measure rate of energy use/flow. You multiply it by time (hour) to get the total amount of energy used in a period of time.
Watts per hour would be a rate of change of the rate of energy flow/use/etc.
Watt-hours are units of Watt times hour.
A Watt is a unit of “power” which is energy per time, commonly used to measure rate of energy use/flow. You multiply it by time (hour) to get the total amount of energy used in a period of time.
Watts per hour would be a rate of change of the rate of energy flow/use/etc.
Watt-hours are units of Watt times hour.
A Watt is a unit of “power” which is energy per time, commonly used to measure rate of energy use/flow. You multiply it by time (hour) to get the total amount of energy used in a period of time.
Watts per hour would be a rate of change of the rate of energy flow/use/etc.
“Per” usually means divided by. As in, A divided by B. But when you combine two units without using “per” it indicates multiplication, as in A times B. For example, “newton meters” comes from multiplying a number of newtons times a number of meters.
“Per” usually means divided by. As in, A divided by B. But when you combine two units without using “per” it indicates multiplication, as in A times B. For example, “newton meters” comes from multiplying a number of newtons times a number of meters.
A Watt already is “per time,” namely one Watt equals one Joule per second. “Per” means divided by, so 1W = 1J / 1s. It’s the rate of usage of energy. If you use that rate for an hour, that is a Watt *times* an hour, or a watt-hour. That’s an *amount* of energy, and in fact is equal to 3600 Joules, since there are 3600 seconds in an hour.
Think about Watts as similar to a speed. If you travel 100 kilometers per hour (100 km / h) for an hour, you go 100 km / h * 1h = 100km, since the h’s cancel out.
A Watt already is “per time,” namely one Watt equals one Joule per second. “Per” means divided by, so 1W = 1J / 1s. It’s the rate of usage of energy. If you use that rate for an hour, that is a Watt *times* an hour, or a watt-hour. That’s an *amount* of energy, and in fact is equal to 3600 Joules, since there are 3600 seconds in an hour.
Think about Watts as similar to a speed. If you travel 100 kilometers per hour (100 km / h) for an hour, you go 100 km / h * 1h = 100km, since the h’s cancel out.
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