this post was submitted on 13 Mar 2024
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Wait wait Wait, can you give me more on this kWh thing? I thought I understood this already.
A single kW is a unit of power, literally 1000 watts.
A kWh is a unit of energy, as in stored or delivered. Draw 500 watts for 2 hours? That's a kWh. Or have a battery that can hold 1 kWh, then assuming 100% efficiency you could draw 1000 watts from it for an hour before it was empty.
All of this is kW times hour, I would say? But in my mind I would interchangeably say per hour as well, they feel the same.
Obviously I'm wrong, but I'd like to know why lol
A watt is a derived unit for a rate of change, an amount of energy used in a unit of time, so P = E / t. A kW per hour would be a rate divided by time, or E / t^2, resulting in another rate.
More colloquially, think of watts/power by analogy to another rate, that of speed. Moving at a speed of 100kph for 3 hours results in 300 speed-hours of distance. Saying 100 kilometers per hour per 3 hours sounds awkward, but is actually a weird way to say acceleration, a rate of change of speed. (And probably a hint to get your car serviced.)
Anyway, the key is to think of a kilowatt as a rate, not a quantity.
Thanks, I guess I still don't understand though.
I see now that watts and therefore kW are rates. So it's silly to add another rate to the end by appending "per hour". But what is the time component of the watt calculation? To me it's essentially instantaneous, even if that's wrong. Even if that breaks the math, it's still essentially true on a macro scale. And if it's instantaneous, or even just close like microseconds, then it doesn't hurt to apend another rate to the end, does it?
So why not use it? Batteries come with capacities rated in Wh and kWh, and it weirdly still makes sense to me because of my usage rate per hour example in my last comment.
And if we shouldn't use it, then what should we use?
Is this problem we're discussing, one that only occurs if you try to get really accurate with the numbers and times? Because for my uses it's always seemed to work well enough.
Not being argumentative, just trying to learn, thanks
Oh, hey, Jerboa is not so good about updating the Inbox tally...
I was responding to your question about kW per hour, and I was going for the intuitive sense of why that's not right. The more "it's just so" reason is that the math just doesn't work, since the word "per" signifies division. So if we discharge a battery at a rate of 100 watts for 3 hours, that's 100W * 3 hours, or 300 Wh used. If we say 100 watts per hour for three hours, that's 100W / 1 hour * 3 hours. The hours cancel, and the result is 300 watts, which is a rate.
It's totally confusing, I know, because people often use "watts" and "watt-hours" interchangeably, but they're as different as speed and position.
Anyway, the watt is a derived unit in SI, and it's equivalent to kg·m^2^ / s^3^. The per-unit-time is hidden when you write it as a watt, but clearly there when you write it in terms of base units. Of course, the joule is kg·m^2^ / s^2^, so energy also has time in the denominator, and I guess could technically also be a rate, but understanding that is way above my pay grade. 😀
If you use exactly 20 kW for an hour, it will translate to 20 kWh. But if your power usage varies over time, you can't keep track of it so simple. It's just how it is.
The unit is really watt [W] and the Greek prefix kilo (k) for 1000. This way it's fast and easy to convert to different scales (like Mega, Giga etc) for comparing numbers