I’m confused why the term watt-hours (Wh) is used. Watts is joules per second. So watt-hours is joules per second per hour. There are 2 time measurements within the same term.
Why not just use joules for total amount of energy used and watts for consistent/average output over a specific time? I assume watt-hours is how many watts are consistently produced over an hour period but it is used often in unhelpful scenarios. When talking about say solar generation, someone could say “my solar array produces 12kWh every day”. So 12kWh for 24 hours means your solar array produces 0.5kW of power for the entire day. How was watt-hours helpful in anyway to describe the solar array’s power output?
Or when talking about a cities power output, the reports are measured in Gigawatt-hours over the entire year. Why is quantifying a cities yearly output over an hour long period helpful?
Now if we compare the 2 given examples, it becomes even more confusing. If I had a solar array that produces 12kWh every day, how many solar arrays would I need to power a city the needs 5000 GWh every year? 5000 GWh every year is around 570.78 MW, so if I just used the standard watts over watt-hours I would have a simple convertion between scenarios while still having the option to say “0.5kWs each day” or “570.78MWs for a year”.
Sorry if this is sounding like a rant post but I’m really annoyed at this term.
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Mild correction: watt-hours is joules per second *times* hours.
A quick conversion of 3600 seconds per hour, and you’ll find that 1 watt-hour is just 3600 Joules.
There’s no real reason not to just use Joules, other than the fact that we already deal with Watts. See, watts may just be joules per second, but a watt is also calculable from current and voltage. 5V at 2A is 10W. If we know the resistance of a load ahead of time, and the voltage it’ll be supplied with, we know the power draw in watts.
So if we leave that item powered on for five hours, we know the energy use is 50 watt-hours. We don’t calculate back by or measure time by the seconds it’s on.
Realistically, it’s just dumb convention.
Watts is a measure of power.
Watt-Hours are a measure of energy, they are Watts x Hours
You get an energy bill
It accounts for voltage of the system when comparing 2 different power sources.
If you want to compare 12v and 24v systems, you can’t just directly compare amp hours, which is what batteries are typically sold on. You need to look at watt-hours because using amp-hours in this case gives you a very slanted comparison; the 12v system puts out twice the current to output the same energy as the 24v system.
A Watt-hour is a watt *times* an hour, not divided by. It is a unit of energy equal to 3600 joules.
It is primarily used to measure and bill for electricity use, because the simplest instruments that measure electricity measure power (watts) not energy (joules), and because electricity is used over long periods of time, so hours are more convenient than seconds.
There are lots of other energy units that are only used in a specific context, such as calories for food, tons TNT for nuclear weapons, or tons (ice) for refrigeration and air conditioning. Each of them only makes sense if you know the history and instrumentation of that field.
Yes, Joule is the proper SI unit and should always be used for any energy calculations.
But kilowatt-hour is trivially understandable with minimal confusion to any idiot wondering why their electricity bill is so high.
So grid power consumption is billed in kilowatt-hours and everything related to that is also described in kilowatt-hours.
A Watt is an instantaneous measurement of energy output. A resistive heater that pulls 10A at 120V is 1200W. At any given instant while it is operating, it is using 1200W of power. Or converting 1200W of electricity into 1200W of heat. Or whatever.
If you want to measure energy output over a period of time, you have to multiply it by the amount of time that energy output was sustained.
So, that resistive heater running for 2 hours would use (or produce, or transform, or whatever) 2400 Wh of power.
> So watt-hours is joules per second per hour.
Joules per second *times* hour. Or more like, (J/s)*3600s, or otherwise, 3600 Joules.
It’s convenience lies in that we measure time in a 60-based system. It’s a bit easier to say “oh this battery is 24kWh, so I can run a 1kW consumer from it for a whole day” than trying to do that estimate in your head with 86.4 Megajoules. Yes, this is a somewhat imperfect situation, much like the mess that are imperial units, and would not be a thing if we used 100-minute hours and 100-second minutes.
I personally think it’s a bit silly, but the reason is probably that people are familiar with watts, but not with joules. And an hour is a more every-day unit of time than a second.
Also I think you are confused about what watt-hour means. It means using 1 watt *for* 1 hour, not anything like 1 watt *per* hour (which would be a rate of change of power, rather than a unit of energy). In other words, since 1 joule is the energy of using 1 watt for 1 second, 3600 joules is the energy of using 1 watt for 1 hour. So 1Wh = 3600J, or with the more common units, 1kWh = 3.6MJ.
So watt hours are basically metric units, just with a slightly weird extra factor. It’s similar to how 1m/s is equal to 3.6km/h, but km/h is the much more widely used unit of speed or velocity, because km and hours are easier to think about in relation to driving and planning a journey.
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