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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watt-hours = watts * hours NOT watts/hours
This is a measure of energy content. It is used for things like batteries. The larger unit kWh (1000 watt-hours = 1 kilowatt-hour) is used as a measure of energy consumption and many utilities price their energy in this unit. The larger units like GWh or TWh are used to describe the size of the energy consumption of countries.
Solar output is rarely consistent throughout the day, however some cases all of the panels output goes to the grid, and is not used by the consumer.
So, they don’t care at all that it made 3kW at 9AM, and 9kW at 11AM, just that over the whole day it put out xkWh, as that is what will be being paid for.
The unit is historical. People know how long an hour is, and if they run a 0.5 kW appliance for a certain number of hours, they can estimate the cost of electricity. A second is too short. Or they roughly how much work can be done by 1 kWh because they have used the unit for so long. If we switched to megajoules, the number would be 3.6 times greater, and might raise suspicion of getting overcharged.
In the case of the solar battery, the amount of energy per longer period might be more useful than instantaneous power because they only produce for a fraction of the day.
Wh and Joules are both units of energy. 1Wh = 3600 Joules. It’s just convenient to use Wh in the context of electrical engineering because Watts = Amps * Volts which are directly specified or displayed on test equipment. For example, a 2Amp Hour, 3.7V Lithium Ion battery has 7.4 Wh of energy.
While physically speaking Watt hours per year and Watts end up being the same unit, it has an important distinction in usage: consistency. When talking about X watts, it is generally assumed to be a constant energy output, unless something like “peak output” is added. Watt hours per year doesn’t imply this, so it kinda helps keeping language clearer.
So yes, 12kWh today is theoretically equal to 0,5kW, when speaking about it 12kWh tends to be preferred.
Also historically electricity is measured in kWh (for at home use, not engineering side) so that just tends to be kept as not to confuse people with kJ.
Watt-hour is not a Watt per Hour, its a Watt x Hour, ie a **cumulation**. It’s useful when you want to know how much energy something uses in an hour, to compare to other energy ratings.
Similarly, it’s useful to know a salary someone accumulates in a year rather than their paycheck, to compare to other salaries. In this comparison, the paycheck would be a watt ($/week), vs the salary (accumulation of $ in a year)
>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.
The last part “watt-hours is joules per second per hour. ” is incorrect it is joules per second times hours. One hour = 3600 seconds so it all works out to 1Wh= 3600J
You measure electric usage over time as energy not as power, Power is just the current rate of consumption
Watt-hour or more often Kilowatt-hour is useful when typical usage time is in hours.
How much energy do a 40W device you run 8 hours per day use each day? 40*8 = 320 Wh =0.32kWh
If you do not use the Wh the calculation is 40*8*3600 = 1 152 000 J
The energy cost starts to be reasonable at kWh too. In the US the average residential energy cost in Feb 2023 was 23 cents. that is 0.00000638cent per joule.
Ther is a reason that the electrical energy is measured in kWh, the number gets reasonable.
For solar power systems, it is important to know that the mount of energy is produce is not constant but changes with the elevation and direction of th sun. Clouds that bloc the sun has also a huge effect
>”So 12kWh for 24 hours means your solar array produces 0.5kW of power for the entire day.”
The last part is incorrect, the output was not 0.5kW during the day it was 0kW during the night. It varies during the day and if you orient the panel to the south the peek will be at noon. 0.5kW is the average power generation for the day not what is produced during the day.
For the city example 5000 GWh every year with power generation of 12kWh every day. The number of generation systems you need is 5000/(12*365/1000000) = 1141000 systems. Ths assumes the power generation is the same all of the year, that it is quite far from current if you are away from the equator.
>5000 GWh every year is around 570.78 MW,
This is the on average power but because power usage is not contend during the day the peak power usage will be a lot higher, you have both daily and yearly variation. It is also not just 570.78 MW you need to say it is “570.78 MW on average during the year” to get energy, the time it is for is important.
The statements “0.5kWs each day” or “570.78MWs for a year” are not correct compared to the rest of the text. a kWs is kilo Watt second = 1000 joule. That means “0.5kWs each day” is an energy generation of 500 joules in a day or 0.14kWh of energy
“570.78MWs for a year” will just mean you generate 570.75 *10^6 Joule each year which is equal to 158 kWh
Watt hour is watts x hours. watts = joules/second. The time part cancels are you are left with Joules i.e the measure of energy.
So that you can know the hourly rate as opposed to the per second.
If you have a battery that outputs 36KW in total and you need to calculate how long it will power a 200W device. Well if you just do 36000 / .2 you now know how many *seconds* it will power your device but we usually don’t think in seconds itd be a lot more convient to know the number of hours. So lets divide it by 3600 for the number of seconds per hour.
Well if we are going to do that, then instead of going through the extra step to calculate the hourly consumption rate, why not just simply, divide the total power by 3600 seconds to directly give you the hourly rate. Now you just have to do 10/.2 a much simpler and more straightforward calculation to tell you the hourly consumption.
The Joule is a unit for energy just like a Watt-hour. It’s defined as a Watt-second. So using Watt-hours instead of Joules is just more convenient when talking about long time scales. It’s easier to think about energy consumption of an appliance based on how many hours it runs for, rather than seconds.
The Watt-hour exists alongside the Joule for the same reason that the hour exists alongside seconds and minutes. Sometimes it’s useful to have more than one unit to describe a quantity.
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