eli5: How is the use of watt-hours useful?

Question

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.

In: 2

Answer 1

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.