They meant 15 watt-hours in both cases, which would be half of 30 watt-hours — or a 50 watt panel for 30 minutes at 60% efficiency. (Which is the stated working premise.)
15 watt-hours -> 5oz would imply that 3 watt-hours -> 1oz, and hence 1 kWh is 333oz.
I generally try to assume best intentions — so if they repeated otherwise the same correct math, but for misnaming a unit, I try to assume they’re correct and typo’d.
However, I think I’d still make the same mistake in the future because I don’t understand how to express what the (assumed) 50 watt rated panel is generating over half an hour.
If it’s running at 60% efficiency then it’s generating at 30 watts per hour.
After an hour of running, we’ve accumulated 30 watts of energy.
If we only ran it for half hour, we’d have 15 watts.
Dividing that out tells me that 3 watts of energy will provide 1 once of water.
Does it matter if those 3 watts are provided over an hour via a 3wh power source or over 10 minutes via a 16wh power source?
“Watt” is the unit of power which is the rate of change in “joules”, which is the unit of energy. In the way that “velocity” is the rate of change in “position”.
1w = 1J/s
A “watt-hour” is another way to represent joules/energy by integrating watts/power over time.
30 watts * 30 minutes = 15 watt-hours or 54 kJ.
30 miles/hour * 30 minutes = 15 miles or 24km
> Does it matter if those 3 watts are provided over an hour via a 3wh power source or over 10 minutes via a 16wh power source?
You have the units backwards:
You can get 3wh from 1 hr @ 3w or 10 min @ 18w.
So let’s create one: the “walk” is the speed a person can walk — 3mph.
Then I can say the store is “3 walk-hours away”, even if I normally drive the 9 miles with my car.
3 walk-hours = 3 hours @ 1 walk = 18 minutes @ 10 walk (or 30mph)
Batteries are rated in “watt-hours” for the same reason:
3wH = 3 hours @ 1 watt = 18 minutes @ 10 watt
I’m assuming what I’m saying is wrong, I just can’t spot it. Thanks for the help.