4 ms·

It wouldn’t be the first time I made a mistake like that by not clearly identifying my units but I think I got it right. Let me rephrase and use watt/hour unit

It wouldn’t be the first time I made a mistake like that by not clearly identifying my units but I think I got it right. Let me rephrase and use watt/hour units. I was assuming it was a 50 watt panel that would only produce 30wh from the weak winter sunlight. Divide 30wh by two (since it was only ru Ning for half an hour) and get 15 watts generated to purify 5 ounces. Had they let it run for the full hour, they’d get 10 ounces from my assumed 30wh of energy the panel produced. 30wh divided by 10 ounces would be 3wh per ounce (or unit of water produced). How many units of water can be produced by 1kwh? Let’s divide 1,000wh by 3wh to get the result of 333.3 units (ounces) produced.

electroly 4y ago
Wh is a measurement of energy, but W is a measurement of power (that is, the rate of energy). They are different units of measurement and you're mixing them up here. Here, you've doubled the original rating of 15 watts up to 30 watts; that's the result of the confusion between energy and power. You don't divide the wattage in half when you run it for half the time, nor double it when you run it for twice the time; that's not how power works. That's how energy works. In your original post, the number was 15 watts, and you figured it produced 5 ounces of water in 30 minutes. So you then figured that 3 watts can produce 1 ounce of water in 30 minutes. So far so good; that's correct. If the panel continuously produces 3 watts of power (remember: watts are a rate of energy) over a period of one hour, that's 3 watt-hours of energy. For 30 minutes, it's 1.5 watt-hours of energy. Thus, it took 1.5 watt-hours of energy to produce 1 ounce of water. (1000 watt-hours) * (1 ounce / 1.5 watt-hours) = 666 ounces. It may make more sense to translate this into distance, which I think is more intuitive for most people. Watts (power) are like miles-per-hour (velocity), and watt-hours (energy) are like miles (position). The panel runs at 15 MPH. After 30 minutes it has run 7.5 miles and produced 5 ounces of water. Thus, it could produce 1 ounce of water by running just 1.5 miles. It still runs at a speed of 15 MPH no matter what.
- zmgsabst 4y ago
  You’re getting confused by their typo on units: 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.
  - MichaelApproved 4y ago
    OP here. Thanks for understanding what I’m trying to say and putting it in the correct terminology. 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?
    - zmgsabst 4y ago
      This is what the other person was trying to say: “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.
      - MichaelApproved 4y ago
        Thanks. I’ll need to read that a few times because I struggle with this constantly.
        zmgsabst 4y ago
        I think part of the confusion is we don’t normally have units for rates of change. 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

electroly 4y ago

Wh is a measurement of energy, but W is a measurement of power (that is, the rate of energy). They are different units of measurement and you're mixing them up here. Here, you've doubled the original rating of 15 watts up to 30 watts; that's the result of the confusion between energy and power. You don't divide the wattage in half when you run it for half the time, nor double it when you run it for twice the time; that's not how power works. That's how energy works. In your original post, the number was 15 watts, and you figured it produced 5 ounces of water in 30 minutes. So you then figured that 3 watts can produce 1 ounce of water in 30 minutes. So far so good; that's correct. If the panel continuously produces 3 watts of power (remember: watts are a rate of energy) over a period of one hour, that's 3 watt-hours of energy. For 30 minutes, it's 1.5 watt-hours of energy. Thus, it took 1.5 watt-hours of energy to produce 1 ounce of water. (1000 watt-hours) * (1 ounce / 1.5 watt-hours) = 666 ounces. It may make more sense to translate this into distance, which I think is more intuitive for most people. Watts (power) are like miles-per-hour (velocity), and watt-hours (energy) are like miles (position). The panel runs at 15 MPH. After 30 minutes it has run 7.5 miles and produced 5 ounces of water. Thus, it could produce 1 ounce of water by running just 1.5 miles. It still runs at a speed of 15 MPH no matter what.

zmgsabst 4y ago
You’re getting confused by their typo on units: 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.
- MichaelApproved 4y ago
  OP here. Thanks for understanding what I’m trying to say and putting it in the correct terminology. 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?
  - zmgsabst 4y ago
    This is what the other person was trying to say: “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.
    - MichaelApproved 4y ago
      Thanks. I’ll need to read that a few times because I struggle with this constantly.
      - zmgsabst 4y ago
        I think part of the confusion is we don’t normally have units for rates of change. 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