So 500 watts could be taken as short hand for “needing 500 watts each hour”. I guess that is what I mean by the per what. So, to make things easy, I will assume 1000W or 24,000 per day per household. Or, very roughly, 8 GigaWatts per household for a year’s supply?
Energy provided by treadmills? Naww, that idea will never take off.
Remember that conserving energy is much more efficient than generating, storing, and transmitting more energy.
So if those joggers would jog to the gym, instead of driving there in gas-guzzling SUV’s, that would save far more energy than you would get from any jogging treadmill generator.
Err…8 Megawatts.
No, you’re still mixed up.
Watts are the rate at which energy is supplied or used. “Watts per hour” is meaningless.
Saying “24,000 W per day” or “8 GW per year” is like saying that you need “60 MPH per hour” to get home, and “1,440 MPH per day”, and “525,600 MPH for a year of driving.”
What you said above is equally meaningless.
A megawatt is enough to run a small railroad locomotive or a moderate wastebasket fire. If you put a guy on a treadmill running a generator, and he turns the light on because he’s going to be in the room, he’s using up about as much power as he can generate. A kilowatt hour is a day’s hard labor for a strong man. A joule is the effort to pick up a book to read it.
In human terms, electric power is incredibly cheap. If you got a job selling electricity you generated, you probably couldn’t make $20 a year.
No, no, no. There’s no such thing as “watts per hour”. Or rather, there could be, but it wouldn’t be useful for describing much. You don’t have watts per time, you have watts. You could say that 500 watts is shorthand for 500 Joules per hour. Or, alternately, you could say that 500 watts is shorthand for 500 watt-hours per hour. Joules and watt-hours are both units of energy, while watts are a unit of power, which is energy per time. For comparison, you would never say that a car has 300 horsepower per second: Horsepower, like watts, is a power measurement.
So if you have a household that needs 1000 watts, to make things easy, call it 1000 watts.
To repeat what I said back in Post #12, if your house uses 500 watts continuously for 30 days, the energy that you have used is (500 W) x (30 days) x (24 hours/day) x (1 kW/1000 W) = 360 kW-hr.
You can also do the calculation this way, keeping in mind that a watt (W) is a joule per second (J/s).
(500 J/s) x (30 days) x (24 hours/day) x (3600 seconds/hour) x (1 MJ/1,000,000 J) = 1,296 MJ (megajoules)
500 Watts = 500 Joules per second.
[I know it was just a slip, but this thread has contained enough misinformation by others that it’s important to keep it straight.]
To understand Watts per what, if it helps, think of Joules – the unit of energy – as being like dollars. You need Joules to do things – run a fan, light up a room for a certain length of time, whatever – just like you need dollars to buy things.
Now think of Watts – Joules used per second, which is power-- like a pay rate (dollars per hour), or rental fee (dollars per hour, or per month or whatever).
If you multiply Watts times hours, you get Joules back, just like ‘two months pay’ gives you dollars again.
If you’re talking about how much energy you need to run your house, you’re doing the same thing as asking how much money you need to live on. The answer is going to be dollars per month, right? So similarly, when you ask how much energy you need to run your house, you’re really asking how much energy per day, which is Power, in Watts (or kilowatts).
Or, if you want to be complicated, kilowatt-hours per month, but that’s kind of like saying ‘weeks of minimum wage pay per month’ in that you’re dividing by time, multiplying by time and then dividing by time again.
Thanks, Antonius. I agree that it’s important to keep straight.
Ok, here’s what I’m thinking: if I had a huge battery that was the only supply of energy from my 500 kw cottage. How much energy would I need to load onto it? Is this measured in watts too?
I guess what started this, is I was wondering if an article says a windmill produces one gigawatt, do they really mean it constantly supplys enough for [1 giga/500kw] households.
They mean “up to 1 gigawatt”. It’s like those ads on telephone poles that say, “Make [sub]up to[/sub] $3000 / week at home!!!”
The windmill will produce 1 GW under ideal conditions (just the right wind speed). At higher or lower speeds, it will produce less.
At lower speeds, it won’t spin as fast and will produce less power; and of course when there is no wind, it will produce no power. At higher speds, windmills are designed to furl their blades (or otherwise shut themselves down) when there is too much wind, so that they are not torn apart.
500 kilowatts? How big is your cottage? 
A standard US/Canada residential electric service from the local utility is 240V centre-tapped*, and can supply a maximum of 100 amps at that voltage. That means that the maximum power supplied to the house at one time can be 100 A x 240 V = 24 000 W, or 24 kW.
You can build a solar or wind system to supply that much power, but it will be expensive. My friends up north have a total of 250 W of 12-V DC supplied by their solar panels, and that cost something like 1500-2000 Canadian dollars. They have an inverter to change that into 120-V AC for appliances when needed, but a lot of the lights are 12-V.
[sub]*You get the 120-volt wall current by connecting between the centre tap and one of the busbars. The Master speaks.[/sub]
Regarding supplying energy from a battery…
We’d have to know how much voltage your battery puts out. Then divide that into the wattage to get the number of amps it needs to supply. Let’s say your battery supplies electricity at 48V (not uncommon for larger setups).
watts = volts x amps; 500 kW = 48 V x <number of amps>; <number of amps> = 500 000 W / 48V; <number of amps> = 10 416 A.
Ten thousand amps. This is up in electric-resistance welder territory, where you need busbars that are solid slabs of metal a foot wide and an inch thick. (I used to work around them. 5V at 20 000 A = a hell of a zap, well capable of welding car parts together. And erasing credit cards, too, but that’s another story.)
Now, the total amount of energy the battery contains (the battery capacity) is a separate issue from the rated battery current. You might have a battery that can put out your ten thousand amps but only for 1 minute. Or you might have a much larger battery that can put out the same 10 000 A for ten hours. Battery capacity is typically rated in A·h (amp hours) or mA·h (milliamp hours).
The first battery would have a capacity of 10 000 A x 1 minute = 10 000 A x 0.0167 h = 167 A·h. The second battery would have a capacity of 10 000 A x 10 h = 100 000 A·h, and would be much more expensive.
Generally, all other things being equal, the greater current a battery can supply, the more expensive it is. And, all other things being equal, the greater capacity a battery has, the more expensive it is. And if you want high current and high capacity, that gets really expensive.
Sunspace: either I am or you are making this more complicated for me, not less. : ) Maybe my example was bad or question is bad. For example, 1000-2000 dollars for what? The setup alone? I don’t even want to get into more calculations, I just wondering (maybe this is not simple to explain) when something says that a windmill (turbine) produces 1 gigawatt, what that means compared to conumption in everyday terms that lay folk like me can understand. I got a C+ in chemistry in college and that was generous, I avoided physics : )
I can’t decide if the fact that no one has groaned at this yet is attributable to the board’s intelligence or their lack of observation.
A single family house uses around 500 Watts to 3000 Watts. A giga watt is 1,000,000,000 Watts and will power a few hundred thousand homes. A mega watt is 1,000,000 Watts and will power a few hundred homes. According to wikipedia The record for most power from a wind turbine is 6 mega watts.
The 500 to 3000 Watts are an average over around 1 months time. At various times during the day you will use a lot more or a lot less. At night in my house the main thing that is running is the fridge. When we get home in the evening things like the clothes drier, oven TV and computers may be on and we will need much more power.
And just so perspective is properly kept, current windmills have a max capacity of 3 MW although there are designs potentially for 5 MW.
Battery energy storage total number of joules in storage) depends on both the voltage and the current capacity in amp-hours.
A little impromptu dimensional analysis:
1 W = 1 A x 1 V.
Therefore, 1 W / 1 V = 1 A.
But 1 W = 1 J/s.
Therefore, 1 J/s / 1 V = 1 A.
1 J/s·V = 1 A.
Now, the battery capacity I’ve been talking about is in A·h. 1 A·h = 3600 A·s.
Since 1 A = 1 J/s·V,
1 A·s = 1 J/s·V x 1 s = J/V.
This makes sense; the more voltage you have in a battery, the more total energy is stored in it. And, also, the greater current capacity the battery has, the more energy you can store in it.
So, for your battery that has a current capacity of 100 000 A·h at 48 V, the total energy stored in joules when it’s full is:
48 V x 100 000 A·h = 48 V x 360 000 000 A·s = 48 V x 360 000 000 J/V = 17 280 000 000 J.
Now, we didn’t do it quite this way when I was in electronics school, but figuring out how the units work lets me know that my answer has a greater probability of being correct. 