understanding watts and alt energy

[Chronos]

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?

How much energy you’d need in the battery would depend on how often you were willing to change the battery. On the other hand, if powered your house with solar panels instead (and we’ll pretend that solar panels don’t wear out; there’s no inherent reason why they must), you’d never have to replace them. The battery has a set amount of energy (measured in, say, joules, or watt-hours), and will eventually run out. The solar panel has a set amount of power (measured in watts), and the longer you run it, the more energy you’ll get out of it, without limit. So batteries and solar panels are not rated in the same kinds of units.

[Sunspace]

@drhess

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?

That was around $2000 for the panels alone. My friends’ solar setup also inclides batteries, an inverter to create AC, a “charge controller” that manages the whole system, and a bunch of wires and outlets and switches and boxes and stuff. I think you could build the whole thing from scratch for 5000. The system can provide electricity at a rate of greater than 250 watts, but it does that by draining the battery. You can use the TV and the computer, but not at the same time. And Og help you if you want to use a toaster or hair dryer.

I think they’re underpowered, but that’s just me. On the other hand, they didn’t know about the big blackout of '03 until I phoned them.

@drhess

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 : )

Well, if the average house can use electricity at a maximum rate of 24 kW, the 1-GW windmill can supply about 40,000 such houses. Bear in mind that 24 kW is 24 hair dryers (at 1000 W each) or 18 toasters (at 1500 W each).

Most houses will not use electricity at that rate. I don’t know what the average usage of houses in Ontario is, but I wouldn’t be surprised if it was a quarter of that. You can probably say that that 1-GW windmill could power 150,000 or 160,000 houses.

And yes, I know that the average big windmill puts out a maximum of 3 to 5 megawatts.

[Sunspace]

@Chronos

How much energy you’d need in the battery would depend on how often you were willing to change the battery. On the other hand, if powered your house with solar panels instead (and we’ll pretend that solar panels don’t wear out; there’s no inherent reason why they must), you’d never have to replace them. The battery has a set amount of energy (measured in, say, joules, or watt-hours), and will eventually run out. The solar panel has a set amount of power (measured in watts), and the longer you run it, the more energy you’ll get out of it, without limit. So batteries and solar panels are not rated in the same kinds of units.

Yes. If your load was always drawing fewer watts than the solar panels were putting out, you could leave it plugged in and turned on indefinitely. Assuming the panels were always in the sun, of course. And assuning the panels were providing the electricity and the same voltage that the load was consuming it. The panels can supply an indefinite amount of energy, as long as the sun shines on them.

If your batteries were could put out energy at the same wattage as the solar panels, you could leave the load plugged in to the batteries and turned on, but not indefinitely. It would only last until the batteries were drained.

So for solar panels, you have a minimum of a wattage and a voltage spec.

For batteries, you have a voltage spec, plus a current and time spec (in amp-hours). The combination of the two gives a maximum quantity of enegy that can be supplied by the battery. (You usually get other specs as well, such as maximum current and power that can be supplied.)

[robby]

@Sunspace

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.

To consider a real-world example, a typical lead-acid submarine battery (they haven’t changed much over the last 60 years) has 126 cells. Each cell produces a voltage of about 2 V, and the cells are wired in series, so the whole battery produces about 250 VDC. At a typical discharge rate of 1,000 A, a submarine battery will produce about 250 kW. Finally, at a discharge rate of 1,000 A, it will last 6-8 hours, so the battery capacity is in the range of 6,000-8,000 A-hr.

Oh, I don’t want to negect to mention that each of the 126 cells is about 1-ft by 1-ft square and 4-feet high, and each cell weighs a bit less than a ton. The whole battery clocks in at about 120 tons, give or take. The battery is kept low in the sub along the centerline, so it can double as ballast.

I’ll leave it as an exercise for the reader to guess how much a 120 ton battery might cost. (Also because I haven’t the slightest idea.)

[Sunspace]

@robby

Oh, I don’t want to negect to mention that each of the 126 cells is about 1-ft by 1-ft square and 4-feet high, and each cell weighs a bit less than a ton. The whole battery clocks in at about 120 tons, give or take. The battery is kept low in the sub along the centerline, so it can double as ballast.

I’ll leave it as an exercise for the reader to guess how much a 120 ton battery might cost. (Also because I haven’t the slightest idea.)

:eek:

Now I know where to get the batteries for my house.

[Tim_T-Bonham.net]

@Sunspace

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.

No, a 100-amp main service is about the minimum you would see in a house built today. Much more common in a new house is 150 or 200 amp service. Some of them are even going to 300 amp service.

So a typical house would be 24kW (minimum), 48kW (typical) to 72kW (mansion). Of course, it’s extremely rare for any house to ever be using the full load at any one time.

[Sunspace]

@t-bonham@scc.net

No, a 100-amp main service is about the minimum you would see in a house built today. Much more common in a new house is 150 or 200 amp service. Some of them are even going to 300 amp service.

So a typical house would be 24kW (minimum), 48kW (typical) to 72kW (mansion). Of course, it’s extremely rare for any house to ever be using the full load at any one time.

Thanks, t-bonham. I was kind of wondering about that, given the size and potential power drain of some of the houses these days.

[drhess]

So a 1 megwatt windmill/turbine would power 1,000 homes that exist at thousand-watt useage? Assuming it continuously was turning at the rate needed for 1 megawatt? I guess I’m thinking of two things: how much power the windmill makes and then at what rate you use it up, but the storage issue is in between the two (since production wouldn’t equal usage on a small close system, so to speak). Essentially, I’m just wondering if I can live in a windmill-home somewhere, assuming the right battery set up and sufficient wind.

[gazpacho]

@drhess

So a 1 megwatt windmill/turbine would power 1,000 homes that exist at thousand-watt useage? Assuming it continuously was turning at the rate needed for 1 megawatt? I guess I’m thinking of two things: how much power the windmill makes and then at what rate you use it up, but the storage issue is in between the two (since production wouldn’t equal usage on a small close system, so to speak). Essentially, I’m just wondering if I can live in a windmill-home somewhere, assuming the right battery set up and sufficient wind.

There are people who do. Those people tend to use on the low side of the range of energy use that we have seen in this thread.

I would just point out that batteries are expensive. They have a finite life span before they need to be replaced. The last time this was discussed on one of the energy email lists where I work it looked like the cost deep cycle lead acid batteries was about 17 cents a KWhr. To keep things in perspective residential electricity costs in California are 10 to 16.5 cents per KWhr. There low population states near hydro sources that pay as little as about 6 Cents per kWhr and some up to 18.

What some people do is stay on the grid and run the electric meter backwards when the sun is shining or the wind is blowing by putting the electricity on the grid and pulling energy off the grid when they are not producing excess power.