Mixcat's cheap AC power idea comments

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Old 07-08-06, 05:49 PM
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Mixcat's cheap AC power idea comments

I would like to be able to discuss this idea that he posted in the Tips forum here.

If the solar panels produce only 70 watts, then why the need for a 3000 watt inverter? Am I missing something here? Can you explain this more?

I realize the battery will yield more wattage right off the bat. But if one solar panel only adds 70 more watts...wouldn't you maybe want more panels than just one? And what kind of total outlay of money are we talking for some idea like this to be of some real use? Can you give an example of how you could utilize this system in powering certain things for X amount of time during an outage, without coming up short in power in the end?

What is the advantage here over say a generator?
 
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Old 07-09-06, 04:53 PM
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I did not find the post you are talking about (I'm too lazy to look very hard...).

I think you are seeing the general problem with solar. It takes a lot of solar panel to get a really usefull amount of energy. Solar panels are not cheap and they only work when the sun is shining.

I am of the opinion that a traditional generator is the best way to go for most uses. You have to maintain it (make sure the gas does not go stale) but you get a lot of power for the money and it can work during a storm and at night.

Solar panels are great for a satellite or sailboat but for household emergency use I don't think they are the best choice. Because the solar panel does not generate much power it must work for a long period of time charging a large bank of batteries (expensive) to provide a usefull amount of current. The electricity is "free" but the solar panels, batteries and inverters are not. And if you think that solar power is green and clean power start thinking about the manufacturing disposal of solar cells, batteries & inverters.
 
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Old 07-09-06, 05:00 PM
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Go back to the main forums page that give all the sub forums, starting with the Automotive forum at the top. Scroll way down by the bottom and you will find his post in the Hints,Tips & Tricks forum down there.
 
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Old 07-09-06, 05:31 PM
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Old 07-09-06, 06:39 PM
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Thanks for the link to the original post.

Let's see... using every drop of a 750 amp battery at 12 volts theoretically gives you 9'000 watts. Most people recommend discharging a marine deep cycle to 50% at the max. so you are down to 4'500 watts. Most inverters are 90% efficient at best so you are down to about 4'000 watts and most are less efficient. It does not sound too bad until you realize that you can only get 4'000 watts for an hour at max.

$200 solar panel
$75 battery
$300-$2'000 inverter depending on size

So for about $600 you can run your TV and some lights for several hours. If you pony up for a big 3'000 watt inverter expect your system to be around $2'500 to be able to start & run your refridgerator for an hour. I still think a cheap generator or a high end Honda and a can of gas is a better way to get you through a three day power outtage.
 
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Old 07-09-06, 07:57 PM
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Pilot Dane I agree with everything you say about solar being uneconomical and inefficient as a home power source. One thing in your calcs though. Marine deep cycle batteries are designed to be drawn to near depletion, unlike your average car battery - draw it down a couple of times and it dies.
 
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Old 07-10-06, 04:21 PM
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Wayne: I was thinking the same thing. A true deep cycle can withstand deep discharges up to 90%, but after a little research it looked like most of the "deep cycle marine" batteries sold today are more a hybrid design. They can take a heavy discharge better than the average starting battery but are not a true heavy plate deep cycle. Apparently the current mass market "deep cycle" design is cheaper & lighter since it uses less lead and fits better with how most batteries are used (running trolling motors and occasionally starting an engine).
 
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Old 07-10-06, 05:33 PM
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Do 9000 watts DC convert over directly to 9000 watts AC, tho'?
 
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Old 07-11-06, 04:58 PM
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Watts are watts so yes the wattage stays the same. AC/DC does not really matter.

The wattage tells you how much power there is while AC versus DC has more to do with how you transmit the power. DC is OK over short distances like in your car or boat but the resistance of wires kills eats DC up over long distances while AC is easily transmitted over long distances especially at high voltages.

Amperage does change with voltage (watts divided by voltage gives you amps). 9'000 watts at 12 volts is 750 amps (9'000/12=750) while the same 9'000 watts is only 75 amps at 120 volts (9'000/120=75).
 
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Old 07-11-06, 05:16 PM
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Well then...that battery packs quite the punch then, just on it's own, without any solar panels to feed any more into it (albeit 70 watts is like a drop in the bucket. That would not be enough to power my 10 inch fan.) But at this wattage you could power various things for up to quite a while. But what is going to happen as the voltage drops off with use? And the fact that only 70 watts of solar power is trying to help it recover? I presume the rating is 70 watts per hour?

Too bad mixcat didn't see fit to comment any more on his idea. That is what I was hoping for.
 
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Old 07-11-06, 06:36 PM
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His instructions also mentioned using a battery with 750 CCA (cold cranking amps) which is not a term commonly used with deep cycle batteries. I just looked at the Trojan battery webpage. Their deep cycle that produces 760 CCA can only deliver 75 amps for 52 minutes. That's only 900 watts for less than an hour.
 
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Old 07-12-06, 05:37 PM
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And weren't we talking 9000 watts in the beginning? Now it's down to 900 for an hour...with only 70 watts in recovery. Hmmm. That is not going to do you a lot of good. You could watch some tv with a fan blowing on you and maybe a few lights on...for an hour or two I suppose. Or, run a window a/c unit for maybe 3/4 of an hour.
 
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Old 07-16-06, 03:53 PM
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Also, after looking at it more closely it becomes clear why an electric golf cart needs so many batteries to last for 36 holes of golf. My gas golf cart goes about a year on 5 gallons of gas where my neighbors electric needs charging after a weekend's use.
 
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Old 07-16-06, 04:56 PM
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Good point. Now we are hitting on what amount of energy it takes to produce "work". And to get a relatively heavy golfcart to "work", it is going to take more energy. Especially when two big out of shape guys are riding in it.
 
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