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Old 30-07-2011, 06:57   #1
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AC Power Questions

how many batteries would it take to power this a/c unit for 8 hours at at time? the specs on the a/c unit are: amps-4.9, volts 115v. i will be using 12v deep cycle batteries, due to fact that i have a good supply of them. what size solor panel would i need to recharge the batteries? i'm going to have a stand alone power system for the a/c.
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Old 30-07-2011, 07:02   #2
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AC Power Questions

how many batteries would it take to power this a/c unit for 8 hours at at time? the specs on the a/c unit are: amps-4.9, volts 115v. i will be using 12v deep cycle batteries, due to fact that i have a good supply of them. what size solor panel would i need to recharge the batteries? i'm going to have a stand alone power system for the a/c.
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Old 30-07-2011, 07:47   #3
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Re: A/c power questions

You'll need lots and lots of batteries. To get 5 amps 115vac from 12vdc batteries, you'll need to be producing at least 60 amps. To run 60 amps for 8 hours, that's over 400 amp hours. Since you never want your batteries to get below 50% charge, and they rarely charge over 80%, you'll need a battery bank of at least 1,400 amp hours.

There's no possible way a solar panel on a sailboat would be able to recharge that amount of electricity. Sorry to be the bearer of bad news.....
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Old 30-07-2011, 08:13   #4
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Re: A/c power questions

The base formula to use is Watts = Volts * Amps.

Thus you are using 563.5W.

That means, at 12V, you will be using 47amps. 8 Hours of that would use 376Ah of energy. Assuming your batteries are 90% full (charging to 100% is far too expensive in terms of efficiency) and that you never want to go below 50% charge, your absolute minimum battery bank size would need to be 980Ah. Considering the voltage goes down as the battery nears 50% plus the standard conversion losses in going between AC and DC you'd need to add 10% - so now you will need about 1100Ah battery capacity. But it get worse - Peukert's Law. The capacity of batteries is given in 24 hour discharge times, if you draw energy out of the battery at a faster rate, the actual capacity of the battery goes down; so add another 20% on top.

You are now at a 1300Ah battery bank (call it 12 Group 34 's).

If you need to replenish 600Watt for 8Hrs of energy via solar : at 8 hours per day of usable sunlight (I'm being generous and assuming the boat is on Lake Pleasant in Arizona) that would be around 600W per hour, at 12V that would mean the solar cells would need to produce 50Amps, adding 10% losses during charging for Dr. Peukert plus other losses and the occasional cloud we arrive at 55amps.

I just checked and see that the average for top-end solar panels seems to be 11.5 watts per square foot, so that would mean you'd need 52 square feet of solar panels. But this is an absolutely best-case scenario - lots of sunshine, no boom or mast to cast shadows, new batteries, an efficient top-end inverter, big cables, an efficient AC unit, etc.

Just as hogfighter has already stated, running AC off batteries is a difficult and expensive (and heavy) task. A genset or little generator is the only real option here. On a side note, I've got 800Ah (at 24V, so 1600Ah at 12V) on board and a top-end Victron inverter and wouldn't dream of running the AC. I'm in the tradewinds and the wind generator just keeps up with the fridge power requirements.
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Old 30-07-2011, 08:16   #5
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You would use 500 amp-hours or a bit more from your 12v batteries. You would probably expect to cycle your batteries between 90% and 50% full charge for reasonable battery life. That means a battery bank of about 1250 amp-hours for the A/c alone. You would probably need a 1200 watt solar array to put this back into your batteries on a full sun day. These are just rough numbers to give you an idea what you are up against. You would be better off (cheaper,easier) using a small generator to run the a/c or recharge the batteries.
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Old 30-07-2011, 09:53   #6
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Re: AC Power Questions

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Originally Posted by SoonerSailor View Post
You would use 500 amp-hours or a bit more from your 12v batteries...
... You would probably need a 1200 watt solar array to put this back into your batteries on a full sun day ...
500 AH x 13V = 6,500 Watt-Hours
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Old 04-08-2011, 11:16   #7
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Re: A/c power questions

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Originally Posted by hogfighter View Post
You'll need lots and lots of batteries.
Or just one REALLY BIG battery!
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Old 04-08-2011, 12:01   #8
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Re: AC Power Questions

Well, not impossible, just unlikely verging on really awkward. You could do this with about 8 Shell 80 watt panels ($350 @ plus tax and shipping), a heavy duty solar regulator ($600), an arch to mount the panels on ($2000), mounting hardware and big gauge DC wire for connections ($800), an inverter which could handle the load, especially the start-up load ($1200), and of course the energy-efficient 6000 Btu air conditioner (window mounted unit - $250). Good thing the batteries are cheap ;-)

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Old 06-08-2011, 03:47   #9
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Re: AC Power Questions

As others posted, it is an unlikely method. The only way I see that working is by using a small unit and something like a Honda 2000i generator. If you can find a very heavy duty charger (80 amps or so) you could use a few batteries and a 3000 watt inverter to overcome the transient power surges while continuously charging the batteries with a smaller (1000i) Generator
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Old 06-08-2011, 03:59   #10
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Re: AC Power Questions

The real solution to this problem is to move the boat to a cooler climate.
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Old 06-08-2011, 06:33   #11
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Re: AC Power Questions

Quote:
500 AH x 13V = 6,500 Watt-Hours
How many AA cells would that be Gord? Well, could use the rechargeable ones, right...
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Old 06-08-2011, 08:18   #12
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Re: AC Power Questions

Since a “AA” NiMH recharable might be rated as high as 2000 mAh (2 Amp-Hours), that might be as few as 500 batteries.
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Old 06-08-2011, 08:33   #13
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Re: AC Power Questions

HONDA GENERATOR
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Old 06-08-2011, 08:43   #14
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Re: AC Power Questions

A/C unit
5 amps * 120 volts = 600 watts
Use for 1 hour is 600 watthours

Inverter efficiency = 90%
Total watts used at inverter = 600watts/.90 = 667 watts
Use for 1 hour and that is 667 watthours

Battery at 12 vDC = 667watts/12v = 55.6 amps
Use for 1 hour and that is 55.6 amp hours
Use for 8 hours and that is 55.6*8= about 445 amp hours.
As you can only discharge a battery 50% of its rating you will require 900 amp hours battery capacity. Get a generator
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Old 06-08-2011, 17:06   #15
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Re: AC Power Questions

While I'm in the camp that thinks it's silly to try to run an air conditioner on batteries, we're forgetting that an air conditioner doesn't draw the same power all the time. When the temperature drops to the point set on the thermostat, the compressor shuts off. At that point power consumption drops to just what ever it takes to run the pump and fan. 200 watts or less? How much of the time is the compressor running, 50%, 75%? How would duty cycle of the compressor affect the needed battery size?
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