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Old 03-12-2011, 07:54   #1
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Amps vs Watts

First I want to thank everyone for helping me with my questions. As you might guess I am in the process of buying a boat and I am trying to figure out how to outfit it.

The boat is 12v throughout. I have decided to have a 12c dc generator, solar, and wind. I am also going to have a/c. At first I was looking at only 12v a/c units. But it seems to me that a 6k btu 110v unit uses about the same amount of energy as a 6k btu 12v unit. For instance a CruiseAir 6000 uses 42 amps (high). That is 504 watts. A Domestic STQ6k uses 4.6 (full load) at 115v. That is 529 watts. What I don't know is if you can use watts to compare how much energy one appliance vs another is taking out of the batteries. If it is true that a 115v unit does not use much more energy than a 12v unit then it makes sense to me to get a 110v and use an inverter - there are more choices and they are cheaper.
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Old 03-12-2011, 08:12   #2
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Re: Amp vs Watts

You just have to take into account the efficiency of the inverter, which isn't 100% but rather around 90% at full load.

Then, to provide 529W in 115V, the inverter will draw 529/0.9=588W on the battery. The difference (588-529=59W) is heat released in the boat.

And, if the inverter doesn't operate at full load, its efficiency will be less. In fact, if it delivers no power, it will still draw on the battery. Then, you would have to size it precisely to the consumption of the a/c, to reduce the loss.

Alain
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Old 03-12-2011, 08:56   #3
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Re: Amp vs Watts

"...What I don't know is if you can use watts to compare how much energy one appliance vs another is taking out of the batteries...."

You absolutely must use watts (or some other unit of POWER) to compare how much power is taken out of the battery. The simple formula for power is P=IE. Where power is watts, I is amps and E is voltage. Since in most cases we assume 12vdc for the battery, often we just compare amps.
Considerations for comparing different ways of powering the same type of appliance include lost efficiency in converting from 12 vdc to 110 vac, the type of inverter you will use, and whether or not that appliance will run on the inverter. Not all appliances will work with out a full sine wave. Motors have significantly larger starting current than running current, which might mean that your precisely sized very efficient inverter may not be able to actually start it.
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Old 03-12-2011, 09:02   #4
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Re: Amp vs Watts

Yes, use watts to compare how much energy one appliance is using vs another.

See also Nes' accurate & detailed comments (#3); and
"Ohm's Law & Boats"
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Old 03-12-2011, 09:14   #5
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Re: Amp vs Watts

Clayzone,

I knew there was a reason why I studied physics...

For a given cooling load, ignoring differences in efficiency, you will need the same amount of watts (which is power needed to turn the compressor).

Now where do we get the Watts? Fairly easy - in Electricity Watts is Voltage times Amps. So when you compare a 115 Volt and a 12 Volt system of similar wattage, your amps are gonna be 10 times higher.

And that really is the crux of a battery operated A/C system. Even at 500 Watts, which is fairly low, you will need 40 Amps to run it. Do that for a decent night of 10 hours and you have 400 Watt hours - enough to drain all but the most powerful batteries.

Hence the rule seems to be - A/C either runs off a Generator or off the land line, and both come in high voltage, whether that's 115 or 230 Volts.

Oliver
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Old 03-12-2011, 10:14   #6
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Re: Amp vs Watts

40 amps for 10 hrs=400 watthrs?
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Old 03-12-2011, 10:32   #7
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Re: Amp vs Watts

Quote:
Originally Posted by Oliver L. View Post
Clayzone,

I knew there was a reason why I studied physics...

For a given cooling load, ignoring differences in efficiency, you will need the same amount of watts (which is power needed to turn the compressor).

Now where do we get the Watts? Fairly easy - in Electricity Watts is Voltage times Amps. So when you compare a 115 Volt and a 12 Volt system of similar wattage, your amps are gonna be 10 times higher.

And that really is the crux of a battery operated A/C system. Even at 500 Watts, which is fairly low, you will need 40 Amps to run it. Do that for a decent night of 10 hours and you have 400 Watt hours - enough to drain all but the most powerful batteries.

Hence the rule seems to be - A/C either runs off a Generator or off the land line, and both come in high voltage, whether that's 115 or 230 Volts.

Oliver
That should be 400amp-hours, which at 12v would be 4800watt-hours.

If all loads and supplies operate at 12volts, then you can just as easily compare things with amp-hours as watt-hours. If different items operate at different voltages watt-hr is the way to track energy use.
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Old 03-12-2011, 10:34   #8
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Re: Amps vs Watts

Some of us are confusing energy with power. One of the units for power is called watts. Power is the rate at which energy is being created or consumed.

One of the units for energy is watt-hours. (power times time). Watt-hours is an amount of energy that has been used or created over a period of time. Watt-hours can also be an amount of energy that is being stored.

A good liquid analogy is energy is the amount you can hold in a tank and power is the amount of energy flowing into or out of that tank.
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Old 03-12-2011, 10:41   #9
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Re: Amps vs Watts

Was not aware one could create energy,only changed?
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Old 03-12-2011, 10:49   #10
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Re: Amps vs Watts

OK, you got me - it is 400 Amp-hours, which at 12 Volt is almost 5 kWh.

The water analogy is a good one - the amps are the amount of water, the Volts correspond to how much this water will drop. Hydropower would be P=m*g*h, electrical power would be P=U*I.

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Old 03-12-2011, 12:36   #11
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Re: Amps vs Watts

Thanks for all the info. I am not one of the people that believes they can run 10k btu a/c without running a generator. In the beginning, due to ignorance, I thought I had to have a 12v a/c since I had a 12v system. Now I realize that with an inverter that is not correct. Actually having an inverter on board opens up other options as well. BTW my water analogy is volts is water pressure, amps is the diameter of hose, and watts is how fast the bucket fills up.
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Old 20-09-2012, 23:59   #12
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Re: Amps vs Watts

Here is a simple chart I produced as a fast reference on the boat. It seems I'm always needing to convert stuff so I know what wires and fuses to use. Copy it for yourself if you deem it useful.

It's in my gallery too!


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Old 21-09-2012, 01:14   #13
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Re: Amps vs Watts

A couple of points that need to be considered.

Batteries become much less efficient at higher discharge currents. For most electrical loads on boats this is a small consideration, but for high draw appliances is becomes very important. This is covered by Purkets equation. If you are planning on running A/C off battery power this becomes a very significant factor.

Watts is the official unit of power and WHrs is the correct unit for energy. However on boat the technically incorrect AHrs is much more useful. Batteries are rated in AHrs, meters read in AHrs, and finally using AHrs helps reduce the fudge factor for battery inefficiency. Amps into a battery are at a higher voltage than Amps out.
If you use WHrs you need a larger factor for battery inefficiency.
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