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Old 05-03-2012, 20:53   #1
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Inverter Sizing

I'm a bit an electrical dummy: I have a Laptop where the transformer says the following: Input 100-240V -2A, 50-60Hz - output 18,5V 6,5A (120W)

I learned once that the Watt is always the same - not matter of voltage. So, would an 120Watt Inverter (12V to 120V) be enough to run the Laptop from 12 Volt source?? And if yes, why the 2A on the input side?

Thanks! Marco
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Old 05-03-2012, 21:20   #2
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Re: Inverter Sizing

Marco,

A Watt is a Watt is a Watt. However some energy is lost in the transformer.Therefore you really only to pay attention to the input side. The input wattage is (120V*2A) = 240 W. If the laptop is your only AC load then I would suggest finding a "car" charger. Car chargers transform DC voltage to DC voltage with much less energy wasted.

If you have more than one AC load, then I would suggest sizing your transformer to run two separate loads at the same time. This will insure that you always have enough juice.

Simply put, the 120 Watt transformer will not be enough to power the laptop. you will need at least 250 Watts.
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Old 05-03-2012, 21:27   #3
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Re: Inverter Sizing

Quote:
Originally Posted by CruisingFan View Post
Marco,

A Watt is a Watt is a Watt. However some energy is lost in the transformer.Therefore you really only to pay attention to the input side. The input wattage is (120V*2A) = 240 W. If the laptop is your only AC load then I would suggest finding a "car" charger. Car chargers transform DC voltage to DC voltage with much less energy wasted.

If you have more than one AC load, then I would suggest sizing your transformer to run two separate loads at the same time. This will insure that you always have enough juice.

Simply put, the 120 Watt transformer will not be enough to power the laptop. you will need at least 250 Watts.
Oki - Thanks a lot, just another question for education purpose
- That a Watt is a Watt is clear - what confuses all the time me is the fact that it states 120 - 240V -2A. I mean I would have 2 Options: 120Vx2A=240W or 240Vx2A= 480W. Or ist there a rule with multi volt transformers?
Thanks anyway!
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Old 05-03-2012, 21:28   #4
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Re: Inverter Sizing

Inverters below 500 watts are dirt cheap thanks to China jumping in the marketplace. I would go with a 500 watt unit, plug an AC outlet strip into it, and use the little guy for all your wall cubes for charging cell phones, shavers, etc.
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Old 06-03-2012, 06:57   #5
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Re: Inverter Sizing

Quote:
Originally Posted by swisscraft View Post
Oki - Thanks a lot, just another question for education purpose
- That a Watt is a Watt is clear - what confuses all the time me is the fact that it states 120 - 240V -2A. I mean I would have 2 Options: 120Vx2A=240W or 240Vx2A= 480W. Or ist there a rule with multi volt transformers?
Thanks anyway!
You're right to be confused.
They're not actually giving you accurate technical information.
I expect that the (not more than) 2A is intended for (not less than) 100V input (200W).
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Old 06-03-2012, 07:27   #6
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Re: Inverter Sizing

Gord's right (as usual). But so is Cruising Fan when he says that it's MUCH more efficient to get a 12v power supply for your laptop & run it directly from your boat's batteries. They cost ~$50 & plug into a cigarette lighter type socket.

Strangely enough, most general purpose DC to AC inverters have a difficult time with computer power supplies (which are all switching supplies inside). Most boats will have an inverter (for running power tools, blenders, etc) but they're not an efficient way to run a laptop.
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Old 06-03-2012, 11:14   #7
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Re: Inverter Sizing

Quote:
Originally Posted by swisscraft View Post
Oki - Thanks a lot, just another question for education purpose
- That a Watt is a Watt is clear - what confuses all the time me is the fact that it states 120 - 240V -2A. I mean I would have 2 Options: 120Vx2A=240W or 240Vx2A= 480W. Or ist there a rule with multi volt transformers?
Thanks anyway!
The 120-240 V means you can plug in that unit into any voltage between 120 v and 240 v.

There is a direct relanship between voltage, curent and resistance (load or what any electronic/electric unit will use)

If the load (watts used) stays the same, less current will run thru the wires if the voltage is increased. Vice-versa as less voltage more current will be seen thru the wires.

The 2A (amps) will be the maximum draw at the lower voltage.

Back to your original question

Quote:
Originally Posted by swisscraft View Post
So, would an 120Watt Inverter (12V to 120V) be enough to run the Laptop from 12 Volt source??
It should work. Most electronic things are designed to work in a given range. I would try to run in the middle of that range. Some electronics get pretty hot when running near the high end.

If you are only interested in running your laptop, it's better to just get a DC (12V) to DC (18-20V or what your laptop needs) as others have suggested.

Inverters come in different grades. Most things with transformers/switching power supplies are designed to run on a pure sine wave. Lots of electronics/electrical things behave strangely and can be destroyed with modified sine wave inverters.

What I did was get a 400 watt electronic grade inverter. It's capable (not all at once) of running LCD tv, laptop and all the other little chargers.

Inverters, DC-DC converters and transformers are not the same. They each have a special purpose, so be careful when using their names in corespondance.

Gordon
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Old 07-03-2012, 18:23   #8
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laptop power converters

Take exception to the previous discussion regarding power required for your laptop. The laptop converter is a "universal input" switch-mode converter, not a transformer (unless you have a very old laptop). Pay no attention to the converter input current, that is listed so that when powering up one does not nuisance trip a supply fuse or breaker. Modern switch-mode converter efficiencies are quite good, on the order of 90% or so and, therefore, you can just about ignore that conversion loss.

Note only the OUTPUT current, which will be a maximum and perhaps a lot more than your particular laptop will ever draw. Modern switch-mode laptop converters work fine with non-sinwave power inverters and, in many cases, are most convenient to use than trying to convert dc to laptop dc, after all, the laptop supply will have proper output regulation for that device. With most inverter/charger/transfer switch units driving laptop converters you may not realize a significant loss in conversion effeciency, especially if you are running other conveniences such as LCD tv and sound system.

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Old 08-03-2012, 05:11   #9
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Re: Inverter Sizing

Even though I agree with most of what has been said in this thread, my real world experience differs:

A cigarette lighter based inverter which is typically rated at 120-150 watts can run most laptops. The specs on the back of the brick are often overstated and the real input current is often much lower and within the capability of these 120-150 watt inverters.

A DC to DC converter can work but it won't be much if any more effiient than an inverter. I bought a cheap Chinese made converter to run my netbook. It draws more current than my inverter while powering the same netbook. Go figure.

David
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Old 08-03-2012, 18:21   #10
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Re: Inverter Sizing

I don't know anything about electrics. What I do know is:
My 175 watt cigarette lighter plugin inverter will power my laptop.
My 160 watt cigarette lighter plugin inverter will not.
Now you know what I know.
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Old 08-03-2012, 19:30   #11
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Re: Inverter Sizing

Is there a blown fuse in the 160 watt unit?
Might not be... What power requirements does the laptop have - often written on the bottom somewhere.
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Old 09-03-2012, 11:58   #12
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Re: Inverter Sizing

Yes, one cannot tell in advance if a switch-mode inverter will start up the laptop even if the continuous power rating covers the laptop running requirements. One must try the inverter to tell.

I had a 75 Watt inverter that would start any laptop when some 100 Watt units would not. The offshore manufactured units that cost very little often do not have much of a surge power rating. Most well designed and manufactured units can deliver a surge rating 2X the continuous rating. Non switch-mode, non sinewave units can surge 5X to 7X yet you don't see those units much any more.
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