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I have several laptops of various brands. Each has an AC adapter that's designed for the specific computer it came with. I also know there are universal adapters can adjust their voltages automatically to be compatible with multiple models.

I'm able to use AC adapters from different companies that have the same voltage regardless of their amperage rating (assuming they're physically compatible), yet if I use two adapters with different voltages and the same amperage, it either fails completely if it's too low or, from my experience, if it's too high something inside the laptop sizzles and pops and the computer's now a brick.

I know that you can use a power supply with a higher amperage rating than you need because electronics don't have to draw 100% of this to function. They need at least their rated amperage, but any extra won't hurt.

A lot of people shop for power supplies based on wattage. That seems to be the golden cow of computer power supplies. When looking at laptop power supplies, however, it isn't very helpful.

The only laptop adapter I have that even lists wattage says:

12V --- 4.0A
- MAX 49W -

I understand that there's a formula for determining watts from volts and amps, yet the max wattage on this power brick is very close to watts times amps. Many laptops have desktop grade parts, yet they come with these same power supplies.

Edit -

Clarification: How can a small AC Adapter give a laptop enough juice to run a desktop grade GPU when a desktop PSU is much larger and requires a fan and heatsinks?

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Wattage is equal to volts * amps –  wizlog Aug 26 '11 at 20:34
    
Not everyone shops for watts - stability of the load is more important to me. Wattage does matter a bit with desktop systems in that you can add a lot more hardware (multiple hard drives, dual optical drives, high powered sound cards, etc), some of which can be very power intensive (high end video cards, sometimes more than one). Since you won't be adding these types of devices to a laptop, the wattage of the adapter isn't an important consideration - it is designed to work perfectly with the hardware in the laptop. –  MaQleod Aug 26 '11 at 20:34
    
Could you clarify what you're asking? I see a lot of text about your AC adapters and then something about wattage, but I fail to find the connection between the two. –  digitxp Aug 26 '11 at 20:36
    
I'm asking how wattage works in relation to laptops. –  Tyler Faile Aug 26 '11 at 20:54
    
It's important to note that, when it comes to power supplies, more power is not necessarily better. An oversized power supply will waste more energy, and will likely regulate the output voltages more poorly. It's best to "right size" a power supply, and pick one that's adequate but not excessively large. –  Daniel R Hicks Aug 26 '11 at 21:11
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closed as off topic by techie007, digitxp, surfasb, Tom Wijsman, Nifle Aug 27 '11 at 7:41

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5 Answers

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You can easily deduce wattage by taking into account the following formula :

1 watt = 1 amper * 1 volt

When trying to use a different power supply it is best to use one with the recommended voltage and at least the minimum recommended amperage.

I can tell you for sure that the power consumption of a laptop is much lower than that of a desktop computer. All the components inside a laptop are optimised for low power consumption while in a desktop power draw is not the main priority but performance and "bang for the buck" :).

I have a power consumption meter and a PC and a laptop, and in normal operation the PC draws about 200 watts while the laptop stays under 50 watts. Both have Intel Core 2 duo CPUS E7300 (pc) and T7300 (laptop). And I've never seen the laptop reach 100W consumption.

Later Edit: As for the clarification question, the GPU is also optimized and draws less power, the whole platform is optimised so that the laptop has only a fraction of the power requirement of a desktop, and that's why the AC Adapter doesn't need any fans and stuff.. on the other hand, the AC Adapter might get warm to hot from time to time, but it still doesn't need to have the output of a PC PSU and it doesn't need the fans..

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In every case, the voltage of a mains adaptor must match that required by the device to which it is going to be connected - if you apply too much voltage to a device it can damage it - a bit like trying to put too much air in a balloon - in fact, in electrical/electronic terminology, voltage is sometimes described as electrical pressure - too much pressure and things go bang, and, conversely, supply something with too little voltage (pressure) and it hasn't got the energy needed to work properly, if at all.

Once switched on, the voltage from the mains adaptor will be used to drive a current flow - measured in Amps - through the device. The amount of current that flows will be determined by all of the bits and pieces inside the computer and provided you apply the right voltage you cannot force more current through a device than it wants to take - that's why it's OK to use, say, a 19V 5A power supply on a laptop that originally came with a 19V 4A unit - in effect the 5A unit is being underworked, but that's fine. Turn things around, though and use a 19V 4A adaptor with something that needs a 19V 5A supply and the adaptor is going to struggle.

You are right that for a DC supply, the power (in Watts) used is equal to the rated voltage x the maximum current it can supply, so a 19V 5A unit can also be called a 95W unit, but because the voltage is an important value, it's best to call it a 19V 95W unit - after all, a power supply for something else that's rated at 48V, 2A is also a 95W (OK, 96) unit, but it will almost certainly damage anything expecting only 19V.

Volts, Amps and Watts for a desktop PC total up in exactly the same way as for a laptop, but as has been pointed out, laptops tend to use more power efficient components to maximise battery life.

In terms of your edit and clarification, a desktop power supply is generally rated to allow you to add extra disks and expansion cards to the computer, so the overall wattage of the power supply is higher than that of a laptop to allow for expected expansion - still then, you hear of people having to uprate from a 450W power unit to a 600W one because they have added some scary powerful graphics adaptor.

Compare this to a laptop that knows exactly what's inside it and can't be changed by the user except perhaps for adding a bit more RAM or a USB device or slot-in expansion card - in this case, the PSU can be rated accordingly and without needing to assume much more is going to be powered from it. When it comes to GPUs, though, yes even laptop power supplies need to cater for them - for example, our corporate laptops (mostly Lenovo), have 19V 65W or 90W power supplies, but we have a couple of HP laptops with 'better' graphics subsystems that we use for product demos and they come with 19V 120W power supplies - and darn big they are too!

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No, watts are relevant for a laptop, since that is the measure of electrical power that will be available for the laptop to use from the power supply (aka brick). Since laptops are designed to operate from a battery, the PC components installed in the laptop are chosen for electrical power efficiency (as well as light weight). These factors are typically not considered at all for desktop PCs.

The "volts x amps" product (aka VA with units of volt-amps) is a crude measure of power. It is related to the more refined power unit of watts. When dealing with DC and a resistive load, there is no difference between watts and VA. But when dealing with AC and non-resistive (i.e. reactive) load, the sinusoidal nature of the voltage and current should be accounted for. The VA product ignores any sinusoidal (phase) mismatch between voltage and current, whereas the wattage does. The VA number will typically be higher (and never less) than the wattage number.

You might consider acquiring an electrical power meter such as the Kill-A-Watt (under US$20 when on sale). You would then be able to measure the actual power consumption from the wall outlet used by each device. Both wattage and VA can be displayed.

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Because wattage is determined by multiplying the amps by the volts, watts are the most crucial figure.

In general, you can interchange amps for volts (because,again, its multiplication).

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Not sure what you mean by "you can interchange amps for volts" - can you clarify. –  Linker3000 Aug 26 '11 at 21:04
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The listed wattage/amperage on an electronic device is rarely even close to the real value. The listed value will be a peak value, seen maybe at power-on and at a few other times when load is especially high, and even then manufacturers are likely to overstate the wattage/amperage just to be safe.

You can really only tell how much juice a computer draws by measuring it. But I'm guessing that most desktops (except the souped-up gaming models) draw 70-150 watts when "working", and moderate-power laptops less than half that. The display will contribute another 10-30 watts on a desktop system, unless you have a fancy plasma unit or some such (which would be more).

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