The short answer is that you may be able to use that 130-watt power supply with your new machine, which came with a 65-watt supply, if a couple of conditions are met.
Let’s review those conditions.
Become a Patron of Ask Leo! and go ad-free!
Swapping power supplies
It can be kind of complicated and confusing if you’re not familiar with how a power supply (and to some degree, electricity itself) works.
The short rules boil down to this:
Input versus output
People sometimes get confused about which set of numbers to pay attention to. Somewhere on the power supply itself, there’ll typically be a confusing jumble of information and logos, all often in very small print.
There are two sets of numbers we care about: input and output.
Input specifies what power your power supply itself requires from wherever you plug it into. Normally, it will say something like “AC 100V-240V”, and may or may not include watts or amps. This is simply what your power supply takes as its input from your wall socket.
Electrical systems around the world use different standards – 120 volts or 240 volts are the most common. A label on your power supply that includes something similar to “100-240V” as part of the input specification means that the power supply will work with whatever it’s given in that range, be it 100 volts, 240 volts, or anything inbetween. In other words, it’ll work just about anywhere.
Naturally, you should check to make sure your power supply includes “100-240V” if you’re travelling to a location with a power standard different than your own.
Output specifies the characteristics of the power provided by the power supply to your computer. This is is often something like “DC 19.5V 4.62A”, meaning that this power supply provides electricity at 19.5 volts and is capable of providing a little over 4.5 amps of electricity.
The input numbers are all about connecting to the wall socket. It’s the output numbers that need to meet your computer’s expectations.
On plugs, sockets, and polarity
Particularly when you use a power supply from a different vendor, it’s critical that the polarity be correct. The connections have positive (+) and negative (-) wires, and those must match what your laptop expects, or you can do damage.
Among that jumble of logos and small print on the power supply label, there is typically a diagram that indicates polarity.
This must match what your computer expects. Unfortunately, while there are generally accepted standards for labeling power supplies, there’s no equivalent for the laptop or computer itself. Look for similar information, either on the computer case or in the computer’s documentation.
But if you’re not sure, don’t try until you have someone knowledgable look at it.
The other way to determine what your computer requires is to look at any power supply you already have that works, like the one it came with. For example, if both of your machines are Dell laptops, as long as they use the same kind of power connector (you wouldn’t be able to plug it in if not), they’ll be polarized the same way. In Dell’s case, that means the positive lead is in the center and the negative lead is the outside of the connector.
Output voltage must match
The output voltage provided by your power supply needs to match. There is absolutely no substitute for that.
If your computer needs 19.5 volts, or the prior power supply provided 19.5 volts, then your power supply must be 19.5 volts. Period.
Normally, if you’ve got the same kind of power supply from the same manufacturer and it can plug in at all (meaning the plugs match), voltage isn’t something you have to worry about. I’ve used different versions of Dell power supplies across a number of different Dell machines.
But regardless of the source or the computers involved, the voltage output by the power supply – the number associated with “V” – must match the voltage expected by the computer you’re plugging it into. If you’ve got an existing power supply you’re replacing, you can read the output voltage from its label.
Amps or watts must be the same or more
I’ll just refer to watts, since it’s perhaps more common, but the same comments apply to amps. (Often the power supply label will include only one or the other.1)
Unlike voltage, which is a constant, the watts rating of a power supply indicates what it’s capable of. A power supply rated for 130 watts can provide up to 130 watts. It can easily provide anything between zero and 130 watts of power; it just can’t provide more than that.
Your laptop will require a certain number of watts. The power supply you use must provide at least that many, and it’s OK if it provides more.
Since your laptop came with a 65-watt power supply, it’s safe to assume your laptop needs no more than 65 watts of power. That means:
- The 130-watt power supply will work just fine and will have plenty to spare.
- The 65-watt power supply will work, because that’s what came with the laptop.
So in the scenario you’re running, things are just fine.
What if watts or amps are less?
If you use an under-powered power supply – one that cannot provide the number of watts or amps that your laptop might require – results are unpredictable.
While rare, it is possible you could damage the laptop, or even the power supply itself.
More commonly, your laptop simply won’t charge, or won’t charge as quickly.
Some years ago I had one of the higher end, more powerful Dell laptops. I had the opposite scenario of yours. It came with a 130-watt power supply, but I also had a 65-watt power supply.
This machine would, of course, work properly with a 130-watt power supply. But, if I connected a 65-watt power supply instead, the laptop would actually tell me at boot-up, “The power supply that’s connected isn’t powerful enough. I may not be able to recharge; I may only be able to recharge at a very slow rate.”
So in my case, the laptop was able to detect that I had put in a power supply that wasn’t really strong enough. In my experience, most laptops aren’t as informative.
The bottom line
The bottom line, once again, is:
- Connectors must match
- Polarity must match
- Voltage must match
- Amps or watts must be greater than or equal to what is needed
Meet those requirements, and you’re good to go.
If you found this article helpful, I'm sure you'll also love Confident Computing! My weekly email newsletter is full of articles that help you solve problems, stay safe, and give you more confidence with technology. Subscribe now and I'll see you there soon,