45W, 65W, 90W or 130W: which charger a laptop needs

Watts are volts times amps

On a barrel charger the voltage is fixed by the brand. Dell and HP run at 19.5V, Lenovo at 20V, and Toshiba, ASUS and Acer at 19V. What changes from one adapter to the next is how much current it can supply at that voltage, and current multiplied by voltage gives you watts.

At 19.5V, a 45W adapter supplies about 2.31 amps, a 65W adapter about 3.34 amps, a 90W adapter about 4.62 amps and a 130W adapter about 6.67 amps. The plug and the voltage stay the same, but the largest adapter supplies nearly three times the current of the smallest. Every adapter prints this on its label as an output line, for example 19.5V 3.34A, so you can always work out the wattage even when the label does not state it.

Use the same wattage or higher

A laptop draws the current it needs and no more. Put a 90W adapter on a laptop that shipped with a 65W one and, as long as the plug and the voltage match, it simply works, runs a little cooler and has headroom to spare. The only costs are a larger brick and a higher price.

The other way round causes trouble. An undersized adapter cannot meet the laptop's demand when it is working hard, so the battery drains even while plugged in, charging takes hours or stops partway, and the laptop may slow its processor to stay within the power available. Dell and HP machines also identify the adapter through the centre pin and warn at start-up that it is too small. The adapter itself runs hot all day, which shortens its life, and that comes back to you as a return.

The counter rule that follows is simple: match the wattage or go higher, never lower.

What each size is for

45W suits thin and light machines with low-power processors, including many 13-inch and some 14-inch business laptops. It is the smallest and cheapest brick, which matters to customers who carry one every day, but it is the most likely to be undersized if you guess.

65W is the workhorse of business fleets. Most 14 and 15-inch office laptops with mainstream processors ship with one, and if you only stock one wattage per connector, this is it.

90W covers larger laptops, machines with faster processors or entry-level dedicated graphics, and laptops that also power accessories through a dock. It will also run any 45W or 65W machine with the same plug and voltage.

130W and above is mobile workstation territory: Precision and ZBook class machines with dedicated graphics. Some of these need 180W or more, which a 130W adapter will not satisfy, so always read the label on the laptop. On our current list, HP's top barrel adapters are rated 120W rather than 130W.

Finding the right number

The quickest source is the rating label on the base of the laptop, which gives the input voltage and current. Multiply them: 19.5V at 4.62A is 90W. If the label has worn off, the original adapter's label gives the same figures as an output.

Many Dell laptops, and some other brands, also show the attached adapter's wattage in the battery or power page of the BIOS setup screen, which is useful when a customer is unsure whether their current charger is the right one. Failing that, the model number on the base leads to the manufacturer's specification sheet, which lists the adapter it shipped with.

For laptops bought by the lot, the charger column on each lot's spreadsheet shows which units include an adapter. Most do not, so budget for chargers when you price units for resale.

USB-C wattage works differently

A USB-C adapter does not have one fixed voltage. Through USB Power Delivery it offers a set of voltages, typically 5V, 9V, 15V and 20V, and the laptop asks for the one it wants. Laptops charge at 20V, so a 65W USB-C adapter supplies 20V at 3.25A, and a 90W adapter supplies 20V at 4.5A.

Two things follow. First, the cable matters: an ordinary USB-C cable is rated for 3A, which caps it at 60W, so a 90W or 100W adapter needs a 5A cable to deliver its full output. Second, laptops react to small USB-C adapters in different ways. Some charge slowly and show a slow charger message, some charge only when switched off, and some refuse to charge below a minimum. A larger USB-C adapter is safe on smaller devices, phones included, because each device asks only for what it needs.

Stocking by wattage

Order by connector first and wattage second. Within each connector you sell, hold 65W in depth, add a smaller stock of 90W to cover customers who want headroom or run larger machines, and keep 45W for the thin models where size and price are what the customer cares about. Hold 130W only if you serve customers with workstations.

Because a larger adapter always covers a smaller laptop of the same connector, a few 90W adapters can rescue a sale when the 65W shelf runs empty. The reverse never works, so when stock is tight, run out of 45W before you run out of 90W.