Start with your exact laptop model
Laptop manufacturers reuse similar-looking chargers across ranges with different specifications. 'Lenovo IdeaPad charger' is not precise enough — find the full model number, usually printed on a sticker on the underside of the laptop (for example, IdeaPad 330S-15IKB rather than just 'IdeaPad 330').
The original charger's label is equally useful: it lists output voltage, amperage and wattage, plus a part number you can cross-reference. Photograph both labels before you start shopping.
Voltage must match exactly
This is the non-negotiable rule. A laptop designed for 20V must be powered by a 20V supply. Too high risks damage; too low usually means the laptop refuses to charge or behaves erratically. USB-C Power Delivery chargers negotiate voltage automatically, but barrel-connector chargers do not — the number on the label is what you get.
If a listing only says 'compatible with Lenovo' without stating output voltage, treat that as a red flag and look elsewhere.
Wattage: match or exceed, never go below
Wattage (volts × amps) is the charger's capacity. Using the same wattage as the original is ideal. A higher-wattage charger of the correct voltage is generally safe — the laptop draws only what it needs — while an under-powered charger will overheat, charge slowly, or shut off under load.
Common laptop ratings are 45W, 65W and 90W. Gaming and workstation laptops may need 120W or more. When in doubt between two options, choose the higher wattage at the correct voltage.
USB-C vs barrel connectors
Modern thin laptops usually charge over USB-C with Power Delivery (PD). For these, any reputable USB-C PD charger meeting the wattage requirement should work, though some manufacturers show a warning with third-party chargers. Older and budget models use round barrel connectors, where both the outer and inner diameters must fit — a 7.9mm plug will not work in a 4.5mm socket and vice versa.
Measure barrel size from the specification sheet, not by eye. Listings for replacement chargers normally publish both diameters and a compatibility list of laptop models.
Polarity and the fine print
Barrel connectors have a polarity — almost always centre-positive for laptops — indicating which part carries positive voltage. A centre-negative supply of otherwise identical spec can damage the machine. Reputable replacement chargers state polarity explicitly; if it is not stated, do not buy.
Also confirm the mains side: a UK plug (or a detachable kettle lead you already own), and input rated 100–240V if you travel.
Common purchasing mistakes
Buying by brand and plug shape alone is the classic error — voltage and wattage still have to match. Second is 'upgrading' to a cheap no-name high-wattage unit with no safety certifications; look for CE/UKCA marking from a traceable brand. Third is ignoring the connector dimensions on barrel chargers. And finally, remember that a charger that gets uncomfortably hot or smells of plastic is telling you something — stop using it.