Choosing a right charger isn't actually as hard as it seems - You only need to follow some guidelines. The most important thing when choosing a charger, is to identify the plug size (in mm, for example - 7.4 x 5.0 mm), voltage (V) and amperage (A). You can calculate Watts from amperage and voltage. For example, when Your old charger is 20V and amperage is 3.25A, then You need 20 x 3.25 = 65W charger as a replacement. The new charger can also be up to 2x more powerful than the original one. This means that if Your old charger is, let's say - 65W, then 90W charger is also suitable. It doesn't harm the computer. But we strongly do not recommend buying a less powerful charger - it just doesn't supply enough.