You can but you should try to avoid using a lower wattage charger than what your laptop requires. A 30W charger can’t supply sufficient power for a 90W laptop’s needs. At the least, your laptop is likely to run out of power while you are using it. At the worst, you could damage the laptop or create dangerous problems for the charger.

The under-powered charger won’t have enough power to give to your laptop and it therefore has to work harder to try to match the needs of your laptop. The danger is that this can cause overheating and potentially fire.

For more details read our explainer What wattage USB-C charger can you use to charge a MacBook?

Yes, you can safely use a laptop charger with a higher wattage as the laptop will only ever draw the amount of power required at the time. If your laptop was supplied with a 35W charger it will never draw more than 35W unless being fast-charged, so you can use a 65W, 90W or higher charger. To reassure you even further, you can safely charge your 5W AirPods with a 240W charger if you wish.

If the charger has multiple ports that extra juice will mean you can charge other devices at the same time.