You can use a lower-rated adapter to charge your laptop, but it will be considerably slower and will only charge the laptop if it is off or hibernating. If the laptop is running with a lower-rated adaptor, it will pull the power that it needs from the battery, but the charger won’t be able to keep up with the full power draw, so eventually the battery will run out.
It’s best to use a power adapter with the same wattage or higher than what your notebook came with.
In any case, a reputable adapter should cause no harm to your system.
