MacBooks come with power adapters that provide a certain wattage (for example 87 W) required to keep the computer running and charge the battery. If used with a lower-wattage power adapter (for example 61 W), there might not be enough power to keep the computer running. However, could such a lower-wattage power adapter be used to fully recharge the battery if the computer is kept turned off?

Could even a low-wattage iPhone charger be used for this, although it would probably take a long time?

Update: The question concerns newer MacBooks with USB-C chargers.