The Watt capacity (Wattage) of a charger is the amount of power a charger can supply to a device that requests it. Note it is the devices prerogative to request that charge. The Wattage of a deceive is the amount of power a device can source from a suitable charger. There is no danger to either device of charger if the relative wattages do not match. It is ideal for the device (in terms of charge time) that the charger Wattage matches or exceeds the Wattage of the device.