My guess is that this would be a 15W heater without adjustment. If this is the case, the heater should self regulate to the temperature as stated on the packaging. You shouldn't have any worries about how quickly the water obtains this temperature as your temperature should not be off much to begin with.
It is generally advisable to have a heater that is able to heat more than the capacity of your tank as the inverse wouldn't be desirable.
Good buy, happy that you care enough for your fish to provide them with the right environment!