For a tank up to 50 gallons in size, you need a heater that can provide 5 watts per gallon (A 10-gallon tank uses a 50-watt heater)
SO..
5 watts x 15 US Gallons = 75 Watts heating
It really does depend on the make of heater each one seems to be different from what i have researched....
Some say that up to 15 US Gallons which is 13 UK Gallon you need 50W some say 100W, when you read the packaging on the heater it will tell you.