Sadly it's not a simple thing to work out, depends on a lot of variables, including the maximum temperature difference between the room temp and the desired water temp, the shape of the tank, type of cover/hood, the material the tank is made from (acrylic/glass), how thick the material is. etc. etc. etc.
For a fairly typical rectangular glass aquarium, personally I wouldn't go less than 1watt/litre (~4watts/US Gallon) unless I lived in a warm environment or had climate control on in my house 24/7.
shes right need more info because it can vary a lot. i run a 250 on my 180 gallon acrylic but a 150 on my 60 gallon glass. but the tanks in the garage gets up to a 250 on a 40 gallon.