I have always used a 4-5 watts per gallon ratio. However I have always split the total wattage into 2 heaters. Example if I needed 1000 watts I would install two 500 watt heaters. Typically this will give two smaller heaters vs. one larger one and the smaller ones are easier to find room for etc. I also set one of the two to my desired temperature and the other a degree lower. This allows the higher setting heater to kick on by its self when the temp drops and if it does not heat keep up the temp the second heater turns on. The heaters turning on at differnet times is an advantage to many as they do not have 1000 watts of draw on their electrical system all at one time.
Another advantage to the two heaters is if one fails the other can keep the temp somewaht stable until you can get a replacement. (Shame on you if you do not have a spare heater in you water prep container to pre-heat water at water change time
Heaters are fairly cheap compared to the investment in a tank, livestock etc. Keep one on hand.