What wattage aquarium heater do I need?

The common rule cited across reviewers is roughly 3 to 5 watts per gallon for typical tropical setups in a normally heated room. That puts a 10-gallon tank at 50W, a 20-gallon at 100W, a 40-gallon at 200W, and a 75-gallon-plus tank at 300W to 500W. Cold rooms, open-top tanks, or species that need higher temperatures push you toward the upper end of that range.