How many watts does an aquarium heater use per gallon?

In this regard, how many watts do I need for my aquarium heater?

Most aquarium heaters range from 25 watts to 300 watts. Smaller aquariums require smaller heaters. A 10-gallon tank, for example, will typically need a 25- to 75-watt heater, while a 75-gallon tank will require 250 to 300 watts. Larger tanks may require multiple heaters.

Also, can I use a 50 watt heater in a 2.5 gallon tank? That said, most modern heaters are pretty reliable (as long as you stick with the well-established brands) and a 50W shouldn't generate too much heat too quickly for a 2.5-gallon.

Likewise, will a 50 watt heater work for a 20 gallon tank?

Yes you can have a 50 watt heater in a 20 gallon. Another thing is since a betta tank should be kept at 78-80 you might overwork the heater. Normally larger is better so that you don't over work the heater.

How many watts does a 40 gallon tank need?

A 40 gallon fish tank will need at least a 120-watt heater if the room it is kept in is 5 degrees cooler than your aquarium. If the room is 10 degrees cooler then a 40 gallon tank will need a 200-watt heater.