Decent Heater For Five Gallon Tank?

How many watts should a heater for a five gallon be? It'll be for a Betta
I'll be raising the temp of the water by about 15 degrees

 

The general rule of thumb is 2.5 - 5 watts per gallon of water. However it sounds like your room temperature is significantly below the desired water temperature, so I'd probably get something more powerful.

For a 5 gallon, I'd say 50 watts if you're raising the temp 15 degrees Fahrenheit. You may even need 75 watts to raise the temp 15 degrees Celsius.

 

I'm using a 50 watt Aqueon Pro heater in a 7.5 gallon betta tank to hold the temp at 80.
House temp in the winter is around 20°C or 68°F.

 

I use uniclife HT-6050 50 watt for all my betta tanks, 5 and 10 gallon and it works well. $10 on amazon.

 

Aqueon 50 watt is what ive got .It works well .It's the preset 78°F