How to size an aquarium heater
Enter your tank's water volume and how many degrees Celsius you need the water to sit above the room it lives in. As a rule of thumb, about one watt per litre maintains a five-degree rise in a normally heated room, and the estimate scales linearly from there. The calculator rounds up to the nearest standard heater size, and for large tanks it recommends splitting the load across two heaters, which gives more even heating and a safety margin if one fails.
A heater is sized for the coldest the room gets, so measure the winter room temperature, not the summer one. A thermometer in the tank is essential regardless of the wattage; the wattage only sets how quickly and reliably the heater can reach the target, not the target itself.