Compare room and water temperatures
Start with the intended water temperature, the room’s cooler conditions, the tank’s working volume and its cover. These are more useful than a universal watts-per-gallon number. The equipment must also fit the water depth and its required mounting position.
The heater catalog shows maker capacity ranges. These remain manufacturer ratings; they do not certify the temperature performance of your room and aquarium.
Choose wattage from the heater’s own chart
A heater chart may change the wattage selection with the temperature rise above room temperature. Read the conditions attached to the model you are considering. Do not transfer a chart from a similar heater family merely because both heaters have the same wattage.
The Aqueon Pro 100 catalog and manual disagree above 30 gallons. Do not rely on a 40-gallon claim alone: compare its sizing chart with your room temperature and the water temperature you need.
Preset or adjustable temperature?
A preset heater aims for its stated preset condition. An adjustable model provides a control range. Neither replaces an independent thermometer. Check the current operating instructions for submersion, placement, water movement and the settling time before adjustment.
A heater that is physically too long for a shallow tank cannot be fixed by selecting a lower wattage in a calculator. Check the model dimensions and allowed orientation first.
Estimate electricity as a range
The heater’s rated watts describe its load while heating. A thermostat turns it on and off. The percentage of the day it runs varies with heat loss and room conditions; entering 24 hours at full watts is a maximum-use assumption, not an ordinary forecast.
The electricity calculator accepts a low and high duty percentage. Replace its example with a personal measurement or a range you can explain.
Plan removal and replacement
Read the unplugging and cooling instructions before taking a heater out of water. Keep the heater’s water-level marks and condition visible during routine checks. Do not use a damaged electrical device or treat an aquarium tool as a repair instruction for sealed equipment.
Plan dry electrical access so disconnecting equipment for maintenance is straightforward.