How to read the comparison
Three columns matter, and they tell one story from different angles. Heat-up time is how long the tank takes to get within 2 % of the target from a cold start, roughly halving each time you double the wattage. On-time at target is the thermostat duty cycle once the tank is warm: this is where the myth dies, because every heater that can reach the target ends up drawing about the same total energy per day. Energy & cost convert that duty cycle into kWh and money. Watch the daily column stay flat across sizes while heat-up time shrinks.
The practical trade is therefore speed versus safety margin, not price. A smaller heater is cheaper to buy and gentler if a thermostat ever sticks; a larger one recovers faster after water changes and copes with colder rooms. The chart beside the table draws each heater's temperature curve. Hover or tap it to read temperatures at any moment, and click a row or curve to pin a size.
Setting your tank's numbers
If you arrived from the heater calculator, the recommendation arrived pre-selected via the link, and the volume, temperatures and exposure it was based on are one click away under “Tank inputs”. Arrived directly? Open the same section and enter your water volume, room temperature, target temperature and how exposed the tank is; the whole comparison re-computes as you type. Need the volume first? The aquarium volume calculator hands it straight to the heater tools.
The model behind the numbers
The curves use a lump-of-water model with Newton cooling: the heater pushes the water toward an equilibrium set by its power, while the room constantly pulls heat back out through the glass. Consequences the old “litres ÷ watts” rule hides: a heater that is too small may never reach the target (the table flags those as “too small”), and heat-up always depends on the temperature gap, not just volume. Running-cost figures use a thermostat duty-cycle model: the heater only draws power while replacing standing loss. Everything is labelled an estimate; real tanks vary with hoods, stands and seasons.
Frequently asked questions
Does a bigger heater use more electricity?
No. This is the most common misconception about aquarium heaters. Once the tank is at temperature, every heater only replaces the same standing heat loss to the room. A 300 W heater simply runs about one-third as often as a 100 W unit doing the same job, so the daily kWh and the cost stay essentially the same. A bigger heater heats up faster and cycles less; what actually cuts the bill is a lid, insulation and a warmer room.
Is it better to buy the biggest heater I can afford?
Modestly larger is fine and even convenient (faster heat-up after water changes), but there are real limits. In small tanks an oversized heater can swing the temperature quickly if the thermostat sticks, and a 300 W unit in a 10 litre bowl is a hazard, not a comfort. For big tanks prefer two heaters over one giant unit.
Which wattage should I compare against my recommendation?
Compare the size you already own, the calculator’s recommendation, and the next size up or down. If a smaller heater reaches the target in a time you can live with, it will cost the same per day and run gentler cycles; if the recommended size takes longer than an overnight heat-up, the next size up is justified.
How accurate are the heat-up times and costs shown here?
They are model estimates: a lump-of-water heat-loss model (Newton cooling, ~90 % element efficiency) calibrated so results land near published manufacturer sizing charts. Real tanks also lose heat through the stand, hood and evaporation, and rooms change temperature. Treat the numbers as comparisons between sizes, not guarantees.
What electricity price should I enter?
Use the per-kWh price from your utility bill (the energy rate, before fixed fees). The cost columns only appear once a price above zero is entered; leave it empty to compare sizes on heat-up time and duty cycle alone.
Related calculators
Start from real inputs: the aquarium volume calculator measures your tank (including glass thickness and fill level), the heater calculator recommends a wattage, and the electricity cost calculator adds up every device on the tank: filter, lights, air pump and all.