Key takeaways
- Heater wattage should be calculated from the gap between your coldest room temperature and your target, not from tank volume alone.
- A delta of 6C (11F) needs roughly 1 watt per litre, or about 4 watts per gallon, on a lidded glass tank.
- The old 5 watts per gallon rule can specify nearly three times the heat a normally heated room requires.
- Tanks under 40 litres need proportionally more watts per litre because they lose heat through more glass per litre of water.
- Above 200 litres, two smaller heaters give better distribution and a survivable failure in both directions.
A customer brought in a 60 litre nano last winter with a 300W heater clamped inside it. The tank had been sitting at 30C (86F) for a week and every shrimp in it was dead. Nothing had broken.
The heater was simply so far oversized that each thermostat cycle dumped enough energy into that small volume to sail past the setpoint before the contacts opened. The unit was working exactly as designed. The sizing was the fault.
Wattage is the spec most people copy off a forum post and never check against their own house. That is the mistake, because the house is half the calculation.
Size the gap, not the tank
A heater has one job: replace the heat your tank loses to the room. If your living room holds 22C (72F) and you want 25C (77F), the heater only has to make up 3C (5F). Put the same tank in an unheated garage at 12C (54F) and it has to make up 13C (23F), which is a different appliance entirely.
So the number that drives everything is the delta: your target temperature minus the coldest room temperature you will realistically see. Not the yearly average. The coldest, because that is the night the heater has to hold the line alone.
Measure it rather than guessing. A cheap digital min and max thermometer left on the shelf beside the tank for a week in the coldest month you can manage will tell you more than any sizing chart.
Watts per litre and watts per gallon
With the delta in hand the arithmetic is close to linear for tanks between roughly 60 and 400 litres. The figures below are working estimates for a standard glass tank with a lid, not laboratory values, and an open top tank with a surface skimmer will sit at the high end of each row.
| Temperature gap | Watts per litre | Watts per gallon | 200 litre (55 gal) tank |
|---|---|---|---|
| 3C (5F) | 0.5 | about 2 | 100W |
| 6C (11F) | 1.0 | about 4 | 200W |
| 9C (16F) | 1.5 | about 6 | 300W |
| 12C (22F) | 2.0 | about 8 | 400W, split across two units |
Round up to the next standard size. Most makers step through 25W, 50W, 75W, 100W, 150W, 200W, 250W and 300W, so rounding rarely costs you more than twenty percent.
Why five watts per gallon runs hot
The old rule ignores the delta completely. It assumes one room temperature for every reader in every climate, which is why it survives: it is easy to repeat and impossible to check.
Run it on a 55 gallon (208 litre) tank and it tells you to buy 275W. In a 21C (70F) room holding 25C (77F), that tank needs closer to 100W. You have bought nearly three times the heat the job requires.
Excess wattage is not free. A larger element raises a given volume faster, so each cycle overshoots further past the setpoint before the thermostat opens, and the tank oscillates instead of holding steady. On volumes under 100 litres that overshoot is measurable in whole degrees.
Oversizing raises the stakes of a stuck thermostat. A jammed 100W heater in 200 litres climbs slowly enough that you may catch it on a morning water check. A jammed 300W in the same tank can add 4C to 5C (7F to 9F) in an afternoon, which is well past the point where a swing starts doing real damage.
Small tanks break the ratio
Below about 40 litres the maths stops being linear. A nano loses heat through proportionally more glass per litre of water, so the watts per litre figure climbs even though the absolute number stays modest.
You are also limited by what exists. The smallest dependable adjustable heaters start at 25W or 50W, so a 20 litre shrimp tank in a 21C (70F) room will be running something technically oversized no matter what you buy.
That is exactly where a separate thermostat sitting between the heater and the socket earns its price. It takes the decision away from the bimetal strip inside the heater and switches on a probe reading the open water, typically holding within 0.3C of the setpoint.
One heater or two
Above roughly 200 litres, splitting the load is usually the better build. Two 100W units in a 200 litre tank give the same total output, better distribution along the tank, and a failure that is survivable in both directions.
If one sticks off, the other holds the tank a degree or two low instead of letting it fall to room temperature overnight. If one sticks on, only half the wattage is trying to cook the fish. Most of the argument for running a pair rests on that second scenario.
The cost is two points of failure instead of one, and two units to inspect at every clean. On tanks under 100 litres it is rarely worth the extra clutter.
Sizing table by tank volume
The table assumes a lidded glass tank, moderate flow, and a target of 25C (77F). The mild column covers a heated room that holds 20C to 22C (68F to 72F). The cold column covers a room that drops to 15C to 17C (59F to 63F) on winter nights.
| Tank volume | Mild room | Cold room |
|---|---|---|
| 40 L (10 gal) | 50W | 75W |
| 75 L (20 gal) | 50W | 100W |
| 110 L (30 gal) | 75W | 150W |
| 150 L (40 gal) | 100W | 200W |
| 200 L (55 gal) | 100W | 2 x 150W |
| 300 L (75 gal) | 150W | 2 x 200W |
| 450 L (120 gal) | 250W | 2 x 250W |
Manufacturer charts such as the one published for the Eheim Jager range land in the same territory, though most assume a fixed gap of around 6C and do not tell you so on the box. Read any chart as a starting point, then adjust for your own room. The Eheim product documentation is one of the few that states its assumptions.
Placement changes the answer
A correctly sized heater sitting in dead water behaves like an undersized one. The thermostat reads the warm pocket around its own element, shuts off early, and the far end of the tank sits a degree cooler than the display suggests.
Mount it in moving water, near the filter outflow or the intake strainer, and the volume equalises within minutes. Getting the position right matters far more on a 120cm long tank than on a tall cube.
Wattage does not set the temperature
Wattage decides whether the heater can hold a setpoint against the room. It says nothing about what the setpoint should be. Most community tropicals sit comfortably at 24C to 26C (75F to 79F), discus want 28C to 30C (82F to 86F), and true coldwater species should not be on a heater at all.
Check the range your stock actually wants before you buy, because a discus tank in a cold room can push the delta to 14C (25F) and change the wattage bracket entirely.
Sizing a 180 litre tank in a 19C room
A 180 litre (48 gallon) planted community tank, room dropping to 16C (61F) on winter nights, target 25C (77F). The delta is 9C (16F), so 1.5 watts per litre gives 270W.
Rounding to a single 300W glass heater would work and is what most shops would hand you. The owner went with two 150W units instead, one at each end of the tank, and a probe thermometer at the opposite corner from each.
Measured spread across the tank came in at 0.4C. Total draw is identical to the single 300W, no single element is hot enough to scorch a leaf or a sleeping loach, and a stuck thermostat now has half the power behind it.
What the money buys at each level
Under about $30 you are looking at basic glass adjustable heaters with a bimetal thermostat and a printed dial. They work, they are usually accurate to within 1C to 2C once you have found the offset, and the dial markings should be treated as suggestions rather than measurements.
Between roughly $40 and $80 you get shatter resistant polymer bodies and electronic thermostats with tighter deadbands. Units in this bracket, such as the flat polymer Cobalt Neo-Therm or the Fluval E series with its own display, typically cycle in a narrower band than a bimetal unit.
Above that you are buying a titanium element with a separate controller, which is a different design philosophy rather than simply a better heater. The tradeoffs between glass, titanium and inline units come down to whether you want the sensing done inside the heater or outside it.
Check your sizing before you buy
- Log the coldest room temperature over a full week, not a single evening
- Subtract that from your target to get the delta
- Multiply tank volume in litres by the watts per litre figure for that delta
- Round up to the next standard heater size
- Above 200 litres, split the total across two units
- Confirm the target suits every species in the tank
If the number you land on is more than double what the shop chart suggests, your room is the problem and insulating the tank lid will do more good than another 100W of element.
Questions readers ask about this
Undersize slightly rather than oversize heavily. A heater 20 percent under spec runs longer cycles and may sit 0.5C below target on the coldest nights, which most fish tolerate. A heater at triple the needed wattage overshoots on every cycle and can add 4C to 5C in an afternoon if the thermostat sticks closed.
In a room holding 20C to 22C (68F to 72F) with a 25C (77F) target, a 20 gallon (75 litre) tank needs about 50W. If the room drops to 15C (59F) at night, step up to 100W. Below 25W nothing dependable is manufactured, so very small tanks are always somewhat oversized.
Yes, and more than most people expect. Evaporation is the largest single heat loss path on an open tank, so a glass lid or a tight hood can cut the required wattage by roughly a quarter. If your calculated figure looks high, fitting a lid is cheaper than buying a larger heater.
You can, but set them to the same temperature and accept that the larger one will do most of the work. Matched pairs are easier to reason about when one fails. Two 150W units in a 300 litre tank behave predictably; a 200W paired with a 50W means losing the big one drops you to a quarter of your heating capacity.
A heater draws its rated wattage only while cycling, typically 15 to 30 percent of the time in a normally heated room. A 200W heater at 20 percent duty averages 40W, or roughly 29 kWh a month. At common domestic rates that lands somewhere around $4 to $9 depending on your tariff and room temperature.
