Aquarium heater size calculator
A heater has to replace what the tank loses to the room, which depends on the temperature difference and the glass area.
m2
The total glass area, roughly.
W/(m2 K)
per kWh
Heater wattage
165W
- Heat loss at that temperature difference
- 110.0 W
- Heater duty cycle
- 66.7 %
- By the watts-per-litre rule
- 60 W
- Energy per day
- 2.640 kWh
- Annual running cost
- 269.99
- Hours to raise the tank two degrees
- 2.82
- Temperature difference
- 8.0 K
Show the working
- Water volume = 200
- Target temperature = 26
- Coldest room temperature = 18
- Tank surface area = 2.5
- Glass U-value = 5.5
- Electricity price = 0.28
- Temperature difference = 26 - 18 = 8
- Heat loss at that temperature difference = 2.5 * 5.5 * max(8, 0) = 110
- heater_watts = 110 * 1.5 = 165
- By the watts-per-litre rule = 200 * (8 > 8 ? 0.5 : 0.3) = 60
- Heater wattage = max(165, 60) = 165
- Heater duty cycle = 165 > 0 ? 110/165*100 : 0 = 66.6667
- Energy per day = 110 * 24 / 1000 = 2.64
- Annual running cost = 2.64 * 365.25 * 0.28 = 269.993
- Hours to raise the tank two degrees = 165 > 0 ? 200 * 4.184 * 2 / 3.6 / 165 : 0 = 2.817 51
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- heat loss = surface area x U-value x temperature difference
- heater sized at 1.5 times the steady loss
Two smaller heaters are safer than one large one: a stuck-on heater cooks the tank, and a failed one leaves you some heat.
A tank in a cold room needs far more heater than the volume rule suggests.