The wattage a tank needs depends on how much heat it loses, which comes down to the gap between room and tank temperature, the size of the tank, and whether it has a lid. The chart below uses the same heat-loss model as the heater calculator, for a tank held at 78 °F.
Heater wattage chart (target 78 °F, with a lid)
| Tank | Room 60 °F | Room 65 °F | Room 70 °F | Room 75 °F | 65 °F, no lid |
|---|---|---|---|---|---|
| 10 gallon≈ 9 gal of water | 50 W | 50 W | 25 W | 25 W | 75 W |
| 20 gallon high≈ 18 gal of water | 100 W | 75 W | 50 W | 25 W | 150 W |
| 29 gallon≈ 25 gal of water | 150 W | 100 W | 75 W | 25 W | 200 W |
| 40 gallon breeder≈ 40 gal of water | 2 × 150 W | 200 W | 100 W | 50 W | 2 × 150 W |
| 55 gallon≈ 51 gal of water | 2 × 150 W | 200 W | 150 W | 50 W | 2 × 200 W |
| 75 gallon≈ 70 gal of water | 2 × 200 W | 2 × 150 W | 200 W | 75 W | 2 × 300 W |
| 90 gallon≈ 80 gal of water | 2 × 300 W | 2 × 200 W | 200 W | 75 W | 2 × 300 W |
| 120 gallon≈ 107 gal of water | 2 × 300 W | 2 × 300 W | 2 × 150 W | 100 W | 2 × 500 W |
Sizes are rounded up to standard heater wattages. Above 200 W the load is split across two or more heaters.
What about "5 watts per gallon"?
The classic rule is 2.5–5 watts per gallon. It is a fair starting point for a tank a few degrees above a heated room, but it ignores the two things that matter most. In a warm room it oversizes, and in a cold garage or basement the low end of the range is too small: a 10 gallon in a 60 °F room needs about 4 W per gallon, because small tanks lose heat faster for their volume. For a 55 gallon the rule says up to 275 W; with a lid in a 70 °F room the chart above shows far less is enough.
Is a bigger heater a problem?
Not for your power bill: a heater replaces the heat the tank loses, so a larger one simply runs for shorter bursts and uses the same energy. The risk is failure. A heater whose thermostat sticks on can overheat the tank, and a much larger heater does it faster. That is one reason to stay close to the recommendation, or to use an external controller on large heaters. The heater size comparison shows heat-up time and running cost for different sizes side by side.
When to use two heaters
On tanks that need more than about 200 W, two smaller heaters at opposite ends spread the heat more evenly and give you a backup: if one fails off, the other keeps the tank from dropping fast; if one fails on, it is too small to cook the tank alone.
The cheapest upgrade: a lid
Most heat leaves through the water surface by evaporation. A glass lid or cover cuts that sharply, which is why the "no lid" column needs noticeably more wattage. A lid also reduces evaporation top-offs. Then see what the heater costs to run with the electricity cost calculator.