Size your reef heater from your tank's real heat loss, then see how long the tank stays safe if the power goes out. Enter the display's inside dimensions, the sump water, and your room and tank temperatures.
How it's calculated
Heat leaves through the glass and the open water surface. We estimate the tank's heat-loss coefficient as K = h × A, where A is the area of all glass faces plus the water surface and h = 10 W/m²·K, a middle value for still air that includes radiation and surface evaporation (Incropera, DeWitt, Bergman & Lavine, Fundamentals of Heat and Mass Transfer (Wiley): Newton's law of cooling and free-convection coefficients (about 2–25 W/m²·K)). The heater must replace K × (tank − room temperature); we add 50% headroom and split it across two heaters. A 36 × 13 × 24 inch tank (about 49 gal) in a 68 °F room needs roughly 120 W of steady heat to hold 78 °F, so we suggest about 180 W total, or two 100 W heaters, close to the familiar 3–5 W per gallon rule.
For outages, the water cools toward room temperature as T(t) = Troom + (Tstart − Troom) × e−t/τ, where τ = mass × specific heat ÷ K. Saltwater is about 1.023 kg per liter with a specific heat near 3.99 kJ/kg·K (Sharqawy, Lienhard & Zubair (2010), Thermophysical properties of seawater, Desalination and Water Treatment 16). Sump water adds thermal mass, so it lengthens the time. Lids, foam insulation, and a battery air pump all help in a long outage; keep the water moving and the lid on.
Related: reef tank temperature guide and electricity cost calculator.
FAQ
What size heater do I need for a reef tank? Most reef tanks need about 3–5 watts per gallon when the room is 8–10 °F cooler than the tank. This calculator estimates your tank's actual heat loss from its glass area and the temperature gap, adds a 50% margin, and suggests splitting it across two heaters so one failure can't cook or chill the tank.
How long will my reef tank stay warm in a power outage? Water cools exponentially toward room temperature. A 36 × 13 × 24 inch tank (about 49 gallons) in a 68 °F room takes roughly 9 hours to fall from 78 °F to 72 °F by this estimate. Big systems and sumps cool more slowly; small tanks and cold rooms much faster. Measure your own tank once with the heater off to calibrate.
Why two heaters instead of one? Heaters fail on and off. Two half-size heaters mean a heater stuck on heats the tank more slowly, and a dead heater still leaves half the heating. Pair them with a controller as the failsafe.
Sources
- Incropera, DeWitt, Bergman & Lavine, Fundamentals of Heat and Mass Transfer (Wiley): Newton's law of cooling and free-convection coefficients (about 2–25 W/m²·K)
- Sharqawy, Lienhard & Zubair (2010), Thermophysical properties of seawater, Desalination and Water Treatment 16