Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
How fast the tank cools with the heater off, and how long you have before it matters.
| Hours without power | Temperature | Dropped by |
|---|---|---|
| 1 h | 77.5 °F | 0.48 °F |
| 2 h | 77.1 °F | 0.93 °F |
| 4 h | 76.2 °F | 1.78 °F |
| 8 h | 74.8 °F | 3.25 °F |
| 12 h | 73.5 °F | 4.45 °F |
| 24 h | 71.1 °F | 6.92 °F |
Water holds a lot of heat, so a reef tank cools far more slowly than most people expect. Oxygen is the more urgent problem in a long outage. A battery air pump costs little and matters more than a generator for the first day.
U = 90 W ÷ 5.56 K = 16.20 W/K · τ = 284 L × 4186 ÷ 16.20 = 20.4 h · T(t) = T_room + ΔT·e^(−t/τ)
A tank losing heat follows Newton's law of cooling. The rate of loss is proportional to how far above the room the water sits, which gives exponential decay toward room temperature: T(t) = T_room + ΔT₀ · e^(−t/τ).
The time constant τ is the thermal mass divided by the loss coefficient, τ = V × 4186 ÷ U. Water is 4186 joules per kilogram per kelvin, so a large volume gives a long time constant and a slow drop.
The loss coefficient U is measured rather than assumed, using the heater that already holds the tank. If a 300 watt heater runs 30 percent of the time to hold 10 °F above the room, the tank is losing 90 watts across that gap, so U is 90 divided by the gap in kelvin. That calibrates the whole calculation to your actual tank, lids, and room.
The practical result surprises people. A few hundred litres in a normally heated house drops one or two degrees Fahrenheit in four hours. Temperature is rarely what kills a tank in a short outage.
Oxygen is the real risk. With no flow and no skimmer, gas exchange stops while respiration continues, and a heavily stocked tank goes hypoxic within hours. A battery powered air pump is the cheapest insurance in the hobby and does more than a generator for the first day.
Slow the cooling with what you already have. Close the lids, throw blankets over the tank and sump, and leave them alone. Floating sealed jugs of hot water works and is safer than any improvised heater.
Do not feed during an outage, and do not dose anything. Both add demand the system cannot currently handle.