Full freezer efficiency

Why does a full freezer cost less to run than a mostly empty one?

The advice is everywhere: keep the freezer full, and if you have not enough food, fill the gaps with bottles of water. The reason attached is that the frozen contents "help keep each other cold".

That cannot be right as stated. Frozen food is not a source of cold. It is cold because a compressor was paid to make it so, and stays cold only until it has absorbed enough heat to warm. Nothing in the box generates anything. So either the advice is folklore, or it is right for some other reason — and how much is it worth?

Start with where the heat a freezer must remove comes from. It conducts steadily inward through the insulated walls, driven by the difference between kitchen and interior — say twenty degrees against minus eighteen, a gap of thirty-eight — day and night. It arrives with anything warm you put in, paid once per item. It enters when the door opens and cold air is swapped for room air. And in a frost-free model a heater periodically melts frost off the evaporator.

Now the decisive question: which of those is changed by how full the box is? Not the wall conduction, which depends on insulation, area and temperature difference; a cabinet packed with peas conducts exactly as much as an empty one. Not the cool-down of new items — indeed a full freezer got full by paying that bill. Which leaves the door.

So the whole effect, if there is one, lives in what happens when the door opens. Cold air is dense; open an upright and it pours out across the floor while room air flows in above. Filling the volume with food leaves less to swap.

Put a size on that, because the arithmetic does real work. Take a 200-litre cabinet, 0.2 cubic metres, and suppose its free air is replaced once. Room air at twenty degrees has a density near 1.2 kilograms per cubic metre and takes about 1 kilojoule per kilogram per degree, so cooling it through the thirty-eight-degree gap costs 0.2 x 1.2 x 1.0 x 38, about 9 kilojoules. It also carries moisture that must condense and freeze: half saturated at twenty degrees it holds roughly 8.6 grams per cubic metre (from a recalled saturation figure near 17.3), so 1.7 grams enters, and condensing releases about 2,500 kilojoules per kilogram with another 334 on freezing — a further 5 kilojoules. Call one complete air change 14 kilojoules.

That should reset expectations. Fourteen kilojoules is what seven kilograms of frozen food absorbs by warming one degree, so the folk claim of a dramatically cheaper full freezer is not what the physics says. The honest version: a real opening exchanges the air several times over, since the pour-out continues as long as the door stands open, so the saving scales with how long and how often it is open rather than with fullness alone. Opened many times a day, filling is a genuine if modest economy. Opened twice a month, it is worth nothing.

Which yields a test the folk account cannot survive. A chest freezer opens upward, and dense cold air stays in the box rather than pouring over a sill. If the mechanism is air exchange, filling should help an upright appreciably and a chest of the same size and usage hardly at all. The refuting observation: if a full chest freezer measurably outperforms an empty one of the same model, opened equally often, air exchange is not the mechanism — most plausibly it is how the compressor cycles.

That deserves an honest hearing. A loaded cabinet has enormous thermal inertia: frozen food stores something like a thousand times more heat per unit volume, per degree, than the air it displaced, and near melting the latent heat is larger again. The temperature drifts slowly and the compressor runs in long infrequent stretches, plausibly a little more efficient since each start's losses are amortised over a longer run — but "a little" is the operative phrase, and I would not claim a size for it.

What the inertia unambiguously buys is not energy but time. A full freezer that loses power holds temperature for hours where an empty one warms quickly, since every kilogram of ice must absorb 334 kilojoules simply to melt before rising a degree above zero. That is a food-safety benefit, and arguably the better reason for the advice.

The practice is sound and the stated reason is not. "The frozen food helps keep itself cold" treats stored cold as a supply; it is a buffer, charged at your expense. Filling a freezer with bottles of water costs energy once, to freeze them; thereafter they steady the temperature, reduce the air available to spill out, and extend how long the contents survive a power cut. All three are real. None is the food doing the cooling.

The second correction concerns where the biggest term lives. The dominant, always-running load is conduction through the walls, and nothing inside touches it — so siting the cabinet away from an oven, keeping its coils clear and its seal sound are far larger levers than fullness. It also explains a common surprise: filling a rarely opened freezer changes almost nothing, and people who saw no difference on the meter were not imagining it.

There is a limit the other way. Packing a frost-free upright so tightly that circulating air cannot reach the back of the shelves produces uneven temperatures: "full" means space occupied, not airflow obstructed.

One honest refusal: what this is worth in money or kilowatt-hours a year depends on the cabinet's insulation and age, on whether it is frost-free, on kitchen temperature, on door habits, and on a tariff differing by market. Any single figure here would describe somebody else's freezer.