The figure of merit for getting rid of heat in space.
Every kilogram launched costs money, so the useful question about a radiator is how much heat it rejects per kilogram. Two things set the answer. Physics sets the heat per square metre: at 50 °C in low Earth orbit, a white-painted panel radiating from both faces sheds about 1,000 W per m², and only a hotter radiator does better. Engineering sets the mass per square metre: the panel, its tubes and coolant, armour against micrometeoroids and the deployment mechanism, plus pumps and plumbing for every kilowatt it carries. Divide one by the other.
Two things follow. At a given temperature, a lighter radiator is the main lever, until the pumps dominate. And the pumps set a ceiling: with mP kilograms per kilowatt, no radiator, however light, beats 1,000 ÷ mP W/kg.
A companion to watts-to-space, where the same figure appears as the "Thermal rejection" readout for every scenario.
Every point is a radiator: the heat each square metre rejects, against what its cooling hardware weighs per square metre, panel plus pumps. Watts per kilogram is one divided by the other, so each diagonal is one value of W/kg. Down and to the right is better. The dots are the full model's reference cases, each on its own number; yours moves with the sliders.