A radiator's output is effectively a measure of the heat emitted by the radiator. This quantity is calculated as a power output with units of watts, or joules per second. Radiators heat ambient air by means of convection, wherein the water-heated panels exchange heat with the air directly in the vicinity of the panel, the warm air rises -- allowing cooler air to be drawn toward the panel -- and the cycle repeats until equilibrium is reached. A calculation of this quantity depends on the initial water temperature, the temperature of the surrounding air, the final water temperature and the nominal heat emission.

Determine the initial water temperature. This value will be higher than the temperature of the surrounding air, as well as the temperature of the outflowing water. Write this value down and keep track of it, labelling it as TI.

Determine the temperature of the outgoing water. Write this value down to keep track of it, labelling it as TO.

Determine the surrounding air temperature. While a determination of the other temperatures in the calculation will require estimates or references from literature on the components involved in the cycle, this value can usually be read from the temperature monitor in the home. Write this value down, labelling it as TA.

Reference the nominal heat emission of the radiator from the machine's literature. Write this value down and label it as PI (initial power).

Write down the radiator type constant n. If the radiator is a standard panel type, this value is written as n = 1.33. If the radiator is a convector, it will be somewhere between n = 1.3 and n = 1.6. Use the literature on the radiator as a guide for determining this quantity.

Write down the equation P = PI[(TI - TO)/(ln(TI - TA)(49.32))^n. Here, the variable P represents the radiator's heat output.

Substitute the values you have determined in for the representative variables. Punch the entire equation into your calculator to get the heat emission. Ensure your quantities contain all of the beginning and terminating brackets (parentheses) exactly where they are written, or the result will be inaccurate.