Duct heat gain formula

[Drew]

Which formula should i use to measure the capacity of a ducted unit running in heating.

I can measure the air quantity and the temp on the return air grill and then the temp of the supply air.
In metric please!

[Drew]

Would the following be correct:

Kw = l/s (air velocity) x kj/kg K (SHC air) x kg/l (air density in liters) x (air temp on coil - air temp off coil)

Therefore

Kw = l/s x SHC x air density x temp diff

or

Kw = cubic mters air/hour x SHC x air density in cubic meters x temp diff .all divided by 3600.