airconditioning

http://en.wikipedia.org/wiki/Chiller

You need to measure the water flow , and diferential temperature incoming outcoming water at the evaporator.

Flow is hard to get , but you can check from the owners manual's FLOW DIFERENTIAL

PRESSURE chart, at the in and out evaporator.

Then for 1 ton = 3000 Cal , or 12000 BTU , rounded.