You can't realistically measure and quantify cooling consumption. There are two things you can do, however, to achieve your goal:
- Monitor and record power consumption, which is the major variable that correlates to cooling requirements. (Assuming you're not in the outside corner of a building with big windows and poor insulation, the Summer/Winter difference won't be your major variable.) The more specifically you can monitor, the better, right down to individual cabinet power strips, which are now available with both local and remote metering. This will also help you maintain load balance on dual UPS's and phase balance on 3-phase circuits, both of which contribute significantly to power stability. Every 3,000 Watts (approximately 25 amps at 120 volts) requires 10,200 BTU of sensible cooling. If you're using conventional Computer Room Air Conditioners (CRAC's) with humidification, set for 72F Return Air; this will require about one ton of total machine capacity. These are very "round numbers" but should give you a good guideline.
- Install temperature and humidity monitors at key locations in the room and at least in each "High-Density" cabinet, although every cabinet is preferable. There are many systems now available to do this. The system won't tell you how much cooling you need, (the power measurements will,) but will certainly identify when and where cooling is needed, and whether or not the air conditioning additions you made were effective. This will also help you get the most from the air conditioning you already have. If it saves installing just one CRAC unit, the monitors will more than pay for themselves. Additionally, the information provided will be a very good way to know if your cooling design remains stable under all conditions.