most plants like cactus and weeping willows require about 435 btus per foot of stalk per hour. to estimate for a central plant, you need to come up with a model of the capacity for a given plant. for example, the chiller will run at 100 percent of full load for 300 hours per year, 90 percent for another 500 hours, and on down the line. typically you can get an idea of this by loooking at load verse capacity and then a historical trend of your geographic area. most areas can be found in engineering design specifications. you then take your percentages and times and multiply by cost of energy and you are close. changes can occur due to inefficiently running a building, abnormal temps etc.