Last week we installed a split AC in a yoga studio in Carroll Gardens. I was interested to see the price differential between the two in energy costs per year and how long the pay-back period for the split AC/heat pump would be. The split AC/heat pump cost $2100 to cover the cost of the unit and installation. It has a 9000 Btu/h output and a 26 SEER rating. For this exercise I used the most energy efficient,Energy Star rated, and most purchased window AC unit at Sears that costs around $210. The unit has an output of 8000 Btu/h output and a 10.8 EER rating. I converted EER to SEER in the exercise below. I found out there was a $47/year price differential in energy costs. That would be a 40 year payback period for the split AC.

The model I used tries to find the cooling load using the cooling degree days. Here is the formula:

Cooling Load = (Btu/h x 24 x CDD) / change in temperature in the day

To compute the change in temperature I used monthly averages from The Weather Channel for Brooklyn.

Here are the results:

**Split AC**

May- (9000 x 24 x 40) / 16 = 540,000 Btu/h

June- (9000 x 24 x 120) / 16 = 1,620,000 Btu/h

July- (9000 x 24 x 322) / 15 = 4,636,800 Btu/h

August- (9000 x 24 x 207) / 14 = 3,195,714 Btu/h

September- (9000 x 24 x 88) /14 = 1,357,714 Btu/h

Total = 11,350,228 Btu/h

(11,350,228 Btu/h) / (SEER= 26 BTU/ Wh) = 436,547 Wh/year

(436,547 Wh/year) / (1000) = 436.547 kW/year x .18 kW/hour = $78.57 a year

**Window AC**

May- (8000 x 24 x 40) / 16 = 480,000 Btu/h

June- (8000 x 24 x 120) / 16 = 1,440,000 Btu/h

July- (8000 x 24 x 322) / 15 = 4,121,600 Btu/h

August- (8000 x 24 x 207) / 14 = 2,838,857 Btu/h

September- (8000 x 24 x 88) /14 = 1,206,857 Btu/h

Total = 10,087,314 Btu/h

Calculation from EER to SEER = -0.02 × 10.8² + 1.12 × 10.8 = 14.4

(10,087,314 Btu/h) / (SEER= 14.4 BTU/Wh) = 700,508 Wh/year

(700,508 Wh/year) / (1000) = 700.508 kW/year x .18 kW/hour = $126 a year

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