My WaterFurnace Series 7 tracks energy use by kWh and cost. The past several days have been in the high 90's to 100 degrees. My average cost to cool my 3,500 sf home averaged $1.73 per day! Pretty amazing!
I can confirm these numbers.. WF Series 7. 2400 Sq Ft home. Last month it cost $1.62 per day to cool my house.
http://welserver.com/WEL0713/ $23/month to cool a 2600 sqf house with southern exposure. http://welserver.com/WEL0834/ $24/month for a 2,800 sqf house http://welserver.com/WEL0447/ $50/month, but also 5,200 sqf leaky farmhouse http://welserver.com/WEL0663/ 5 series dual stage, 3,300sqf, from a time when we did not have the 7 series, $62/month. Not bad, but also not as good as the variable speed.
it will not reduce your cooling cost, only your how water cost. The heat pump does not care if it discharges the BTUs in the ground, or your hot water tank. The amount of BTUs it removes from the house is the same.