While there are many variables to account for, such as climate, home insulation, HVAC type, etc., research conducted at Auburn University found that with 17% shade on your house, your power bill will decrease by $10 per summer month (when your air-conditioner is running). If shade were to increase to 50% over your house, this translates to a savings of $30 per month.
Arbor Day has a neat tool you can use to calculate savings based on customized situations set by you.
Go here to see what kind of energy savings you can accomplish at home!
http://energysavingtrees.arborday.org/?PartnerCode=06903