From the journal, Energy and Buildings (Volume 41, Issue 6, June 2009, Pages 662-668): "The Value of Shade: Estimating the effect of urban trees on summertime electricity use," by Geoffry H. Donovan and David T. Butry.  The journal can be found online at Science Direct.  

Abstract: We estimated the effect of shade trees on the summertime electricity use of 460 single-family homes in Sacramento, California. Results show that trees on the west and south sides of a house reduce summertime electricity use, whereas trees on the north side of a house increase summertime electricity use. The current level of tree cover on the west and south sides of houses in our sample reduced summertime electricity use by 185 kWh (5.2%), whereas north-side trees increased electricity use by 55 kWh (1.5%). Results also show that a London plane tree, planted on the west side of a house, can reduce carbon emissions from summertime electricity use by an average of 31% over 100 years.