Simulations of the impact of tree shade on residential air conditioning in California showed that three shade trees (two on the west, one on the east side) could reduce a home's annual energy use for cooling by 10-50% (200-600 kWh, or $30-$110) and peak electrical use by up to 23% (0.7 kW).
Simpson, James R., and E. Gregory McPherson. (1996). Potential of tree shade for reducing residential energy use in California. Journal of Arboriculture, 22(1), 10-18.