Agresti (1990, 2002, 2013) shows how to compute approximate power for a test of two independent proportions and more generally for the Pearson and likelihood ratio statistics. This information can be used before data collection to select a sample size that yields sufficiently low Type I and Type II error rates, or after data collection and analysis to tell you what the power was for the test. Two sample programs are available:

• PROC POWER or the POWER2X2 macro computes the power of a test that compares proportions from two independent samples for a range of total sample sizes.
• The POWERRXC macro computes the power of the Pearson and likelihood ratio chi-square tests (provided in PROC FREQ) for a range of total sample sizes. A short article that discusses this topic and the programs is available.