The [attachment:prop1s.xls spreadsheet] gives the power for inputted group proportions (which should sum to 1), Type I error and a given total sample size. The expected proportions assume equal group sizes.
The expected effect size, $$\omega$$, defined below is from Cohen (1977,1988, Chap 7).
$$\omega = \sqrt{ \sum_text{1}text{no. groups} \frac{\mbox{(observed proportion - expected proportion)}text{2}}{ \mbox{ expected proportion}}}$$
The thresholds 0.1, 0.3, 0.5 correspond to small, medium and large $$\omega$$ effect sizes.