This link generates R code that can compute (1) statistical power for testing a covariance structure model using RMSEA, (2) the minimum sample size required to achieve a given level of power, (3) power for testing the difference between two nested models using RMSEA, or (4) the minimum sample size required to achieve a given level of power for a test of nested models using RMSEA.

http://www.people.ku.edu/~preacher/rmsea/rmsea.htm