What does it mean to “detect an existing effect” in a Bayesian setting? This sounds like you’re trying to do some kind of frequentist hypothesis testing.
This has come up before, and last time, Andrew Gelman recommended a chapter of his book with Jennifer Hill. Here’s a blog post from Andrew the explains why he doesn’t like traditional power analyses along with some constructive suggestions about what you can do.
In Bayesian analyses, it’s always best to the use the most informative prior you can justify with your prior knowledge. Sometimes you can get away without doing this when the data is informative enough by itself. And you always want to use all your prior information to simulate, even in a frequentist power calculation.