The short answer is "when it accelerates". However, there are some very interesting controversies involved. One involves the equivalence principle and has been much argued over in the literature. I'm really asking about the current state of the arguments. Loosely the equivalence principle says if you are locked in an Einstein cage and cannot look outside, you cannot perform experiments inside the cage to tell whether you are at rest on a very large homogeneous, spherical planet with a downward gravitational acceleration g or whether you are lost in space with a rocket accelerating you upwards at g. If that is so, then why aren't you blinded by the radiation from a charged body sitting on a table in a uniform gravitational field. The short answer is that the radiation at 10 m/s^2 is too small to measure. But others have argued that the radiation is not even there!!! Any one know the current status of this? I apologize if this has been discussed before, but the search engine here could not find any postings involving 'radiation, accelerated charge'.