How do I check the ranking of an engineering conference, and determine whether presenting there will be useful for my CV?
It is a legitimate IEEE conference organised by the local IEEE section in Illinois and its proceedings will get indexed to IEEExplore. Also the University of Illinois is well-known for its Power Engineering - Power Electronics area so you will get to meet some people in the area.

On the other hand, it is not one of the major PES conferences for the year so do not expect many sessions or huge audiences in the conference. you might want to consider that especially if you plan to travel from abroad and need to pay flights - accomodation - registration. Last year's table of contents is available here so you can judge for yourself:http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&amp;arnumber=7064877 

(Probably you should have searched these before you submitted the paper and not after it has been accepted!)