I do not have iPhone, but a Droid X. I do have iPod Touch, so I scanned App Store. There is a bunch to choose from, so I would say it depends on what you do mostly. They have calculators for wire size, conduit and box fill, dwelling loads, motors, etc. They have theory and engineering apps. They have a lot. I also own the ElectraCalc Pro, which I use constantly for conduit fill. Try a few of the free ones to help decide what you want to spend money on.