Do not confuse the intent of our military's use of an EMP device as a tactical device with that of a small 3rd world country intending to use it as a strategic weapon.

Use of an EMP as a tactical weapon against another military system is iffy, the problem being the electrical coupling issues and kill mechanism. Many military systems are hardened and have back-ups. How do you use an EMP against a target that's in your proximity without also using it against friendly systems? It's difficult to develop a weapon system when you don't know the effect it will have on the target.

Using an EMP at high altitude against a large area with the intent to burn out sensitive non-hardened civilian systems and electrical grids might be easier -- if you have a delivery system. Not saying it will destroy those systems, but they are not hardened and our electrical grid makes coupling rather simple. Will it kill the electrical system in a car? Don't know, one study I read says cars might be fine. But lots of other systems may be at much more risk. Lots of unknowns.

90%?? Don't know how they arrived at that number. Maybe assuming the device worked 100% and succeeded in reducing us to an agrarian society with no transportation. Again, lots of unknowns and much of our military would still be operational -- payback could be swift.