I am not convinced that anything less than 2 dozen weapons at high altitudes (nothing like 250 miles) would be necessary to completely destroy our power system and immobilize our population.
It is not a simple matter to evaluate. Depends on the device targeted and the weapon design. Not just Kiloton blast size. EMP weapons are low yield nukes in terms of blast. Frequency of pulse generated. distance does deteriorate the electromagnetic pulse. Especially outside out ozone layer.
This type of weapon is not a magic bullet.
And there is the question of what price the aggressor will pay in our retaliation. That is a serious deterrent. Unless Obozo is successful in reducing the numbers like he plans. Talk of reducing our inventory to 300 weapons. (all types) Insanity!
One of the studies has been posted here already, the State of Washington also has a study posted online.
There are many variables. Any nuclear weapon can generate a EMP spike. They can be designed to create a larger EMP so the smaller bombs are seen as more efficient.
Its one of many threats we face. However it is real and is something to be aware of.