How large would be the transformer for converting a lightning into useful electricity?
According to the Wikipedia article on lightning, a typical lightning bolt has enough energy to light a 100 watt bulb for 5.5 hours.  This is not anywhere near enough energy to make it economically worthwhile to harness that energy.  Especially since the location of a bolt cannot be known before it strikes and since successive strikes will be at different locations.  Also, a transformer will not work since a lightning bolt is basically DC based as it is a discharge of electricity from one point (typically a cloud) to another (typically the earth) that only moves in one direction.