There would be a few different sources of casualties
- The people who were at the time using machines which depend on electricity. This could be people in hospitals who are on life support, people driving electric cars (or any fly by wire car) or people flying. These people would die immediately.
- The second wave of people to die would be because of the collapse of food distribution. The food distribution and manufacturing in developed countries is highly based on computers, knowing where to ship produce and in what amounts without computers (Or even long distance communication) would be impossible. This would lead to people leaving urban centers and creating pressure on the immediate country side.
- Long term effects - without electricity the food and medical supplies we rely on for modern society would not function properly. Without the ability to work the fields using modern technology, using old methods of creating fertilizer the supply of food would decrease dramatically. This will lead to wide spread starvation and eventually to a stable level of population which is much lower than it is today. The same thing applies to modern medicine, people will start dying much more often from diseases which with today's technology are curable
In non-industrialized countries most casualties will come from the first step. As the urban centers and agricultural industry don't rely as much on technology they will be much better poised to deal with these challenges.
If we look at how non industrialized societies have improved their agriculture in recent years, by using modern methods, we can estimate how much population their agriculture can support in the long term. According to this, the wheat production in the least developed countries has grown by 150% in the last 50 years. If we take this as a guide this means the population will decrease by about 60% in non industrialized countries.
If we look at industrialized countries we can see that they have about an urbanization rate of about 70-80%. Given our analysis this share of the population at a minimum will starve out given this event within a few weeks.