Ernst Stavro Blofeld
Good question. The answer will depend on where you are at the time. Are you in the middle of Wyoming, LA, Middle East dessert, etc… If you are driving then you will not see or hear anything, but your vehicle will shut off and roll to a stop. You would check your cell phone after trying to unsuccessfully restart your truck to call someone to come pick you up. Once you think that your cell phone died because you thought you forgot to recharge it, you look at your watch to see how long it will take you to get back to civilization or just a place to make a phone call. Only then realizing that your watch has stopped.
If you were in the city you would notice that there are no lights on at all! People will be frantic and walking around more dumb than normal. If you are near a big city the sounds that you will hear would be the sounds of cars crashing because no stoplights work, the sounds of planes crashing and falling out of the sky, the sounds of panic and mayhem as elevators in big high rises plummet to the ground at incredible speeds.
The best way to knock out the power grid is to have an HEMP. The only difference between an EMP and an HEMP is that an HEMP is detonated in the atmosphere between 20 and 50 kms. They have a line of sight effect, meaning that anything it could see would be reduced to nothing (power wise). So if the terrorists got a few dirty bombs on planes and set them to detonate at a specific time, then about 5 or 6 HEMP’s could knock out the power grid for about 75% of the planet. The power grid in the United States is rated at a D+. It would not take much to disable the U.S. and other countries.
"It is not that I can and others can't. It is that I will and others won't."