How many WATTS a LASER beam must have to penetrate (kill) a human body?

Answer 1

It depends on where you hit the person, obviously.

But let's go for total annihilation. A typical high-explosive grenade has about 1 megajoule of energy. If you can transfer the same amount of energy with a laser, you'll get roughly the same effect.

A watt is a joule per second, so if you want to pretty much completely destroy someone with a laser, you'll either need to use a million-watt beam for one second, or a 6000-watt beam for about 2.7 minutes continuously. And, of course, you can probably kill someone with a fraction of that energy if, say, you penetrate a weak part of their skull through the eye, or hit them between the 6th and 7th vertebrae in the back, or otherwise have a lot of cooperation from your target.

Interestingly enough, you can buy hand-held 6000-watt lasers:

http://www.engadget.com/2005/11/03/handheld-laser-gun-available-for-purchase/

You may not kill anyone with it right off, but you can probably maim a lot of people!