The Elinchrom ELB 500 TTL is a 500 watt pack, giving you 400 Full power flashes, Recycling in 2 seconds, with a Max Flash Duration of 1/20,000th of a second, and active charging which allows you to use while still plugged in and not regularly replace batteries.
All this in an easy to carry pack that allows for effortless mounting and positioning of the head.

More about the ELB 500 TTL:
http://www.elinchrom.com/battery/elb5...