The Elinchrom ELB 500 TTL is a 500 watt second pack, giving you 400 full power flashes, recycling in 2 seconds, with a maximum flash duration of 1/20,000th of a second.
The unit also can be used in 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.