The Quantum ES2 Energy Saver is the size of a AA battery, and instead of using 4 AA alkaline cells in the battery compartment of certain specific flashes, you would insert 3 AA's and the Energy Saver.

The ES2 is compatible with the Canon 430EZ, 540EZ, 550EX; Nikon SB-24, SB-25 (Note: Not required with Canon 580EX due to Custom Function #7 on flash).