My shamans are not geared too well, but I did some experimenting.

I average around 16k dps in heroics. On a couple bosses I made sure to remove lightning shield and my dps dropped down to 9k.

Fulmination seems to be a very big part of the dps. Though I am not sure how to test how much dps is lost from not having lightning shield up and being attacked by any aoe spells, which I assume also trigger a lightning orb.