Conduit

As a reaction, when receiving lightning damage you may cast Conduit, granting resistance to the damage and storing the charge. The charge is expended the next time you cast a spell that deals lightning damage, dealing an additional 1d6 to the damage dealt. 

At Higher Levels. When you cast this spell using a spell slot of 2nd leven or higher, the damage dealt increases by 1d6 for each slot above 1st.