When a Pokémon with Volt Absorb is hit by an Electric-type move, its HP is restored by one quarter of its maximum HP, and the move will have no effect.
It will always be 25% because volt absorb always heals 25% of health of a Pokemon when hit by an electric move. Although there is an exception to this because in 3rd gen, thunder wave is unaffected by volt absorb and will restore 0 health to the Pokemon and also paralyze.
Source: http://bulbapedia.bulbagarden.net/wiki/Volt_Absorb
