We investigate the finite p-energy classes Ep of quaternionic plurisubharmonic functions of Cegrell type. We also construct an example to show that the optimal constant in the energy estimate is strictly bigger than 1 for p>0, p≠1. This leads us to the fact that we can not use the variational method to solve the quaternionic Monge–Ampère equation for the classes Ep when p>0, p≠1.