Lava lamps aren’t significantly more expensive to operate than a standard bulb. Depending on the size of the lamp; 25w to 100w bulbs are used. Normally halogen or some other hot running bulb.

From experience, shaking the bottle when hot will make the wax bits smaller (can burn hands), but they will reconstitute over time. Shaking while cold is the same but you don’t get as many pieces and you don’t burn your hands.

Everything else is a taste issue.