Typically spirits can derive this through exposure to natural cycles like the sun & moon. You can expedite this process through something like a Charging Box or a Charging Spell which provides a source of energy for the spirit to regain their energy to the fullest potential.

Depending on the practitioner & method used to conjure & bind a spirit also depends on the approach you should utilize in providing your spirit energy. Speaking only for Creepy Hollows you can use the methods described above, for any other practitioner it is best to ask them if there are any restrictions in offering the spirits sunlight, moonlight, or enchanted methods of recharging energy.