But there seems to be another method the machines and Zion fleet uses to store the charge, when the Zion ships and machines are "broken open" in Reloaded and Revolutions we see a luminous and highly charged blue gas/fluid spill out of them and rapidly expand, discharging violently against any metal surface.
Besides, the Power Plant only absorbs the energy from the Humans and sends that energy through three massive superconductors into Zero Two. Where presumably the electrical energy is used in various ways. Instead of coal, oil, gas or nuclear energy used to generate power, its simply 6-7 billion Humans with a gel to capture all heat, and wires connected to all nerve clusters to capture bioelectric current. Combined and using a large scale transformer mechanism in the base of the towers to transmit it to the city.
1.15 TW of electrical power coupled with a way of almost eternally punishing Humanity? not logical but then we see them experiencing emotions too. Hostility and pettyness were evident in most of their "programs" as much as it was in any living person.
The Architect did say that without Humans as a power source they were other levels of survival they would be willing to accept, which could refer to turning their efforts back towards other power sources and accepting the loss in energy from the Power Plant in the mean time.