One way that the electric injection molding machine saves energy is that they only use the energy that is needed for the job that is being run. A typical hydraulic injection molding machine wastes energy by centrally recycling the hydraulic oil through the pump even when the process does not require it. This can be overcome by adding a modular valve and control to the hydraulic package, but this brings the price up and you add other maintenance items.
If you are molding with very high pressures and very low velocities, you may need to bump up (oversize) the servo unit on the electric injection molding machine. This is due to the limitation of servo drive performance at very low speeds relative to capacity. This is a specification issue, not a technology problem, but it does add cost to the injection molding machine. This is a major reason why the assessment of high costs of the larger tonnage electric machines. Big servos are not cheep!
Another benefit of electric over hydraulic is that, with no hydraulic pumps, valves, tanks, etc. the unit has far less parts to maintain and can be serviced very easily. Oh, and the injection molding machines are very quiet when running.
Don't get me wrong, when shot sizes are very large (>700 oz. for example) hydraulic units become a better choice because the servo motors begin to get very expensive and ROI starts to get longer. This is why large tonnage machines are often offered as Hybrid machines using both electric and hydraulic technology. With a hybrid injection molding machine, you are able to get the larger capacity while saving some energy and up front costs. This is why most injection molding companies do not offer the higher tonnage machines. However, machines in the range of 100T - 800T are usually right in the wheelhouse for all electrics. They require a larger investment up front, but the energy consumption savings and reduced maintenance costs offset this quickly.