I found a source that says the overall efficiency of EMALS is 15%, pretty much what nlalyst reported.
Says the following: "
1.2 Features of electromagnetic ejection system
...
Compared with the steam catapult, the electromagnetic catapult is more efficient, more flexible, more stable, and more practical. Specifically: First, high efficiency. The efficiency of the American steam catapult is 4%-6%, and a single ejection consumes 614 kg of steam. The total efficiency of the electromagnetic catapult is 15%, which is about 3 times that of the former. The equivalent of a single ejection consumes about 200 kg of steam. In the fully closed state, it takes less than 15 minutes for the electromagnetic catapult to reach the ready-to-eject state from activation, while it takes several hours for the steam catapult to reach the ready-ejected state from the start. The second is that the weight and volume are small, and the volume and weight of electromagnetic ejection are about half of that of steam ejection. The third is more stable performance and easy maintenance. The steam catapult "average non-critical failure usage times" is 405 times, and the electromagnetic catapult is expected to target 1,300 times. At the same time, because the steam catapult does not have a closed-loop control system, the average peak-to-average ratio of the thrust of the carrier aircraft during ejection is 1.25, and the maximum can reach 2.0. The electromagnetic catapult can continuously correct the thrust deviation during the ejection process, and the peak-to-average ratio The ratio is controlled within 1.05. The fourth is a wider range of catapult models. The maximum ejection energy of the steam catapult is about 102MJ, and the electromagnetic ejection system can reach 122MJ, which enables the aircraft carrier to carry more and heavier models while ejecting small and light drones. The fifth is to comply with the trend of full electrification of ships. Electromagnetic catapults use electricity, which is more suitable for the full electrification of aircraft carriers in the future."