Single use ability efficiency caps out at 25% it's original cost (175% efficiency). 50/4 = 12.5

(Invisibility cost = 12.5; first cast 13, second cast 12, then 13, and so on.) Energy Siphon recharges 0.6 energy per second.

12.5 / 0.6 = 20.83

Means you need 20.83 seconds to recharge Invisibility cost with Energy Siphon only. Invisibility at 176% duration lasts 21.12 seconds.

Reference:

StripeyStripe's Loki Prime build, https://overframe.gg/build/100706/loki-prime/banish-loki-the-swiss-army-knife-of-frames/

https://warframe.fandom.com/wiki/Ability_Efficiency

https://warframe.fandom.com/wiki/Maximization#Maximized_Ability_Efficiency

https://warframe.fandom.com/wiki/User_blog:Voqualin/Power_Efficiency_Research

This set up works on Ash with two changes: upgrade Narrow Minded to the next rank, forma aura slot.
v
https://overframe.gg/build/722290/ash/infinite-smoke-screen/