Ability Efficiency

Power Efficiency linearly affects the cost of Warframe abilities. Positive Power Efficiency indicates a reduction of ability cost while negative Power Efficiency indicates an increase in ability cost. Ability cost = (base_ability_cost) * (1 - power_efficiency)
 * Base_ability_cost: the default ability cost.
 * Power_Efficiency: the Power Efficiency of the mods and/or alternate helmet added together.

Mods
Streamline and Fleeting Expertise reduce ability costs linearly based on the percentage shown on the mod card. See the individual mod pages for tables illustrating how they affect mod costs.

Blind Rage also affects ability costs, but it increases them linearly.

Alternate Helmets
Alternate helmets also reduce (or increase) ability costs linearly based on the percentage shown on the mod card. The following helmets affect Power Efficiency:


 * Chorus Banshee Helmet (+10%)
 * Essence Loki Helmet (+15%)
 * Meridian Trinity Helmet (-5%)
 * Scorpion Ash Helmet (-5%)
 * Vespa Nyx Helmet (+15%)

Stacking
Power Efficiency mods and alternate helmets stack according to the below formulas:

Ability cost = base_ability_cost * (1 - S_% - FE_% - BR_% - helmet_%)


 * Base_ability_cost: the default ability cost.
 * S_%: the Power Efficiency of Streamline
 * FE_%: the Power Efficiency of Fleeting Expertise
 * BR_%: the Power Efficiency of Blind Rage
 * helmet_%: the Power Efficiency of the alternate helmet

In other words, Streamline, Fleeting Expertise, Blind Rage and alternate helmets stack additively.*

(*Stacking with Meridian and Scorpion does not appear to follow this formula; this suggests stacking is different when you have a helmet that reduces Power Efficiency. Further testing and calculations are required. See Power Efficiency Research)

Energy Bar Rounding
The in-game energy bar display is rounded down, but the costs of the abilities are not. This results in the discrepancies in the apparent cost of abilities with Power Efficiency mods. For example, an ability with 25 energy base cost will cost 17.5 energy with maxed Streamline (+30%). If this ability is used multiple times in a row, it will appear to cost 18 energy, then 17 energy, then 18 energy, then 17 energy and so on. Energy Siphon can also change the apparent cost of abilities because it does not add a whole number to your energy.

Maximum Efficiency
Power Efficiency maxes out at 75% as of Update 10.3.0 which introduced new Corrupted Mods. This means that stacking bonuses over this point will be wasting mod points. Currently, you can reduce 25 energy abilities to 6.25 energy, a 50 energy abilities to 12.5 energy, 75 energy abilities to 19 energy and 100 energy abilities to 25 energy.