Streamline

The Streamline mod increases your warframe's Power Efficiency, which decreases the energy cost of abilities. The cost of activating an ability when using the Streamline mod is: Streamline is not a reduction in the energy cost needed to activate an ability, but is instead an increase in energy value. It essentially makes 1 point of energy worth 1.3 at max Streamline.
 * Cost = Base Energy Cost divided by (1 + X), where X is the percent power efficiency expressed as a decimal, and the result is rounded up.

Power Efficiency
Below are examples of the base energy cost, and reduced energy cost associated with using a max level Streamline mod, which gives a Power Efficiency of 30%.

Stacking
If used in conjunction with Essence Loki Helmet, they stack such that:
 * Cost = Base Energy Cost divided by [1 + (X+0.15)]
 * where X is the percent power efficiency expressed as a decimal
 * and +0.15 is Essence's Power Efficiency rating.

How exactly the Scorpion Ash Helmet stacks when used in conjunction with Streamline is currently being researched. You can contribute to the research by reading the Power Efficiency page, and by posting on the Power Efficiency Talk page.