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Energy of a Warframe is used mainly to power abilities and melee channeling. All Warframes have 4 abilities that at base, cost up to 100 energy to cast. Every Warframe has their own energypool with varying capacities depending on the types and costs of their various abilities. For channeling, energy is used to provide bonus damage and other effects such as life steal, and to enable parrying while blocking which will nullify all incoming damage, reflect some damage back, and open up melee attackers for finishers, but will drain energy on every blocked attack.
Like health and shields, a Warframe's energy capacity scales with their rank and through the use of mods or buffs. It is also worth noting that the words "power" and "energy" are often used interchangeably by members of the community.Increasing Energy Capacity
Mods
These are the mods currently available in the game. It is worth noting that Flow and Primed Flow cannot be used in the same build.
Formula For Modded Energy Capacity
These mods increase the energy capacity of a Warframe following the following formula:
$ \text{Energy}_{total}=\text{Energy}_{max}+(\text{Energy}_{base}\times \text{Mod Multiplier}) $
So as an example taking Volt Prime with 200 base energy and 300 max energy he will have:
 With Flow: $ 300 + (200 \times 1.5)=600 $
 With Primed Flow: $ 300+(200 \times 2.75)=850 $
 With Primed Flow and Endurance Drift: $ 300+(200 \times 2.90)=880 $
Buffs
The only buff currently available to increase energy capacity is Cephalon Suda's Entropy effect. Entropy will release an AoE radial attack 25m around the player, dealing 1000 Magnetic damage and applying the Magnetic Status Effect to enemies in range reducing their shields. It will also restore 25% of the player's base Energy, and provide a (base) Energy boost of +25% for 30 seconds.
Arcane Helmets
These are retired helmets that in the past used to grant increased energy capacity to their respective Warframes
Warframe Starting Energy In a Mission
Formula
Upon loading into any mission, each Warframe has a fixed amount of energy that they start with. This amount of energy is calculated based on the Warframe's free mod capacity and a base value specific for each Warframe (differs even between normal and prime variants). The formula followed is:
$ \text{Starting Energy}=\text{Initial Energy}+5(\text{Free Mod Capacity}) $
Table of Initial Energies
The initial energy of a Warframe is given from the following table. Do note that this table is currently still under construction and it is still unknown whether these values are calculated off some other attributes or simply fixed parameters for each Warframe. This area of research is still very actively pursued.
Warframe  Energy Pool  

Ash  ??  
Ash Prime  50  
Atlas  50  
Banshee  100  
Banshee Prime  125  
Chroma  75  
Ember  75  
Ember Prime  75  
Equinox  100  
Excalibur  50  
Excalibur Prime  100  
Frost  ??  
Frost Prime  50  
Gara  75  
Harrow  50  
Hydroid  ??  
Hydroid Prime  100  
Inaros  50  
Ivara  75  
Limbo  100  
Loki  100  
Loki Prime  125  
Mag  50  
Mag Prime  50  
Mesa  50  
Mirage  ??  
Mirage Prime  125  
Nekros  50  
Nekros Prime  75  
Nezha  125  
Nidus  50  
Nova  50  
Nova Prime  150  
Nyx  100  
Nyx Prime  150  
Oberon  ??  
Oberon Prime  150  
Octavia  100  
Rhino  50  
Rhino Prime  50  
Saryn  ??  
Saryn Prime  100  
Titania  50  
Trinity  ??  
Trinity Prime  125  
Valkyr  ??  
Valkyr Prime  100  
Vauban  ??  
Vauban Prime  100  
Volt  50  
Volt Prime  125  
Wukong  50  
Zephyr  50 
Effective Energy Capacity
Casting Effective Capacity
Similarly to how one can define the concept of "effective health" by weighting the number of hitpoints with the corresponding damage reduction, one can also define the concept of casting effective energy capacity by weighting the energy capacity using the Ability Efficiency factor:
$ \text{Effective Energy Capacity}_{casting}=\frac{\text{Energy Capacity}}{2\text{Ability Efficiency}} $
Since casting efficiency is currently capped at 175%, the highest possible upperbound for the casting effective energy capacity is:
$ \text{Effective Energy Capacity}_{casting}\leq 4\times\text{Energy Capacity} $
Toggle Effective Capacity
Since the efficiency of toggle abilities depends also on the duration, in that case the formula becomes:
$ \text{Effective Energy Capacity}_{toggle}=\frac{\text{Energy Capacity}\times\text{Ability Duration}}{2\text{Ability Efficiency}} $
For the case where no energy is recovered and a constant rate of energy drain, one can also consider the effective toggle duration as follows: $ \text{Effective Toggle Duration}=\frac{\text{Effective Energy Capacity}_{toggle}}{\text{Base Drain Rate}} $
$ \text{Effective Toggle Duration}=\frac{\text{Energy Capacity}\times\text{Ability Duration}}{\text{Base Drain Rate}(2\text{Ability Efficiency})} $
$ \text{Effective Toggle Duration}=\frac{\text{Energy Capacity}}{\text{Modified Drain Rate}} $
Due to the definition of the terms, there are many ways inwhich the formula for effective toggle duration can be expressed.
Apart from toggle duration, this can be used to work out the effective duration granted per energy pick up.
Examples
Suppose a prowling Ivara has a modded drain of 0.25 energy/sec. An orb of 50 energy would provide:
$ \frac{50}{0.25}=200\text{ seconds} $
Considering the same Ivara with full energy pool of 744 capacity, she can remain in prowl for:
$ \frac{744}{0.25}=2,976\text{ seconds}\approx 50\text{ minutes} $Restoring Energy
In missions, there are various ways to restore energy:
 Energy can be picked up during a mission from Blue Orbs. Blue Orbs dropped by enemies restore 25 energy while containers drops will restore 50 Energy.
 Harrow's Thurible grants players inside its aura a variable amount of energy from kills made by Harrow, depending on how long the ability was charged and whether the enemies were killed by bodyshot or headshot.
 Trinity's Energy Vampire allows a player to regain energy by standing inside the energy pulses the target releases (target is stunned the entire time).
 Limbo's Rift Plane slowly restores energy for all players within it. Can stack with Energy Siphon.
 Octavia's Inspiration passive slowly restores energy over time for all players that were near Octavia when she cast an ability.
 Team Energy Restores are consumables that grant Energy to nearby teammates in varying amounts, up to 400 total.
 Weapons triggering an Entropy effect restore 25% of the player's base Energy, and provide a base Energy boost of +25% for 30 seconds.
 Weapons triggering a Blight effect restore 25% of the player's base Energy, and provide a (base) movement speed boost of +10% for 30 seconds.
 Zenurik's focus nodes have multiple ways in which a player can regenerate energy.
 Notable mods that affect energy generation:
 Rage and Hunter Adrenaline  Converts damage on health to energy.
 Kinetic Diversion  Converts damage on health to energy for Archwing.
 Equilibrium  Health pickups restore a certain amount of energy alongside health recovery (but only if the Orb is picked up).
 The Aura Energy Siphon slowly restores energy for all players based on its mod rank. Multiple copies of this aura allow for faster energy recovery.
 Coaction Drift  Improves the Warframe's equipped Aura, including Energy Siphon.
 Notable arcanes that affect energy generation:
It is important to note that on most occasions, toggle abilities disable energy generation from team abilities, consumables, or passive regeneration methods
Using Energy as Health
Mods
These mods stop lethal damage by allowing hits to drain your energy after your health is depleted in exchange for occasionally staggering the player
Notes
 These mods synergize extremely well with Rage and Hunter Adrenaline since all the health drained before taking lethal damage would ensure that there is enough energy for the mods to consume.