Volts are small white pickups dropped by Enemies when they are destroyed.
Boss phases also drop them - either gradually until they are destroyed, all at once when they are destroyed, or both (depending on the boss)
Collecting them allows you to upgrade your primary weapon up to a maximum of Level 3 (starting at Level 0). Higher reinforcement levels require more Volts.
Volts required for reinforcement
|Reinforcement level||Volts required||Cumulative volts|
Volts dropped by each enemy
|Wasp||1 per 3 killed|
"Volts" is the name used by game files and by the developers. They are also referred to as "Energy" in the tutorial.