Energy is a finite resource on the ship. Thermal energy is produced directly by the reactor, which in turn powers a turbine [and maybe an MPDG] to provide the electrical power to a ship.
Every system on the ship requires at least one of these energy types, with several consuming both at once for operation, so it is an incredibly good idea to ensure that your ship can sustain a balance of them that can ensure it stays operational with as few brownouts as possible.
This guide will go into the details of how the ship's systems are powered, if you would rather skip this and get into the how-to on maintaining a balance of power, you can skip to this section.
Basics of Energy Usage
Take this abstract diagram of a ship's energy cycles:
It doesn't show exactly how each is used, but it is a starting point into describing where energy flows to and from in a ship.
Every active system on the ship (thrusters, weapons, the computer, etc.) relies on electrical power being present, and enough of it in surplus to allow for continuous operation. The turbine itself is the only system completely reliant on the reactor, which itself requires it to be above 2 050 Kelvins to operate. If the reactor drops below that temperature, you will be completely reliant on the ultracapacitor [and if installed, an SMES unit] until it can be regained, which can put you in murky waters quickly.
This next diagram shows a more complex, but still basic, representation of how energy is distributed around the ship:
This diagram provides a better insight into how thermal energy is brought into the equation. Thrusters will draw thermal heat from the reactor by taking superheated propellant from it. If a reactor is unable to heat up the propellant fast enough to provide the active thrusters with enough heat, the temperature will drop with continued use.
This leads to a third and final diagram, which takes into account other ship systems, as well as systems specific to certain optional equipment:
This diagram shows in greater detail the path that both electricity and propellant. The inclusion of the ultracpacitor/SMES stage to the electricity loop shows the buffer that it provides in the event of a negative balance.
Maintaining a Balance
A lot of equipment in ΔV can very easily over-consume the energy that a stock K37 can provide. The equipment menu is especially friendly to display this, as any equipment that by itself will over consume a specific energy type, it will show a marker for what type you will need to be worried about. Equipment that will install multiple for a single type (e.g. thrusters) will have the total consumption of all of the equipment installed taken into account when checking to see if there is enough power. Do note that this marker only takes into account energy production, and assumes no other equipment would be running at the time.
Each equipment will display a power and thermal consumption stat. These will be accurate for nominal usage, as certain situations can use significantly more power than what the equipment is rated for.
The most reliable way to determine how much of a strain active systems are providing on the electrical system is through the power balance section on the HUD. This runs from the equation Power Supplied - Power Drawn, and is especially effective at showing if the electrical power is running on a net positive or net negative at any given time. It is not a bad thing to be in a net negative, however it can very easily be unsustainable dependent on a few factors, which will be gone into more detail later on.
Thermal strain is a little harder to determine whether it's on a net negative, however if the reactor's temperature drops below the usual fluctuations that you see in the temperature.
Brownouts
A brownout happens when an energy reserve runs too low for the current consumption. Most of the time brownouts will occur from the ultracapacitor draining, which causes the computer to be knocked out and you lose HUD items and autopilot controls temporarily. This can also occur from a thermal brownout when the thermal consumption is great enough to drag the temperature below the minimum required to operate the turbine, however most of the time it only is dragged low enough to cause the thrusters to fail to produce optimal thrust, where you would need to stop using the thrusters to bring the temperature up enough to make them usable again.
Power System Upgrades
Each section of the power distribution cycle has it's own selection of upgrades.
Turbine
The turbine is the main provider of electrical power to the ship. It operates while the reactor is online and operating at above 2 050 Kelvins, and power generation scales with the reactor's temperature past that point.
The military turbine is significantly heavier than the difference between the single and dual turbine, and may be better off being upgraded with a MPDG instead of the military variant if mass is an issue.
Reactor Core
The reactor core is the provider of the thermal energy. Cores are provided in two models - SO6 and Yama cores. The S06 cores are cheaper, however are significantly heavier than the Yama cores, and outside of the issue of cost, the Yama cores are preferable once you need more than 8 GW of thermal energy production.
Ultracapacitor
The ultracapacitor array stores electrical energy in banks for times when demand exceeds production.
Unlike turbines, ultracapacitors scale linearly in all forms, and aren't as limited in scalability.
Magnetoplasmadynamic Generator (MPDG)
An MPDG is an external unit that converts thermal energy and some electrical energy into a significantly greater amount of electrical power. These units on their own are much more powerful than a turbine, and weigh less as well, however are not as reliable, as MPDGs require the computer to be on to operate.
You do not need to match the turbine's output to be at least the electrical input, as the ultracapacitor will take some of the initial jolt, and given the MPDG feeds through the ultracapacitor, it provides a loop to constantly provide power.
Superconductive Magnetic Energy Storage (SMES)
An SMES is an extension to the ultracapacitor array, and acts as a significantly larger storage bank than what the capacitors provide. They're good for running a lot of very power hungry equipment where even an MPDG wouldn't suffice, however are a little more expensive to have setup.
Like MPDGs, SMES units require the computer to be online, and are also slower to charge and cannot discharge as much as an ultracapacitor.