Energy

Batteries and Reactors
Batteries and reactors are used to provide power to other ship components such as engines and weapons. Batteries come fully charged but have a limited capacity which will eventually run out, unless recharged by either a reactor or another ship with an energy transfer block. Reactors constantly generate energy but are heavier and more expensive, though they also come with a small stock capacity like batteries.

Firing and moving require energy. If your ship runs out of energy, it won't be able to shoot or move until it generates more

Apart from batteries and reactors, the Energy window also has another tool called "Energy Transfer" which can, as the name suggest, transfer energy to nearby friendly ships. It supplies slightly less than the charge from one 2x2 reactor to every ship within range.

Batteries
Batteries store power and come with a full charge, so you can use them as the only source of power on your ship, or to provide a buffer for your reactors. Batteries are cheaper and lighter than reactors, making them good for fast disposable craft as well as extending the combat time of a ship with a high e draw.

Batteries differ in size but are identical in stats, so stack them any way you please:
 * 2x2
 * 2x1
 * 1x2
 * 1x1

Reactors
Reactors produce energy for your ship which is calculated in Energy per second. They are expensive and heavier than batteries. You can use more than 1 reactor in your ship. They come in the same sizes as batteries, but each of the three size classes has a unique role and function:
 * 2x2, which provides the highest regeneration rate for cost
 * 2x1, which provides addition battery storage to make up for it's slightly slower regeneration
 * 1x2, an upright version of the above
 * 1x1, the lightest of the reactors, ideal for small fast craft, but with the least efficient generation rate and capacity.

Solar Panels
Solar panels produce energy without battery volume. They weigh less than reactors, but add no health and take up more space, leading to a larger hit-box. The were added in update 0.45.12, on May 29, 2017. The different sizes scale evenly, so use whatever fits or looks the best.

Energy Transfer
Energy Transfer transfers energy from the ship with the module to nearby friendly ships up to 800m away. Its range cannot be affected by the Targeting Subsystem.

Energy Transfer, at a cost of $30, consumes 960E per second per ship and has a weight of 40.0T and it offers no HP to the ship. The energy is provided to all ships within 800m, and is not split up. Please keep in mind that the power drain does not show up on the energy bar.