Pellets of RTG Fuel

Plutonium and dense iron plates. Plutonium comes from 9 tiny piles, which the Thermal Centrifuge gives from depleted uranium rods.

EU generators

They produce EU directly, in LV packets (up to 32 EU/t): connect them to any LV machine or to a storage block.

Generator	What it uses	Produces
Generator	furnace fuel (coal, wood...)	10 EU/t
Geothermal Generator	lava: 2 mB per tick	20 EU/t (one bucket = 10,000 EU)
Semifluid Generator	biogas 1 mB/t; coalfuel 1 mB/t; other mods' fuels	biogas 16 EU/t; coalfuel 40 EU/t
Solar Panel	nothing	1 EU/t by day
Radioisotope Thermoelectric Generator	nothing (pellets, never used up)	1 EU/t with one Pellets of RTG Fuel; each extra one doubles it: 6 pellets = 32 EU/t (one per slot)

Generator: the first power source. It can charge a battery in its slot. It can also be made from an Iron Furnace.
Geothermal: load lava by right-clicking, in the top slot (the empty container comes out below) or through pipes. When its energy is full, it uses no lava.
Semifluid: holds 10,000 mB of fuel and 32,000 EU.
Solar panel: works from 6:20 to 17:45 with the sky in view (glass lets sunlight through). It does not work at night, in rain, under a roof or in the Nether or the End. It stores no energy: it charges the battery in its slot and outputs on every side except the top.
RTG: 6 slots for pellets, which never run out and give off no radiation. Pellets are made from plutonium and dense iron plates.

For more power: kinetic generators (Kinetic Generator), Stirling with heat (Stirling Generator) or the nuclear reactor. See the Energy guide.