Energon cubes are large transparent cubes, about 2 meters to a side, used by the Decepticons in the 1980s Transformers cartoon to store raw energy in a high density form called energon. I have no information on whether or not they are used in the other incarnations of the franchise, and have no recollection of the Autobots using them.†
† Thanks to Uberbanana for reminding me that Kup and Hot Rod had a sort of energon snack in Transformers: The Movie which they used to appease the Sharkticons. These appeared to be the robot equivalent of field rations, and were about the size and shape of granola bars scaled up to their height.
Energon cubes are either manufactured or folded and stored by Soundwave in the same chest panel he uses to hold his cassette robots. They have the amazing quality of being able to draw out and store energy from nearly any fuel source: geothermal, unrefined crude oil, electricity, or any of several others. One expects that if left out in the sunlight for an extended period of time, they would slowly fill up. The efficiency of the energy extraction and maximum storage capacity are both unknown, although I would estimate the efficiency to be approaching 100% and their storage capacity to be in the area of 10E9 or 10E10 Joules. Since they were only ever carried by giant war robots, it would be very difficult to estimate their weight and whether or not they weigh more when full of energon. Once full, the cubes can be compressed into a smaller, more dense form for storage or transportation, roughly a quarter their original size. There is no evidence that they could be filled or drained in their compressed form.
The energon stored in the energon cubes exhibits the properties of a liquid. As the cube fills up, the luminous purple fluidlike substance fills them from the bottom up, conforming to the shape of the cube and seeking its own level due to the force of gravity.
Unfortunately, the cubes are fragile and the energon is extremely volatile, prone to exploding violently if the cube is punctured or shattered. However, their cubic shape makes them ideal for stacking and storing in bulk, utilizing available space as efficiently as possible. This can lead to massive chain reaction explosions when they are hit in a firefight with the Autobots, not only posing a danger to anyone nearby but also releasing all the stored energy in the pile. When this happens, the energy quickly dissipates to the surrounding environment as heat and is lost, unrecoverable.
An army fights on its stomach. Being robots, the Decepticons were constantly searching for energy sources to fill their energon cubes to fuel their power requirements. Hydroelectric dams, nuclear power plants, oil drilling platforms, and fuel refineries were common targets for their raids. Once filled, the standard sized (2 to 10 meters tall) Decepticons could recharge themselves from the energon cubes by tilting them to pour the liquified energy from one of the top corners into their oral apertures. The massive city-sized Decepticons, such as Trypticon, simply ate the energon cubes whole.