The Lapotron Crystal is an end-game Industrial Craft 2 item. It's used in crafting the MFSU, the Mass Fabricator, and the QuantumSuit Armor, though it can also be used as portable EU storage if desired. The Lapotron Crystal can store 1,000,000 EU, the size of 10 Energy Crystals or 100 RE-Batteries.
Without Transformer Upgrades, Low and Medium Voltage machines cannot use the Lapotron Crystal as a power source. The only machines that can be powered by a Lapotron Crystal without further upgrades are the MFSU, the Mass Fabricator, and the Terraformer.
When used in crafting the QuantumSuit Armor, the charge level of the Lapotron Crystal determines the amount of charge the item will have. A charged Lapotron Crystal will produce a charged item, while an empty one will produce an item with no charge.