An engery web machine was a device which could be used to find out what sort of things created energy. Since energy couldn't be created or destroyed, there would be some traces left of the energy which would stay there for a while. The machine could find the traces.
Sarah Jane Smith and Maria Jackson created such a machine, using two fuse rods and several other things. Sarah Jane and Maria wanted to use this machine to find out about a storm that Brandon Butler had seen at Haven Green. A storm was created by, and in turn created, energy. The residue hung around for a while, and Sarah Jane knew this could tell them a lot about the storm.
After the machine was used energy streamed out of the contraption into the sky and a lot of entanglement shells were falling out of it. Entanglement shells were a byproduct of transdimensional energy dispersal, so Sarah Jane knew that aliens had used teleportation. (TV: Warriors of Kudlak)