What is it?
The Eddi is an energy management system for use with grid-tied PV or wind turbine systems. Excess energy from the microgenertation system is used to heat water or rooms rather than exporting it to the grid.
How does it work?
Let's imagine for a moment, a solar pv system did not have an Eddi device. So there's just solar panels and an inverter. If your solar pv system was producing more energy than your house consumption required, it would then export the excess energy back to the grid. However, with the Eddi device installed, the Eddi can basically detect the excess energy that's being fed back to grid (through the use of CT's) and redirect it towards the Eddi. So your water is heated with the excess energy instead of being fed back to the grid.
This does not mean you will have no energy exported back to the grid anymore. There will still be some energy exported back to the grid. But the Eddi aims to use as much of it as possible to heat your water.
That's why you commonly see a status on the Eddi device stating 'Waiting for Surplus'. The Eddi's primary job is to sit there and fed off excess energy when it's available. In other words, wait for surplus.