These are Hybrid systems that store the power generated in batteries and allow you to use your solar power in your house even at night. The solar charges the batteries during the day and the inverter feeds your house 24/7. You can configure these to constantly feed your house grid, this would give a constant feed into your grid much like a grid inverter would during the day, but will do so into the evening as the power will come from the batteries. This will lower your electricity costs as you will be using more of the solar power you generate rather than exporting it to the national grid.
The alternative is to take the power directly from the batteries and automatically switch to the grid when the batteries are low. This will also work if there is a grid power cut, your power will be uninterrupted and you will continue having power even when the grid is off. As you will essentially be having an independent supply from the grid. You will have to design your wiring differently if you used it as a grid linked version as above, either having a separate ring main or wiring specific items you want to run to the inverter.