A grid-tie inverter converts direct current (DC) into an alternating current (AC) suitable for injecting into an electrical power grid, normally 120 V RMS at 60 Hz or 240 V RMS at 50 Hz. Grid-tie inverters are used between local electrical power generators: solar panel, wind turbine, hydro-electric, and the grid. To inject electrical power efficiently and safely into the grid, grid-tie inverters must accurately match the voltage and phase of the grid sine wave AC waveform. Some electricity companies pay for electrical power that is injected into the grid. The inverter then synchronizes the two power sources and supplements the mains utility with the solar power. The advantage of this system is that you can drastically reduce your mains utility bill through solar power. This type of system unfortunately does not function if it does not have mains electricity to synchronize with and therefore cannot be used as a backup system. If you're looking to cut costs and have a backup option you need to have a look at the Hybrid Solar Inverters here. An important point to consider when buying a grid-tie or on-grid solar solution is whether it is possible to feed electricity back into the grid (legally). If this is an option your can reap the reward of further savings by supplying excess power to the mains utility grid and receiving credits from your utility provider. If this isn't possible in your area, then you should be sure to purchase a grid-tie inverter with export controls that allow you to disable the feedback into the grid.