I think a lot of people think that when putting in solar power they have to put in the whole system at once. Well I use the Conext 6848 inverter (Schneider Electric) which will do both grid tie and off grid at the same time. The inverter when tied to the grid (but not selling) has a built in charger that will charge the 48 volt battery. The inverter is then able to supply an off grid lighting panel with power. This charging occurs at the same time as supplying the load. You will basically be running off the grid except if the grid goes down you will still have power on your off grid panel until the battery goes dead. You may not have the money to put it all in at once. Solar panels may be added later. Sometimes it take a while to build out the mounting structure for the solar panel array. There are several reasons why you may wish to build out your system in pieces. The important thing is to get started!