Switch from solar system to the grid.

 Hi,
I have a small solar system (solar panel + battery + charge controller
+ pure sine inverter). I am looking for a way to switch automatically
from my output 120v to the 120v grid energy when my battery become to
low and back to my system when the battery is recharged. I use it with
my computer (120v) which have a battery backup so I can plug and
unplug manually from grid to system even if my computer is on but this
system is not efficient. Is there a cheap solution ?

Thank you!