Automatic switch to main power when battery is drained

Hi,

I'd like to have a following design:
1. there is a 12V car battery connected to DC AC Inverter so I get 230V AC.
2. the battery will be charged by the solar system - charger is not a problem.
3. there will be a monitor watching the battery and if the battery is almost drained (probably at night) it will switch to the main power until the battery is not charged by the sun again.

My question is related to the point #3. It is easy to monitor the battery, it is easy to charge it, but could you recommend me a circuit for the switching from the battery power supply to the main power supply? I need to done it without interruption (electrically) so my desktop computer that will run on it will not suffer power failure. Lets say the power consumption is 500W.

Thank you

your idea is good this can be done by using a comparator or using micro. in comparator method we have to set the low voltage cut off when
the battery voltage goes below 11v it will change the relay position. & when battery charges upto 14v then comparator again will change the
relay position.

thanks,
and would you know any circuit that switches one power supply to another on the fly?