The relay has three connections, Common, NO, and NC. The live wire from the lighting circuit is connected to the Common, the live wire from the mains supply to NC, and the live output from the power inverter to NO. The live and neutral outputs from the power inverter are then connected across the relay coil.
When the power inverter is switched on and the batteries are charged, the relay coil has 240VAC current flowing through it. This moves the contact within the relay from the NC to the NO position connecting the live wire from the inverter to the live of the lighting circuit - lights are powered from renewable energy. When the batteries are discharged, or if the inverter is switched off, the lighting circuit live will be connected to the live of the mains supply via the NC of the relay - lights are powered by mains electricity.
This is a fully automatic system with switching from inverter to mains power in a fraction of a second.
To make it safe, a circuit breaker should be used (rated the same as the mains circuit breaker for this circuit), and a manual ON/OFF switch and LED indicator included to enable the user to see which power source is being used and manually switch to mains power if desired.

External Resistor Adjustable
10-288 Volt AC or DC Operation
2 to 600 Seconds Timing Range
Solid State Universal Cube Timer
NTE RLY210 AC or DC Solid State Universal Cube Timer - Delay on Operate. 1k resistor for 2 second delay to 3.9M for 600 Seconds on-delay, or use 5M pot for full range. Switching Load: 40mA min - 1 Amp Max. Utilising 1.2VA (VDE approved) or 1.6VA (UL/CSA recognized) AC coils, the single pole, flange mount T9C AC coil relay is rated at 20A N/O and 10A N/C and features either a 1N/O or 1C/O Cadmium free contact. Further technical details include an operating temperature range from -40 up to +70 degrees Celsius, 250 Vac rated voltage, a maximum breaking capacity of 5000VA and a mechanical life expectancy of 5 million cycles. The T9C AC coil relay is fully RoHS compliant.