The inverter will get the source from the solar panel while the battery is being charged.
How to match solar panel with battery.
Add a solar panel to keep your backup battery topped up step 1.
Solar panels you should design your system such that the voc voltage open circuit of your solar panel s are between 1 4 and 1 8 times your nominal battery bank voltage.
The vmpp of your array or single panel should be 1 15 to 1 3 times the nominal battery voltage.
For example five 100 watt panels in parallel would be 5 29 x 5 26 45 amps.
Match the solar panel to the battery.
Then the moment the cloud dims in which the solar panel can t produce enough voltage the relay will change the inverter s source which is from solar panel to battery.
Solar panel output is measured in watts.
All panels at the low end of wattage output.