To put numbers on it: for a 12 kW system in Chico, CA (a location at ~40 degrees N I chose arbitrarily) the NREL calculator at https://pvwatts.nrel.gov/pvwatts.php gives annual generation of 18500 kWh (40 degree inclination). For a lifespan of 20 years at $0.34/kWh this is $125800 worth of electricity. A 12 kW + 16 kWh LFP battery system costs something like $36K (+ installation and financing).