A car battery with reduced range is still perfectly useable as a storage battery for a PV system. I got a high-voltage hybrid inverter when I built our PV (solar) system specifically for that reason: high-voltage EV batteries will appear on the market which I can use to create energy storage. Commercial storage systems are still too expensive for what they offer but used EV batteries are starting to appear at far more reasonable price levels. If a pack which used to store 90 kWh now only stores 70 kWh but costs 1/10 of a similarly-sized commercial storage system when combined with a suitable controller then I'd just get a bunch of them.