Not OP but I'll run what I plan on doing. In Germany you can get a plug-in ready kit with 2.2 kWh battery and 920W panels for less than 600€ [1]. The panels are about 1.8 m² each (=3.6 m² total), so with on average 1.200 kWh/m² solar radiation and 25% efficiency I'm looking at up to 1080 kWh in energy I'll get out of the system per year.

Electricity here in Germany is expensive at an average 30 ct/kWh, so the panels save 324€ worth of electricity - an ROI of > 50%. Choose some better quality for the inverter and battery and you'll still be north of 30% ROI.

The key thing making this high ROI possible is that it's small. Counterintuitive, yes - but explained by the fact that for larger installations than that, setup costs go up: you need to install the panels on your roof instead of hanging them off your balcony which can be thousands of € in labor, you need to run new power wires...

[1] https://www.idealo.de/preisvergleich/OffersOfProduct/2092828...

[2] https://www.swm.de/unternehmen/magazin/energie/pv-ertrag-im-...