[darkwater]

You don't know if you are going to pay it anyway and depending on the retribution model you can loose them if you don't use them in time.

Cost is not the right mental model here, you can and should count them in the amortization plan of your installation (total cost / (projected generated kWh * lifetime)), so every kWh not used and injected is paying for it, while the kWh consumed should be discounted at the price you would pay for them at that specific moment.

> unless you genuinely keep racking up non-redeemable energy bill credits for feed-in.

I know many many cases of people with overdimensioned PV installations that at some point want to leave their current energy company because prices are changing and have hundreds or more of euros in the "virtual battery".