The cost to run the machine per month is the cost per million tokens.
I think that's just semantics -- unless you genuinely keep racking up non-redeemable energy bill credits for feed-in.
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".
They give you credits for future consumption. You can only spend this on energy. If you always have a surplus, you will just rack up large amounts of credits.
Is the model you're describing (usage credits) in the European market somewhere ?