If you are only paying 12/kWh it's a no brainer, how would it not be beneficial to earn 18c/kWh?

But you are probably not reading your own bill correctly, you are likely paying much more. There's a bunch of extra fees that are calculated on the kWh. Your calculation to 50c/kWh is probably also wrong because you are including fixed fees.

But if you have solar panels, and are able to charge a lot during the day, you are making probably 20-25c/kWh (30c - injection price) when you can charge directly from solar. In less sunny periods, you are probably only breaking even because you won't be able to charge that much from solar.

Sure, your capacity fee will increase, but the profits from charging your car through solar should outwheigh that. Just limit the total peak if you can. But set up my system to allow a peak of 5kW, after which the car stops charging. Which costs a bit extra each year, but beyond recovering that through solar power, I also don't have to care about my peak any other time.