Right MarkD, this one is live: Solar Savings & Payback Calculator South Africa
I took your honesty points straight into it. It shows the optimistic payback an installer quotes next to the realistic one, then lists exactly what eats the gap, the wasted daytime power, panel ageing, the battery replacement down the line, and the higher fixed charge some municipalities add once you go solar. Export credit defaults to zero, because most places pay nothing for what you push back unless you are registered.
Have a proper play and tell me if the self consumption side feels right to you, since you have actually sized these setups. To keep it out of Nebula’s thread I started a dedicated one for it here: New tool: an honest solar payback calculator for South Africa