Thanks eco-gnomist. Direct and important questions. Hard to answer well, because I have done so much experimentation, and so lots of costs are not directly to the current generator. I guesstimate about R3000 - R3500 on parts for the generator, plus about R2500 on batteries, wiring, DC bulbs etc.
I use the power only for a few DC lights currently, mainly as a test, and I don't meter the amp hours (or kWh). I plan to extend the DC wiring to light more rooms. Practically, I should look at the hot water geyser (electric) first to make savings effectively.
The full calculation of cost effectiveness is better done as a 'payback time' - the years/months after which the capital investment is paid back by savings. This requires a figure for interest rates (the money would be in a bank account earning interest if not spent on a wind generator), and also marginal tax rates of the owner (as the domestic savings are 'tax free income', whereas interest is subject to income tax). Also a figure for future grid electricity costs, which is anyones guess here is SA! Then the 'soft' advantage of peace of mind that blackouts or quotas will not plunge your house into darkness - hard to quantify that one.
Lets hope the thread does get going!