For Tshwane the prepaid and normal tariffs are the same price. I switched to pre-paid 3 years ago when I saw a billboard which promised 20% saving. Now, I think I was a fool for not checking properly.
Where it does help, is to buy a lot of electricity before a price increase. I am still using electricity on the pre July 2011 prices, and we are almost at the next increase again. So I made my saving there.
The other nice thing is that there is no stupid readings (happened to my water bill the last 2 months where the first one was double the normal amount, and this month I have to pay nothing for water).
The bad thing is that they block you from buying electricity as soon as there is a payment outstanding on your normal rates/services account.
Yea patents did that too, purchased 30k worth of electricity over about a month or so! Lasted pretty long lol