City of Johannesburg Municipal Tariffs increases are introduced on the 1st June of each year, although, as Schools Municipal Invoices are post-paid (paid after consumption), the increases will appear one month later.
Curiously, the fact that the Higher Electricity Winter Tariffs are charged in June, July and August, (which coincides with the annual tariff increase) the increases appear high initially, where with the September Invoice, the Lower Summer Electricity tariffs apply and invoices appear "less expensive".
Physiologically, the effect is that after the winter (where you expect high costs), the September electricity costs appear lower and the effect of the increase somehow dissipate, and we no longer seem so concerned with the increased tariff. It may however be interesting to look at the compound cost effect of 7 years of increases, to appreciate how much more expensive your Electricity and Water cost really are.
Details of the Electricity Tariffs for the City of Johannesburg can be obtained here and the tariffs applicable to schools can be found on Page 18 of 37.