Most South African municipalities price prepaid electricity using an inclining block tariff, usually shortened to IBT. The first chunk of units you buy in a calendar month is the cheapest, and every block after that costs more per unit. The counter resets on the 1st. That is why buying R500 of electricity on the 2nd of the month can give you noticeably more units than the same R500 bought on the 28th, at the exact same address, on the exact same meter.

On top of the blocks, several metros deduct a fixed monthly service or capacity charge from your first purchase of the month, before a single unit is added. In Johannesburg that charge is R241.50 including VAT, which means a R200 first purchase of the month buys you no units at all — the money goes entirely towards the fixed charge, and you need to top up further before electricity starts loading. Tshwane, Cape Town and Ekurhuleni all apply their own version of this charge.

This is the single biggest reason a simple "amount divided by rate" calculation gives you the wrong answer. This calculator works through each block in order, applies the fixed charge where it is due, and shows you the split so you can see exactly where your money went.

Eskom Direct customers are the exception. Homelight tariffs moved to a single flat rate with no blocks and no fixed monthly charge, so the same amount buys the same number of units regardless of when in the month you buy it.

Roughly half of South Africa's electricity customers are supplied directly by Eskom. The rest are supplied by around 180 licensed municipal distributors, each with its own tariff schedule approved separately by NERSA. Two houses in the same town can be on completely different rates depending on which network they sit on.

If you are not sure which applies to you, your prepaid receipt is the most reliable answer. It normally prints the supplier name, the rand amount and the units received. Dividing the amount by the units gives you your effective rate, which you can compare against the figure this calculator produces.

The calculator currently covers Eskom Direct (Homelight 20A and 60A), City Power in Johannesburg, the City of Cape Town, the City of Tshwane, the City of Ekurhuleni and eThekwini in Durban. Between them these serve a large share of South Africa's prepaid households.

Smaller municipalities are being added over time. Where an area is not yet covered, the calculator says so plainly rather than substituting another supplier's rates, because a confident wrong number is worse than an honest gap when you are budgeting.

Rates are checked against each municipality's own published tariff schedule, and every result shows the date the figures were last verified along with a link to the source document. Municipal tariffs change on 1 July each year and Eskom's on 1 April.