How does a Prepaid Meter Work?

In order to understand Prepaid Meters and their operation, we first need to get an understanding of Electrical Measuring equipment and how electricity is calculated on our monthly electricity bills from our relevant municipalities.  


A kilowatt-hour meter is the meter that the municipality installs at a premises to measure the amount of electricity (or units) used on a premises over the period of time. The municipal meter readers are normally sent out once a month to read these meters and calculate the KWh (units) used over that period.The units or KWh's
measured are then multiplied by the electricity tariff charged by the respective municipality.
To give you an idea of how much electricity one consumes, look at the following basic calculation:
One Kilowatt (or 1 Unit) is equal to 1000 watts. A standard globe used in a house is 100 Watts. If that globe is to burn for one hour it will consume 0.1 KWH (or units) of power

(100 Watts / 1000). The current price for electricity in Pretoria is R 1.0066 a unit or KWh. A 100 Watt globe burning for one hour will thus cost 10c (0.1KWh x R 1.0066). If that same globe were to burn for 24 Hours it will therefore cost R2.40 since it has used 2.4KWh or 2.4 Units of electricity.
Municipalities can only read meters after electricity has been consumed. The municipal accounts are therefore billing you for electricity that has already been used. Municipal accounts are therefore referred to as post-paid electricity.


A prepaid electricity meter is a KWH (Kilowatt Hour) meter, measuring electrical consumption. The main
difference is that this KWh meter counts backwards as the electricity is consumed and has a  a relay (an automatic switch) which disconnects the power when the KWh reading on the meter reaches zero. It further incorporates hardware which has the ability to decipher the pin number entered and convert it to KWh.
All prepaid meters in South Africa are STS (Standard Transfer Specification) compliant. This means that they all use the same coding system. In SA this is a 20 digit encrypted code, preventing fraudulent vouchers from being generated. For more on STS click here
Prepaid Meters (the hardware) needs software that have the ability to generate a token which can be deciphered by the meter and converted to KWh.  The software is programmed to only generate a token if the meter is in credit. In South Africa all prepaid software functions by using STS technology.

An STS compliant prepaid meter has the ability to function on any STS compliant software.

Since there are numerous prepaid meter manufacturers as well as numerous software providers, a specific prepaid meter has to be registered on a specific software package before it will function. All STS compliant prepaid meters are identified by an 11-digit code, and can only be linked to one software package at a time.

In order for a prepaid meter to switch to a different software package, the current software supplier as well as the new software supplier need to issue a code, which is physically punched into the meter, before the meter will accept tokens generated by the new software company.
It is this software, as well as the various ways in which these meters are wired into a circuit, that differentiates Municiple Prepaid Meters from Prepaid Sub Meters.