More than 200 million prepayment meters are in service worldwide. In sub-Saharan Africa, they represent the dominant metering model. Understanding how they work — and where they are heading — is essential for any metering professional.
What is a Prepayment Meter?
A prepayment (or Pay-As-You-Go) meter requires the customer to purchase credit before consuming utilities. When credit runs out, the meter disconnects supply. It eliminates bad debt for the utility and gives low-income consumers control over their spending.
STS: The Token Standard
The Standard Transfer Specification (STS / IEC 62055-41) is the most widely used prepayment standard globally. It defines how credit tokens are generated, encoded, and accepted by meters.
How STS Tokens Work
- Customer purchases credit at a point-of-sale (mobile phone, kiosk, bank)
- The vending system calculates a 20-digit token using the meter’s unique keys (DKGA02 algorithm)
- Customer enters the token on the meter’s keypad
- Meter decodes and validates the token cryptographically
- Credit is added to the meter’s internal balance
The token is tied to one specific meter — entering it on any other meter fails. This is the anti-fraud mechanism.
STS Token Classes
- Class 1 (01): Credit token — adds kWh credit
- Class 2 (02): Configuration token — changes tariff, emergency credit threshold
- Class 3 (03): Clear credit — removes all credit (utility use)
- Class 10 (0A): Set meter SGC (Supply Group Code) — re-keying
CTS: Contactless and Keypad-Free
CTS (Contactless Transfer Specification) is the AMI evolution of STS. Instead of a customer entering a token, the HES pushes credit directly to the meter over the AMI communication network. This enables:
- Remote top-up (no keypad entry required)
- Emergency credit via mobile app
- Automated tariff changes
- Eliminating the vending infrastructure cost
Friendly Hours and Emergency Credit
Most STS meters implement Friendly Hours — a configurable window (typically evenings and weekends) when the meter will not disconnect even if credit reaches zero, to prevent households losing supply at night. Emergency Credit provides a small buffer (e.g., 5 kWh) usable once per month.
The African Market: Dominance and Evolution
Sub-Saharan African utilities (KPLC Kenya, ZESCO Zambia, ZETDC Zimbabwe, UMEME Uganda) run almost entirely on STS prepayment. The transition from STS keypad to CTS smart AMI is the defining infrastructure story of the next decade in these markets. Key challenges:
- Mobile money integration (M-Pesa and similar) for vending
- GPRS/NB-IoT coverage gaps in rural service territories
- Consumer trust in remote disconnection
STS Association and Key Management
The STS Association manages the standard and the Key Change Token (KCT) infrastructure that allows utilities to re-key meters when they change vending system provider — a critical interoperability safeguard. Membership and technical documentation: sts.org.za
Frequently Asked Questions
What is the DKGA02 algorithm and how does it secure STS token generation?
DKGA02 is the cryptographic algorithm used by STS vending systems to generate 20-digit tokens that are mathematically bound to a meter’s unique keys, preventing tokens generated for one meter from being accepted by another. This creates the anti-fraud mechanism that makes STS tokens non-transferable between meters.
How do STS token classes differ and what is the operational purpose of Class 10 (0A)?
STS defines multiple token classes: Class 1 adds credit, Class 2 modifies configuration/tariffs, Class 3 clears credit for utility disconnection, and Class 10 sets the Supply Group Code (SGC) for meter re-keying when utilities change vending providers. Class 10 tokens are critical infrastructure tokens used infrequently for interoperability management.
What is the technical advantage of CTS (Contactless Transfer Specification) over STS keypad entry in rural AMI deployments?
CTS eliminates the need for manual token entry by pushing credit directly from the Head-End System (HES) over the AMI communication network, enabling remote top-up and automated tariff changes while reducing reliance on point-of-sale vending infrastructure in areas with poor GPRS/NB-IoT coverage. This is particularly valuable in sub-Saharan African rural service territories.
How do Friendly Hours and Emergency Credit function at the meter firmware level to prevent supply disconnection?
Friendly Hours is a configurable time window (typically evenings/weekends) when the meter’s disconnection logic is suspended even if credit balance reaches zero, while Emergency Credit provides a small usable buffer (e.g., 5 kWh) triggered once per month to prevent household supply loss during critical periods. Both are firmware-based protections implemented within the meter’s control logic.
What role does the STS Association and Key Change Token (KCT) infrastructure play in prepayment meter interoperability?
The STS Association manages the Key Change Token (KCT) infrastructure that allows utilities to cryptographically re-key meters when switching vending system providers, ensuring meters remain functional across different vendor ecosystems and preventing vendor lock-in. This is a critical interoperability safeguard managed at sts.org.za.