South Africa’s electricity debate often centres on a single question: Do we have enough power? The answer, however, is more complex than the country’s installed generation capacity suggests.

Eskom, as the operator of the national power system, does not judge system health purely by how many megawatts exist on paper.

Instead, it relies on a set of operational indicators that reveal whether power stations can actually deliver electricity reliably, when it is needed.

At the heart of this assessment is a distinction that is frequently misunderstood in public discourse: the difference between capacity and availability.

What Does “Power System Health” Mean in South Africa?

In Eskom’s context, power system health refers to the ability of the electricity system to meet demand consistently, safely, and at an acceptable cost. It is not simply a measure of how many power stations have been built, but how well those assets perform day after day.

South Africa has a large installed generation fleet, largely made up of coal-fired power stations commissioned decades ago. On paper, this fleet represents tens of thousands of megawatts of capacity.

In practice, however, not all of that capacity can be used at any given time. Power stations require maintenance, experience breakdowns, and operate under technical constraints that limit output.

This is why Eskom places greater emphasis on usable power rather than headline capacity figures. A system with high nominal capacity but frequent breakdowns is far less reliable than a smaller system that performs consistently.

Why megawatts on paper don’t equal electricity on the grid

Installed capacity is a static number. It does not account for whether units are offline due to planned maintenance, unexpected failures, or operational deratings. Electricity, however, must be produced and consumed in real time. If generation units are unavailable when demand peaks, the system becomes vulnerable, regardless of how much capacity exists in theory.

This gap between theoretical capacity and actual delivery is one of the central challenges facing South Africa’s power system.

Capacity vs Availability Explained Simply

To understand how Eskom measures system health, it is useful to separate three related but distinct concepts.

Installed capacity

Installed capacity refers to the maximum possible output of all power generation units connected to the grid, assuming they are operating at full output. It includes coal, nuclear, hydro, wind, solar, and other generation sources.

This figure is often cited in national energy discussions, but on its own, it says little about day-to-day system performance.

Available capacity

Available capacity is the portion of installed capacity that is actually ready to produce electricity at a given time. Units taken offline for maintenance or affected by breakdowns are excluded.

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Eskom tracks this closely through metrics such as the Energy Availability Factor (EAF), which measures the percentage of the fleet that is available to generate power. A higher EAF indicates a healthier, more reliable system.

Energy actually delivered

The final measure is the electricity that is ultimately produced and supplied to consumers. This depends not only on availability but also on demand patterns, grid constraints, and system losses.

A power station may be available, but if demand is low or transmission capacity is limited, it may not operate at full output.

Why availability matters more than capacity

From a system stability perspective, availability is more important than total capacity because it reflects real operational conditions. Load shedding, for example, is not triggered by a lack of installed capacity, but by insufficient available power to meet demand at a specific moment.

Improving availability often involves extensive maintenance, refurbishment, and operational discipline, especially in an aging fleet. While these efforts may temporarily reduce available capacity due to planned outages, they are intended to improve long-term reliability.

In contrast, adding new capacity without addressing performance issues in existing plants can leave underlying problems unresolved. A system with many power stations but low availability remains exposed to disruptions.

For Eskom, measuring power system health is therefore about understanding how the fleet performs under real-world conditions. Capacity provides a sense of scale, but availability determines whether the lights stay on.

By Thuita Gatero, Managing Editor, Africa Digest News. He specializes in conversations around data centers, AI, cloud infrastructure, and energy.