Energy and the National Electricity Grid

Generating electricity in South Africa

South Africa's national electricity grid is supplied by electricity generated at power stations spread across the country. Most of our electricity comes from coal-fired power stations, where burning coal boils water into steam, and the steam spins turbines connected to generators. South Africa also generates electricity from:

Hydroelectric power — falling or flowing water spins turbines (e.g. the Gariep and Vanderkloof dams).
Nuclear power — Koeberg Power Station near Cape Town, South Africa's only nuclear power station, uses nuclear fission to generate heat that produces steam.
Renewable energy — a fast-growing share from solar photovoltaic farms and wind farms, especially in the Northern and Eastern Cape.

Key idea: whatever the source, a power station's job is the same — convert some other form of energy (chemical, nuclear, kinetic, or light) into electrical energy that can be sent out on the grid.

The journey from power station to home

Electricity travels a long way before it reaches your wall socket. The journey happens in clear stages:

1. Generation — electricity is generated at the power station, typically at 11–22 kV (kilovolts).
2. Step-up transformer — at the power station, a transformer increases ("steps up") the voltage to around 400 kV for transmission.
3. Transmission — electricity travels over long-distance pylons and cables at this very high voltage, sometimes hundreds of kilometres, to reach the areas where it is needed.
4. Step-down transformer (substation) — at a substation nearer to towns and cities, transformers reduce ("step down") the voltage to safer, more manageable levels.
5. Distribution — the electricity is distributed to homes and businesses at a safe, usable voltage — 230 V in South African homes.

Why transmit electricity at such high voltage?

It seems strange to push electricity to such extreme voltages just to reduce it again later — but there is a very good reason. When electrical energy is transmitted, some of it is always lost as heat in the cables due to their resistance.

For the same amount of power delivered, transmitting at a much higher voltage means a much lower current is needed. Since the heat lost in a cable increases sharply as current increases, a lower current means far less energy is wasted as heat over those long distances.

Qualitative idea: less current for the same power = less energy lost as heat in the transmission cables. This is why power stations step voltage UP before sending electricity long distances, then step it DOWN again before it reaches your home.

Transformers: stepping voltage up and down

Transformer type	What it does	Where found
Step-up transformer	Increases voltage, decreases current	At the power station, before transmission
Step-down transformer	Decreases voltage, increases current	At substations, before distribution to homes

Using electricity wisely at home

Generating electricity uses resources and, for coal-fired power stations, releases greenhouse gases. South Africa has also experienced load shedding — planned power cuts used when the demand for electricity is greater than the supply available from the grid. This makes the efficient, careful use of electricity in the home especially important.

Switch off lights and appliances when they're not in use.
Use energy-efficient LED bulbs instead of old incandescent bulbs.
Use a geyser blanket and timer to reduce water-heating energy use (geysers are one of the biggest users of electricity in a home).
Unplug chargers and appliances on standby, which still draw small amounts of power.

Local relevance: understanding the grid helps you understand why load shedding happens — the demand for electricity across the country can exceed what the power stations and grid can safely supply at that moment, so supply is deliberately reduced in some areas to protect the whole system.

Links forward: in later grades you'll study the formula P = I²R which explains mathematically why high voltage/low current transmission minimises energy loss.

Answer in your exercise book. Write in full sentences and pay attention to the mark allocation for each question.

Question 1 · (3 marks)

Name THREE different ways that South Africa generates electricity, and for each one, state what form of energy is being converted into electrical energy.

Question 2 · (5 marks)

List, in the correct order, the FIVE stages electricity passes through on its journey from a power station to a home. Include an approximate voltage for at least three of the stages.

Question 3 · (3 marks)

Explain, in your own words, why electricity is transmitted over long distances at very high voltage rather than at the voltage used in homes.

Question 4 · (2 marks)

Describe the difference between what a step-up transformer does and what a step-down transformer does.

Question 5 · (4 marks)

What is load shedding, and why does it happen? Suggest TWO ways a household can reduce its electricity usage to help ease pressure on the grid.

Question 6 · (4 marks)

The table below shows the approximate current needed to deliver the same amount of power (100 MW) along a transmission line, at three different voltages.

Transmission voltage	Approx. current needed to deliver 100 MW
11 kV	≈ 9 090 A
132 kV	≈ 758 A
400 kV	≈ 250 A

(a) Using the table, calculate roughly how many times higher the current is at 11 kV than at 400 kV, for the same power delivered. Show your working. (1)
(b) Based on the lesson's key idea that heat loss in a cable increases sharply as current increases, which of these three voltages would waste the LEAST energy as heat over a long transmission line? (1)
(c) Explain why a power company would choose to transmit electricity at 400 kV instead of simply sending it straight from the power station at 11 kV. (2)

Energy and the National Electricity Grid

Generating electricity in South Africa

The journey from power station to home

Why transmit electricity at such high voltage?

Transformers: stepping voltage up and down

Using electricity wisely at home

The grid pipeline

💡 Why does the voltage rise so high, then drop again?