Chapter 2

Introduction

Every solar power system begins with the same source of energy: the Sun.

Yet despite the Sun shining on the entire Earth, not every location receives the same amount of usable solar energy. Some regions enjoy long, cloudless days for much of the year, while others experience frequent rain, shorter winter days or lower Sun angles. Even two houses only a few hundred kilometres apart can produce noticeably different amounts of electricity from identical solar systems.

Before an engineer can decide how many solar panels are needed, where they should be installed or how much electricity they are likely to generate, they must first answer one fundamental question:

How much sunlight is actually available?

This chapter explores how engineers measure the Sun's energy at the Earth's surface and why geography, seasons and climate play such an important role in the design of every solar power system.

Measuring Sunlight

Earlier in this handbook, we discovered that the Sun continuously emits an enormous amount of electromagnetic radiation in all directions. By the time that energy reaches the Earth, however, not all of it arrives at the ground.

Some sunlight is reflected back into space by clouds and the atmosphere. Some is absorbed by atmospheric gases such as ozone and water vapour. The remainder reaches the Earth's surface either directly from the Sun or after being scattered by the atmosphere.

For solar engineers, the important question isn't how much energy leaves the Sun, but how much actually reaches a solar panel.

To answer this, two closely related terms are used.

Solar irradiance is the instantaneous power of sunlight falling on a surface and is measured in watts per square metre (W/m²). It tells us how intense the sunlight is at a particular moment.

Solar irradiation, by contrast, measures the total amount of solar energy received over a period of time. It is usually expressed in kilowatt-hours per square metre (kWh/m²) and tells us how much solar energy has accumulated throughout a day, month or year.

A useful way to think about the difference is to imagine rainfall. Irradiance is like the rate at which rain is falling at this moment, while irradiation is the total amount of rain collected in a bucket over the course of a day.

For designing solar systems, irradiation is often the more useful quantity because it determines how much electrical energy a solar installation can realistically produce over time.

Peak Sun Hours

Tracking the intensity of sunlight every minute of every day would make estimating solar energy production unnecessarily complicated.

Instead, engineers use a much simpler concept known as Peak Sun Hours (PSH).

One Peak Sun Hour represents the equivalent of receiving 1,000 watts of solar irradiance per square metre for one hour. Importantly, this doesn't mean the Sun shines at its maximum intensity for exactly one hour. Instead, it combines all the varying sunlight received throughout the day into a single, easy-to-use value.

For example, imagine a location experiences weaker sunlight during the morning and evening but much stronger sunlight around midday. When all of this energy is added together, it may be equivalent to receiving five hours of perfect sunshine at 1,000 W/m². The location is therefore said to receive 5 Peak Sun Hours.

This simple concept allows designers to estimate energy production without analysing the changing intensity of sunlight throughout every hour of the day.

For example, a 6 kW solar system installed in a location receiving an average of 5 Peak Sun Hours has a theoretical daily energy production of approximately:

6 kW × 5 h = 30 kWh

Real systems will produce slightly less than this due to unavoidable losses, but Peak Sun Hours provide an excellent starting point for system design.

Why the Seasons Matter

If the Earth always received the same amount of sunlight, solar systems would generate almost identical amounts of electricity every day of the year.

In reality, they do not. The changing seasons are caused not by the Earth's distance from the Sun, but by the 23.5° tilt of the Earth's axis.

During summer, the Sun follows a higher path across the sky, daylight lasts longer and sunlight strikes the Earth's surface more directly. During winter, the Sun remains much lower above the horizon, daylight hours become shorter and the sunlight is spread over a larger area.

As a result, solar panels generally receive more energy and generate significantly more electricity during the summer months than they do during winter.

Designers must therefore consider the performance of a solar system throughout the entire year rather than judging it by a single sunny afternoon.

Why Location Matters

The amount of sunlight reaching the Earth's surface also depends on where a solar system is installed.

Locations closer to the equator receive relatively consistent sunlight throughout the year because the Sun remains high in the sky for much of the day. As you move further north or south from the equator, seasonal differences become increasingly pronounced. Winter days become shorter, the Sun remains lower above the horizon and the amount of available solar energy decreases.

Local climate also has a major influence. Frequent cloud cover, prolonged rainfall, dust, humidity and air pollution can all reduce the amount of sunlight reaching the solar panels. By contrast, regions with consistently clear skies generally produce much higher annual energy yields.

Australia provides an excellent example. Although both Perth and Hobart are excellent locations for solar power, identical systems installed in each city will not produce the same amount of electricity. Perth enjoys more clear-sky days and higher annual solar irradiation, while Hobart experiences lower winter sunlight levels and greater seasonal variation. Designers account for these differences when estimating long-term energy production.

Understanding both geography and climate allows engineers to predict how a solar installation is likely to perform over its lifetime rather than simply assuming every location receives the same amount of sunshine.

The Sun Sets the Limit

No matter how efficient a solar panel becomes or how carefully a system is designed, one fact can never be changed. A solar installation cannot generate more energy than the Sun provides.

The role of the designer is therefore not to create sunlight, but to make the most effective use of the sunlight available at a particular location.

Everything that follows in the design process, from choosing the roof orientation and selecting equipment to estimating annual electricity production, depends on understanding this available solar resource.

The Sun sets the upper limit. Good engineering ensures we get as close to that limit as possible.

Looking Ahead

Knowing how much sunlight is available is only part of the challenge.

The next question is just as important: Where should the solar panels be installed to capture as much of that sunlight as possible?

In the next chapter, we'll explore how roof orientation, tilt angle, available space and structural considerations influence the performance of a solar power system, and discover why choosing the right location is one of the most important decisions in the entire design process.