Chapter 8

Introduction

Over the previous chapters, we've explored each stage of solar system design separately. We've learned how designers assess sunlight, choose the best location, account for shading, select equipment, create the electrical layout and estimate how much energy the finished system is likely to produce.

Now it's time to bring everything together.

Imagine a homeowner has asked you to design a solar installation for their property. They don't want the largest system that can fit on the roof. They want a system that reduces their electricity bills, supports their changing energy needs and provides good long-term value.

Your task is to turn that broad objective into a complete engineering design. The process begins not on the roof, but with the customer.

Understanding the Customer

The customer is a family living in a detached home in Brisbane. Their electricity bill shows an average daily consumption of approximately 24 kWh.

However, that single number doesn't tell the whole story.

Smart meter data reveals that the home uses relatively little electricity during the morning. Demand rises during the afternoon as the air conditioning begins operating, then reaches its highest level in the evening when cooking, lighting and entertainment systems are all in use.

The family currently owns a petrol car, but they expect to purchase an electric vehicle within the next few years. They are also interested in battery storage, although they are not yet certain whether the additional cost is worthwhile.

Their main objectives are clear:

• reduce electricity purchased from the grid

• generate enough solar energy to support future electric vehicle charging

• retain the option to add a battery later

• avoid unnecessary upfront cost

Already, these objectives begin shaping the design.

A very small system may reduce today's electricity bill but leave little capacity for future demand. A battery may improve evening self-sufficiency, but it is not essential to the first stage of the project.

The design must therefore work well today while remaining flexible enough for tomorrow.

Assessing the Solar Resource

Brisbane receives a strong solar resource throughout the year, making it well suited to rooftop solar. Long-term climate data indicates that the site receives approximately 5 Peak Sun Hours per day on average, although the exact amount varies with season and weather conditions.

Summer generally provides longer days and higher total solar irradiation, while winter produces less energy because the Sun remains lower in the sky and daylight hours are shorter.

The solar resource gives us a first indication of what the system might produce.

For example, an 8 kW solar array receiving an average of 5 Peak Sun Hours per day would have a theoretical daily energy potential of:

8 kW × 5 h = 40 kWh per day

This is only a starting point. Real-world losses from temperature, shading, cabling and inverter conversion will reduce the final delivered energy. Even so, the calculation shows that a system of this size could generate more energy on an average day than the household currently consumes.

The next question is whether the property can physically accommodate it.

Assessing the Roof

The house has three main roof sections. The largest faces north and receives strong sunlight throughout most of the day. A smaller section faces west, making it useful for extending energy production later into the afternoon. A third section faces south and receives significantly less direct sunlight, so it is rejected early in the design process. 

The north-facing roof contains enough space for fourteen panels. The west-facing roof can accommodate six more. Together, the two usable roof sections could hold twenty panels.

There are, however, several constraints. A skylight reduces the available area on the north-facing roof. A nearby tree casts a small morning shadow across one corner of the west-facing section. Installation and maintenance clearances must also be preserved around roof edges and access routes.

Rather than filling every available space, the designer arranges the panels only where they can operate safely and effectively.

The final layout uses:

• fourteen panels on the north-facing roof

• six panels on the west-facing roof

• no panels on the shaded corner or south-facing roof

This produces a total of twenty panels while avoiding the poorest locations.

Choosing and Sizing the Equipment

The available roof space and customer objectives now guide the equipment selection. The chosen solar panels are rated at 400 W each. With twenty panels installed, the total array capacity becomes:

20 × 400 W = 8,000 W (or 8kW)

This size provides a useful balance. It is large enough to offset a significant proportion of the household's current electricity use while providing extra generation for future electric vehicle charging. At the same time, it remains within the available roof area and avoids unnecessary oversizing.

Because the roof is divided between north- and west-facing sections, the selected inverter includes at least two independent MPPT inputs. This allows each panel group to be optimised separately.

A hybrid inverter is chosen so that battery storage can be added later without replacing the main conversion equipment.

The customer decides not to install a battery immediately. Instead, the design preserves the option to add one once evening demand increases or the financial case becomes more attractive.

The final equipment selection therefore includes:

• twenty 400 W solar panels

• an 8 kW solar array

• a compatible multi-MPPT hybrid inverter

• provision for future battery storage

• monitoring equipment for tracking system performance

The most expensive equipment has not automatically been selected. The equipment has been chosen because it matches the site, the roof and the customer's future plans.

Designing the Electrical Layout

The physical roof layout now becomes an electrical design. The fourteen north-facing panels form one electrical group, while the six west-facing panels form another. Because these groups experience different sunlight patterns throughout the day, they are connected to separate MPPT inputs on the inverter.

This allows the inverter to find the best operating point for each group independently. The north-facing array is expected to produce the largest amount of energy overall, with output rising through the morning and peaking around midday. The west-facing group contributes less annual energy but continues producing later into the afternoon, when household demand begins increasing. This makes it valuable despite its lower total output. 

The designer also plans the cable routes, equipment locations and access requirements. The inverter is installed in a shaded, ventilated and accessible location close to the main switchboard. DC cables are routed through protected pathways from the roof to the inverter, while the AC connection carries converted electricity into the home's electrical system.

The completed single-line diagram shows how the two panel groups, inverter, switchboard, meter and grid connection form one integrated system.

Estimating Energy Production

The theoretical daily energy estimate was approximately 40 kWh. However, the system will not deliver this amount every day.

The north- and west-facing panels receive different amounts of sunlight. Panel temperatures rise well above standard test conditions during hot weather. Small losses occur through cabling, inverter conversion, soiling and equipment mismatch.

To account for these effects, the designer applies a realistic overall performance ratio. If the system achieves a Performance Ratio of approximately 85%, the estimated average daily energy becomes:

40 kWh × 0.85 = 34 kWh per day

This does not mean the system will generate exactly 34 kWh every day. On clear summer days, production may be considerably higher. During cloudy winter days, it may be much lower. The annual simulation provides a more useful estimate because it accounts for seasonal changes in sunlight, temperature and weather.

Suppose the design software predicts an annual output of approximately 11,000 kWh. Compared with the household's current annual consumption of around:

24 kWh × 365 = 8,760 kWh

The system could generate more energy over the year than the household currently uses. However, annual generation and annual demand do not occur at the same time. Some electricity will be consumed directly. Some will be exported during sunny periods.

Additional electricity will still be imported at night or during poor weather. This is why understanding daily energy patterns is just as important as comparing yearly totals.

Reviewing the Final Design

The completed design can now be reviewed against the customer's original objectives. The 8 kW system should significantly reduce electricity purchased from the grid. Its west-facing panels extend generation into the afternoon, improving the overlap between solar production and household demand. The hybrid inverter allows battery storage to be added later. The system also provides additional energy capacity for future electric vehicle charging.

Most importantly, the design reflects the actual property and customer rather than following a standard template.

The final proposal includes:

• a twenty-panel, 8 kW solar array

• north- and west-facing panel groups

• separate MPPT control for each orientation

• a hybrid inverter

• provision for a future battery

• an estimated annual production of approximately 11,000 kWh

• a clear roof layout and single-line diagram

• realistic expectations for self-consumption, imports and exports

There may still be alternative designs. A smaller system could reduce the initial cost. A battery could increase evening self-sufficiency. Higher-efficiency panels could reduce the required roof area.

Good engineering does not pretend that only one solution exists. Instead, it identifies the option that best balances the customer's goals, the site's constraints, expected performance and long-term value.

Looking Ahead

Until now, the solar system has existed only as information. Electricity bills. Roof measurements. Weather data. Equipment specifications. Drawings and performance estimates.

We've followed the complete path from understanding the customer's energy use to producing a finished solar system design. The equipment has been selected, the panel layout has been finalised, the electrical system has been drawn and the expected energy production has been estimated.

Now the design must leave the page and become a real working installation.

Panels must be mounted securely to the roof. Cables must be carefully routed and protected. Electrical connections must be completed safely, and the inverter must be configured to operate correctly with both the solar array and the electricity grid.

Once construction is complete, every part of the system must be inspected and tested to confirm that the finished installation matches the original design and operates as intended. Even the best design can fail if it is installed poorly. Likewise, a well-installed system must still be commissioned correctly and maintained throughout its operating life.

The design process may now be complete, but the journey of the solar installation is only beginning.

In Part 6, we'll explore how solar systems are installed, inspected and brought safely into operation. We'll follow the process from mounting the first bracket and panel to testing the final electrical connection, before discovering how commissioning, monitoring and maintenance help ensure the system continues operating safely and reliably for decades.