A solar design report shows exactly what an installer intends to put on your roof and how it will be wired. It is the best tool you have for judging whether a quote was genuinely designed for your home or simply adapted from a template. You do not need to be an engineer to read one. You need to know what each section is for, which numbers should agree with each other, and which gaps deserve a polite question before you sign.

What a complete design report usually contains

Formats vary, but a thorough residential design package typically includes the parts below. If a quote arrives with only a price and a panel count, ask whether a design is available.

Reading the panel layout

The layout drawing is usually a plan view of your roof with a rectangle for each panel. Start by counting them and multiplying by the panel wattage: 15 panels at 440 W is 6.6 kW. If the drawing, the quote and the equipment list give different totals, ask which one is correct.

Then look at placement:

• Roof faces used. Each section should be labelled with its orientation (north, east, west) and pitch.
• Edge clearances. Panels need a margin from ridges, gutters and roof edges to suit the mounting system and wind requirements. Panels drawn hard against an edge are worth a question.
• Obstructions. Vents, skylights, antennas, evaporative cooler units and flues should appear on the drawing. If your roof has them and the plan does not, the design may have come from an aerial image alone.
• Portrait or landscape. Either can be right; the choice affects how many panels fit.

If you want context on how the panel count was reached in the first place, the guides on how many panels you need and system sizing cover that step.

Understanding the string plan

Panels are connected in series into groups called strings, and each string feeds an inverter input known as an MPPT (maximum power point tracker). The string plan shows which panels belong together, often with colour coding or labels such as "MPPT 1, String A".

The main principle is simple. Panels facing different directions, or with very different shading, should generally not share a string, because a string tends to perform at the level of its weakest panels. A north face and a west face would normally sit on separate MPPT inputs. If your roof has three usable directions and the inverter has only two inputs, ask how the designer has handled it: module-level electronics, a different inverter, or leaving one face out.

A good report also confirms that each string's voltage stays within the inverter's limits on hot afternoons and cold mornings. You do not need to check the arithmetic, but you can ask whether it was done.

Reading the single-line diagram

The single-line diagram (SLD) is the electrical schematic. It uses one line to represent each circuit and standard symbols for switches and protection devices. Follow it from the array to the grid:

1. The solar array and strings, with DC cable sizes.
2. DC isolation for the array at the inverter.
3. The inverter, and the battery if one is included.
4. AC isolation and a dedicated circuit breaker at the switchboard.
5. The meter, plus any export control meter or current transformer (CT) used to limit exports.
6. A backup circuit or sub-board, if blackout backup is part of the quote.

The most useful check is whether the diagram matches what you were promised. If you were offered backup power, the SLD should show a backup output and the circuits it supplies. If your network requires export limiting, a meter or CT should appear. If your switchboard needs work, the diagram or notes should say so, because that is a common source of later variations. The electrical work must be done by a licensed electrician, with design and installation by SAA-accredited professionals.

Checking the yield estimate

The yield estimate converts the design into kilowatt-hours: annual generation, usually a monthly chart, and assumed losses for temperature, cabling, inverter conversion, soiling and shade.

As a sense check, a 6.6 kW north-facing system in Sydney typically produces roughly 24-27 kWh a day on average across the year. East or west arrays, shaded sites and unusual pitches will differ. Questions worth raising:

• Is the estimate for year one only, or does it allow for gradual panel degradation, commonly around 0.4-0.55% a year?
• Does the loss list include shading, and does that match what you see on your roof in winter?
• Is inverter clipping accounted for where panel capacity exceeds the inverter rating?
• Where do any savings figures come from? Yield is physics; savings depend on how much you use yourself, your retail rate and your feed-in tariff.

An estimate well above other quotes for a similar system on the same roof reflects a more optimistic assumption, not a better system.

Interpreting the shading analysis

Shade is where desktop designs most often go wrong. A proper analysis considers nearby trees, neighbouring buildings, chimneys and the roof's own ridges across all seasons. Winter matters most, because the sun sits lower and shadows are longer. Look for a solar access percentage per roof face or per string, plus a note on how it was measured, such as a survey taken at the roof during the site visit.

If a large tree is involved, the report should say whether it was modelled at its current height or allowing for growth. Where pruning could make a real difference, the market on this site lists an Arborist & Shading Assessment from $450 (indicative, confirmed after a site assessment), combining shade analysis with qualified arborist advice under council tree rules.

What to query before you sign

Keep your questions in writing, and ask for any agreed change to appear in a revised design. Useful questions include:

• Does the layout reflect an inspection of my roof, and are all obstructions shown?
• Which panels share each string, and why?
• Does the diagram show isolation, export control and any backup I was offered?
• Is the yield estimate after all losses, including shade and clipping?
• Will I receive the as-installed design at handover?

If one of your quotes came without any design at all, the guide to common installation mistakes explains why skipping a proper site assessment tends to cause problems later.

Next steps

Ask each installer for their design report and compare the documents section by section, not just on price. To see a design prepared after a genuine in-home inspection, you can request a free assessment from Blue Energy Solar. If you would like your roof, usage and likely yield reviewed first, the market also lists a Solar Feasibility Report from $149 (indicative).

Frequently asked questions

Should I expect a design report with every solar quote?

You should at least expect a panel layout, the exact equipment models and a yield estimate before you sign. A full string plan and single-line diagram are sometimes finalised only once you proceed, because they take time to prepare. It is reasonable to ask that the final design be confirmed before installation day and that an as-installed version be handed over with your warranty documents at completion.

What happens if the installed system differs from the design report?

Small layout adjustments on installation day are common, such as shifting a panel to clear a vent the drawing missed. Changes to the panel count, equipment models or system size should be agreed with you in writing first, with the price adjusted if needed. Compare the as-installed documents with your contract at handover and raise any differences before paying the final balance.

Can I use one installer's design report to get quotes from others?

A design report is the work of the installer who prepared it, so check any terms before sharing it. It is usually more useful to ask each installer to prepare their own design after inspecting your roof, then compare the documents. Differences in layout, stringing and yield assumptions often reveal more about each installer's approach and care than the price alone.