The system you need, and the roof you have

By Oliver Hill

Usable area is not roof area

Panels have to sit in continuous runs on a single plane, with clearance at the edges and room to work. Every interruption, every change of pitch and every shaded corner takes usable space out of the total. A roof measured at a given number of square metres will usually carry less than that arithmetic suggests.

What takes the space

The usual culprits are unremarkable individually and add up quickly:

• Hips and valleys, which break one plane into several smaller ones
• Chimneys, which sit where they sit and cast shade as well as taking area
• Vents, flues, skylights and whirlybirds
• Edge clearances, which are required rather than optional
• Any face too small to take a worthwhile run

Extensions create several roofs at once

A house that has been extended, which is common through Footscray, Seddon and Yarraville, often ends up with two or three roof sections at different pitches and orientations. Sometimes this helps: a newer rear extension can be the best solar roof on the property. Sometimes it means the array is split across faces that each perform differently, which is fine as long as the design accounts for it and you are told what each section contributes.

Narrow blocks and access

Older inner-west housing sits on narrower blocks than the outer suburbs, and the houses sit closer to their boundaries. That affects the roof itself, and it affects getting equipment to it. Neither is a barrier, but both are reasons an installer might quote a higher site cost, and it is fair to ask them to say so rather than fold it into a total.

Segmented project-home roofs

The opposite problem shows up on newer estate housing through Tarneit, Truganina and Point Cook. Those roofs are usually in good condition and simple to work on, but they are often broken into several faces for the look of the thing. Total area can be generous while the largest single usable face is modest. This is a common reason a new, large-looking house ends up with a smaller system than the owner expected.

The neighbours

Shade is not only about trees. On compact lots a neighbouring double-storey house can cross part of your roof at some point in the day, and a roof that looks clear at the time of the site visit may not be clear at four in the afternoon. A proper design accounts for the whole day.

Orientation trade-offs

North-facing panels produce the most over a day. That does not automatically make a north-only array the right answer. Splitting an array east and west spreads generation across the morning and afternoon rather than concentrating it at midday, which can suit a household that uses power at the ends of the day better than a smaller north array would.

The right layout depends on when you use power, not only on which way the roof points. If an installer has proposed an unusual layout, ask what it is designed around.

Why a big house can have a small usable roof

Put the last few sections together and the pattern is clear. A large modern home in Caroline Springs can offer less continuous usable area than a plain older house half its size, because the area is cut into more, smaller faces. House size is a poor predictor. Roof shape is a good one.

If the roof cannot carry what you need

This happens, and there are real options rather than dead ends. Higher-output panels fit more capacity into the same area. A garage or outbuilding on the same property may add usable space. Storage lets a smaller array cover more of your consumption by moving it rather than growing it. And in some cases the honest answer is a smaller system than you hoped for, which is still worth having.

What none of those change is the sizing question itself. If you want to work out how much you should be aiming for before worrying about whether it fits, What size solar system do I need in Victoria works it through from your bill, and what a 6.6kW system costs in Victoria has what the most common size actually costs.

Common questions