A Solar Calculator Gave You a Number: What It Cannot See on Your Landed Roof in Singapore

You typed your address into a solar calculator, outlined your roof, and a number came back: so many panels, so much a year, and a payback period. It is a useful number. It is also a number about a picture of your roof, not about your roof.

That difference is where most landed solar projects go wrong or go slow. Nobody was dishonest. The calculator did what it was built to do. It simply cannot see the things a builder walks onto a roof to find out. This guide is about those things: what a calculator reads, what it does not, and what to check yourself before you let a figure on a screen turn into a deposit.

What a good calculator actually reads

Take our own solar calculator as the example, because we can say exactly what it does. It pulls roof imagery and orientation data for your address from Google's Solar API, you drag an outline until it hugs your roof, and the numbers are worked out from the area you outlined. It then applies Singapore tariff assumptions: for landed homes it models about 35% of the power as used in the house, valued at SP residential rates, with the rest exported to the grid.

That is a fair first screen. It tells you roughly how many panels the visible roof area can hold, which way that area faces, and what the economics look like if everything on the roof is as simple as it looks from above.

The calculator's own page is honest about its limit. Under "What the calculator can't see" it lists three things: the roof's structural condition, the age of the trusses, and future shading from neighbouring rebuilds. Those three are the start of a longer list.

What the picture cannot show

1. What the roof is built from, and how old it is

A panel array is not weightless. A typical installation adds roughly 15 to 25 kilograms per square metre of sustained load once you count the panels and the mounting frame, and that is before wind uplift or anyone walking the roof to clean it. A flat reinforced-concrete roof usually has the capacity. An older roof may not.

Landed homes built before the late 1990s were commonly designed with roofs that carried roughly the weight of the roofing material and little else. From the street, and from a satellite, a roof like that looks identical to one that could carry an array. The only way to know is to ask when the house was built, what has been done to it since, and then have someone look at the structure.

Extensions complicate this. Where a house has been extended, the main roof and the extension roof are often two different structures with two different answers, even though they look like one continuous roof from the ground. If your outline on the calculator crosses from one to the other, treat it as two roofs.

2. Where the panels want to go versus where the structure can carry them

The calculator places panels where the sun is. The structure does not care where the sun is. Wind uplift acts upward and concentrates at roof edges and corners, which are exactly the places an installer is tempted to fill. A ballasted frame on a flat roof can be considerably heavier than a fixed one, so "it is a flat roof" is not the end of the question.

The rear extension or car porch is the classic trap. It is usually the sunniest and flattest part of the roof, and usually the lightest structure, because it was built as a covering rather than as a load-bearing roof. In our experience that is the roof most often refused as drawn, or needing steel upgrading before anything goes on it. If the best-looking part of your outline is a metal roof over an extension, ask about the structure before you believe the panel count.

3. What happens to the waterproofing

On a flat roof, a bolted frame means every foot is a penetration through a membrane that was continuous until that morning. Whether each one is sleeved, sealed, dressed back into the membrane and left draining is invisible once the panels cover it, and the failure shows up two monsoons later as a stain on a ceiling. On a tiled roof the equivalent is a lifted or cracked tile that only leaks in wind-driven rain, and the stain appears nowhere near the crack.

No calculator has a field for the age and remaining life of your waterproofing. It matters, because a membrane near the end of its life is much cheaper to redo before an array sits on it than after. We set the full list of these checks out in is your landed roof ready for solar.

4. Shading that has not happened yet

Google's imagery is a snapshot. It shows the neighbour's roof as it was when the image was taken. If the house next door, or behind, or across the lane is about to be rebuilt higher, the shadow it will cast is not in your number. Our own page calls shading from neighbouring rebuilds the variable that ruins payback, and says it is the part the calculator cannot see and has to be checked on site.

You can do part of this yourself. Look at the houses around you: are any older, lower houses on tight plots that look due for a rebuild, or any rebuild already under way? A rebuild next door is a question for your solar plan as much as it is a question for your boundary. Our note on what you can ask when a neighbour is building next door covers the other side of it.

5. Getting up there, and getting the power down

Two practical items sit outside the outline. First, access: scaffolding to reach the roof safely, a safe route to walk the roof once panels are on it so it can be cleaned, and whether anything already up there, such as a water tank, a solar water heater, aircon condensers or a roof terrace, takes the space or the load you assumed was free. Second, the electrical run: the cable has to get from the roof to your distribution board, and the route and the space at the board are real constraints on an existing house.

These are exactly the items we check at the free site survey. Our calculator's quick-estimate section says the final fixed price is confirmed at that survey, where we look at roof access (scaffolding), roof condition and the electrical run. That sentence is the clearest summary of what the number on the screen is not yet.

6. How your household actually uses power

The 35% self-consumption figure is an assumption about landed homes in general. Yours may be higher or lower. A household with someone home in the daytime, aircon running and a pool pump or two will use more of the output directly. A household empty from morning until evening will export more, and exported power is worth less than power you did not have to buy. The calculator cannot know which household you are, so look at how your own days run before you lean on the payback figure.

How to use the number properly

Do not throw the calculator result away. Use it for what it is good at, and write down what it assumed. Then do this before anyone comes to the roof:

• Note the age of the house and of the roof. Roughly when it was built, and whether the roof was ever replaced or extended.
• Mark which parts are original. Main roof, extension, car porch. If your outline spans more than one, split it into sections in the tool, since it lets you add another roof section.
• List what is already on the roof. Water tank, solar water heater, condensers, a terrace, anything that takes space or load.
• Take photos of the roof surface and the ceilings below. Existing stains or soft spots are information a vendor should see before they quote.
• Look at the neighbourhood. Any rebuilds under way or likely nearby that could shade you.
• Note where the distribution board is. The run from roof to board is part of the job.

Then ask whoever quotes you five things in writing: who has assessed whether the roof can carry the array; whether the mounting is ballasted or fixed and how many penetrations that makes; how each penetration is waterproofed and who is responsible if it leaks; whether any reinforcement is inside their price or someone else's separate job; and whether the layout leaves a safe route to walk the roof. A vendor with a good answer to all five has looked at the roof. A vendor who answers with the panel count has looked at the calculator.

If the roof cannot take it as drawn

This is not usually a flat refusal. There are three routes. You can reinforce the structure where the panels want to go, which is often straightforward on a rear extension. You can move the array to the part of the roof that can carry it, accepting fewer panels or a poorer orientation. Or you can wait and do it as part of building works you were planning anyway, which is much the cheapest: the structure can be designed for the load from the start, the cable can run inside the building rather than down an external wall, and space can be left at the distribution board.

If the roof cannot take panels at all, there is another structural answer: solar carried on a frame instead of the roof. The steel, and why wind uplift rather than panel weight sizes it, is in solar on a canopy instead of the roof. And if you have not yet read the vendor-side questions, what to ask a solar vendor that they will not volunteer is the companion to the list above.

What we would do in your position

Run the calculator once to see the shape of it. Take the panel count and the payback as an opening position, not a quotation. Then get the roof looked at by someone who builds, before you compare solar prices, not after. Prices compared before anyone has checked the roof are prices for an array that may not be the one that gets built.

Wong Lye Pte Ltd holds a BCA General Builder Class 2 (GB2) licence. We look at the structural side of a roof as a builder, which is the part that pure solar vendors tend to miss. Whether your installation needs a structural submission depends on what is being fixed, to what, and how, and that is a question for the Qualified Person on your project, so raise it early.

Already have a number from a calculator and want the roof behind it checked? WhatsApp us on 9107 2601, or try the solar calculator first and bring us the result.

Related reading: is your landed roof ready for solar · solar on a canopy · questions for your solar vendor