Flat roofs and low tilt panels

What do you lose with flat mounting?

A flat panel receives the same sunlight as the ground beneath it. At a tilt of 0°, the module's power and the power on a horizontal surface are equal, since the module is lying flat.¹ It isn't turned toward the sun at all.

Across a full year, a fixed panel gives its maximum when tilted to match latitude.¹ A flat panel isn't at that angle. How big the gap is depends on where you live, and these notes don't have a figure for it.

There is a small silver lining. Low tilt angles use a greater fraction of the light in summer.¹ If most of your power is used in the warmer months, flat or nearly flat isn't a silly choice. Winter is the season to worry about.

What does dirt do to a panel at a low angle?

Dirt, dust and other grime on the glass is called soiling. It reduces the light that strikes the cells, and so it cuts power.² Losses add up over time without cleaning.²

The best-known comparison comes from a study in a snow-rich part of Canada. Cleaning flat-mounted panels after 15 months raised their output by almost 100%, yet arrays tilted at just 5° were adequately cleaned by rainwater.²

That's snow country, so don't treat the number as your own. The lesson is the pattern: flat lets grime sit, and even a small tilt gives rain something to work with.

Will rain keep flat panels clean?

Sometimes, and not on its own. In the study above, a small tilt was enough for rain to do the job. A fully flat panel was the one that needed cleaning.²

Where you live matters too. In many cases, especially in arid regions, or near deserts, roads, industry or farming, regular cleaning of the panels is cost-effective.² Soiling losses vary a lot from region to region.²

So watch your own glass. If you see dust or droppings building up, that's your sign. Cleaning means reaching the panels, and roofs are dangerous places. Don't climb on a roof without proper access gear, and don't clean from a position where you could fall.

Does heat matter on a flat roof?

Yes, a bit. Module output degrades faster at higher temperatures, and letting air flow over and, if possible, behind the modules reduces the problem.²

A panel laid hard against a flat roof leaves no gap behind it for air. Where you can, mount it a little proud of the surface so air can move underneath.

Can I add a tilt on a flat roof?

Yes, with a frame that holds the panels at an angle. The rules for picking a tilt apply, and in Australia the panels should face north.

There's a catch. A tilted frame is more exposed to wind than a panel lying flush. Unless the panel is fixed flush, the mount needs careful design for wind loading, because it's more exposed.³

Follow the frame maker's instructions for your model, and ask an installer or engineer to confirm the roof and fixings suit your wind conditions. Roof fixings can also affect waterproofing, so ask a roofer if you're unsure.

Is the wiring part of this?

It is, sooner or later. Australian rules reserve mains wiring, switchboard and grid work for licensed electricians, so none of it is a DIY job. Before anyone connects or disconnects a battery, shunt, fuse or inverter lead, isolate the battery and read the maker's manual. If that's beyond you, book a licensed electrician.