An insulated box with a glazed lid.

A dark absorber plate inside it.

With fluid channels bonded to the plate.

That is essentially all of it.

Which is why it is cheap and robust.

Because roof area is often not the limit.

Two cheap collectors can beat one dear one.

For the same money and the same output.

Efficiency per panel matters less.

When you can simply fit more panels.

Generally yes, and noticeably.

One sealed panel with toughened glass.

Very little to go wrong.

Good under hail and wind loading.

And it looks like a rooflight.

From the absorber to the glass.

The plate sits in air inside the box.

So heat conducts and convects away.

And the loss grows with temperature difference.

Between the plate and outside.

On a cold bright day.

Wanting hot water rather than warm.

The difference is largest exactly then.

So it gives up much of what it gathers.

Which is when tubes do better.

Warm ones, and summer-weighted demand.

With modest target temperatures.

It performs very well there.

And costs far less to install.

Cold year-round demand favours tubes.

On-roof or in-roof.

On-roof sits above the covering.

In-roof replaces it, like a rooflight.

Which looks better and weathers in.

But is a roofing job as well.