Can an Electric Rooflight Run on Solar?

People ask this in two ways, and the answers differ. The first is a rooflight that has its own solar cell and needs no connection to the house. The second is an ordinary mains rooflight in a home that has solar panels on the roof.

The second case is simple. The motor is on the household circuit, so it takes power from whatever the house supplies, whether that comes from the grid or from panels. The rooflight itself does not know the difference.

The first case has the design decisions in it, and the sections that follow work through it.

A small photovoltaic cell sits on the frame or the glazing bar, where it can see the sky. It charges a rechargeable battery during the day. When you press the remote or the wall switch, the battery drives the actuator that pushes the sash open or pulls it shut.

A rain sensor, where fitted, shares the same battery. It closes the unit when it detects water, which means the battery needs enough charge to move the sash even after a run of dull days. The maker's instructions describe how the unit behaves when the charge runs low, so read them alongside the data sheet. The guide to rain sensors explains how the sensing side works.

• The cell needs daylight, not direct sun, to keep the battery topped up.
• The battery lasts through short spells of low light and charges again afterwards.
• The control is a handheld remote or a wall unit that talks to the rooflight by radio.

Fixings and wiring inside the unit are factory work. On site the frame and flashing go in as for any opening rooflight. The remote is then paired with the unit, and the sash is run through its full travel, open and shut, before the job is handed over.

The saving is a cable. A mains rooflight needs a supply cable run to the actuator, and in a finished house that can mean lifting floorboards, chasing plaster or running trunking. A solar model needs no cable, so it can go into a roof with a finished ceiling and no wall space to hide a run.

Remote control also solves a reach problem. A rooflight set high over a stair or a double-height space cannot be opened with a pole, and a handheld remote or wall switch puts the control where you stand. That works the same whichever power route is used.

That suits a bungalow with a sealed loft, a barn conversion with exposed beams, or a ceiling that has just been decorated. With no mains connection to make, there may be no electrician needed at all, which the answer on electricians looks at in more detail.

Check the stroke too. The distance a sash travels depends on the actuator, and the answer on how far a rooflight opens explains what the figure means for airflow.

The cell has to see daylight. A rooflight on a slope that is shaded for most of the day by a taller neighbour, a chimney stack or a tree gets less charge. The survey notes the slope direction and what stands near it, so a shaded roof is spotted before the unit is ordered.

Large or heavy units can need more power than a small battery gives. A big sash, or several units driven together from one control, may be better on mains. A lantern with a bank of vents is one case; the roof lantern installation page covers those.

Cold and cloud do not stop a cell charging, but a unit that is buried in snow or leaf litter stays dark until it is cleared. A slope with an overhanging tree sheds leaves onto the glass, and those need brushing off. The survey sheet records what stands near each slope, so you know before ordering.

If your house already has solar panels, the roof area they use matters when a rooflight is planned. Panels and rooflights both need unshaded slope. We measure the slope before anything is ordered, which shows where a window can sit without sharing space the panels need.

The rooflight's own power draw is tiny. A motor that runs for a few seconds, a few times a day, uses almost nothing from a household supply, so it will not change an electricity bill in a way you could see.

A mains rooflight in a house with panels is still wired as a normal circuit. In some systems the motor and controls run at low voltage from a transformer, while others take a mains supply at the unit itself. The data sheet says which, and the electrical side is covered in the answer linked above.

The electric and opening rooflight installation page covers the choices between models and controls.

If panels are planned for later, say so at the survey. Knowing where they will go lets us place the rooflight on the other side of the slope.