A solar-powered rooflight is an opening roof window or rooflight that runs on a small photovoltaic cell in or on its frame, charging a rechargeable battery that drives the motor. There is no mains cable to the window. You open and close it with a wall switch or a remote, and the sun keeps the battery topped up. It works well where a power route would be awkward, and less well where the roof gets almost no daylight.
If you have seen a solar skylight advertised and wondered whether a solar powered rooflight would suit your own roof, it comes down to the slope, the shade and the ceiling below. The idea is simple, but the choice is not always obvious. This guide covers how the cell and battery work, where solar suits a roof, where mains is the better answer, and how the decision affects the finished ceiling in the room below.
How a solar rooflight works
A solar rooflight has three working parts: a cell, a battery and a motor. The cell is a small photovoltaic panel, usually fitted into the outer frame or fixed above the glass where it can see the sky. It turns daylight, not only direct sun, into a low electric current. That current charges a battery housed in the frame, and the battery runs the motor that opens and closes the sash.
A control unit sits between them. It manages the charge, protects the battery from over-charging and tells the motor what to do when you press a button. On many systems it also looks after the rain sensor, so a solar window can still shut itself in a shower.
The motor draws very little power in use, because it only runs for the few seconds it takes to move the sash. That is the reason a cell the size of a playing card can run a window for weeks. The battery spends most of its life topping up, not working.
Does it work on a grey day?
Yes, within limits. A photovoltaic cell responds to daylight, not only to bright sun, so it still charges under cloud, though more slowly. A Devon winter, with short days and low cloud over the Culm and the moor, charges a cell more slowly than a July afternoon, but opening and closing a window uses so little that the battery usually holds.
Where it struggles is not weather but position. A rooflight on a north-facing pitch, in the shadow of a taller building, under a large tree or at the foot of a dormer receives much less light than one on an open south or west slope. The cell sees less, the battery charges less, and in a poor spot it may not keep up if the window is cycled several times a day.
That is why we look at the roof before we recommend solar. Where the cell has a clear, open view of the sky for much of the day, it will do its job for years. Where it does not, a mains unit is more reliable and a solar one is a risk.
Solar or mains: which suits your roof
The choice often comes down to the route a cable would have to take.
| Point | Solar | Mains |
|---|---|---|
| Cable to the window | None | A cable from a spur or consumer unit |
| Best suited to | Finished ceilings, retrofits, awkward routes | New builds, extensions with open ceilings, lofts being boarded |
| Power source | Daylight and a battery | The house supply |
| Weak point | A shaded roof slope, an ageing battery | A power cut, unless there is a battery back-up |
| Disturbance to the ceiling | Minimal | A cable run, sometimes a chase |
| Rain sensor and remote | Usually available | Usually available |
Neither is better in the abstract. A new roof with the ceiling still open is the perfect moment for mains, because a cable costs nearly nothing to lay while the joists are exposed. A finished room, in a house where you do not want plaster chased out, is where solar is the better answer.
For a wider view of that decision, our guide to electric or manual opening rooflights sets powered windows against pole-operated ones, and the question of wiring is covered in whether electric rooflights need an electrician.
The ceiling a cable would have crossed
In our work, the strongest case for solar is not the electricity. It is the ceiling.
Take a first-floor landing in a Georgian terrace in Southernhay or St Leonards, with an original lath and plaster ceiling in good order. Running a mains cable to a new rooflight means getting a flex from the nearest supply point to the opening. That could be through the joists, down a wall or across the ceiling, and each route risks the plaster. In lime over lath, cutting a channel can crack a large area around it, and making good takes longer than the rooflight itself.
The same applies to a 1930s semi in Heavitree with a skimmed ceiling you have just redecorated, or a thatched cottage where the roof structure makes cable routes difficult. A solar unit leaves all of that alone. The cable question disappears, and the finish around the opening is limited to the lining we form for the window.
That fits how we work. The opening is cut and framed, the lining is formed and finished to meet your ceiling, and nothing else disturbs the plaster. It is also why we note the power choice in the finish schedule that comes with the quote, alongside the lining and the finish. More on how the inside is formed is in how a rooflight is finished inside.
The battery: life, replacement and access
The battery is the one component that wears out, and it should be thought about from the start.
A rechargeable battery loses capacity over the years, whatever the cell is doing. Batteries on solar rooflights are designed for long service, and makers specify replaceable packs, but you cannot assume it will last as long as the window. The question to ask is whether it can be replaced without taking the window apart, and from where.
On a roof window, the battery is normally in the frame or the control housing on the room side, within reach from a stepladder. That is the tidiest arrangement, because a replacement is a quick job done from inside. A battery on the outside of the roof is a different matter, because it needs safe access.
We make a point of positioning solar rooflights where the battery can be reached, and of telling you where it is. If your rooflight sits over a stairwell or a double-height space, ask at survey how the battery will be reached, because the answer affects whether solar is sensible there.
Rain sensors, remotes and controls
A solar window is not a lesser one. Most systems offer the same conveniences as the mains versions.
- Rain sensor
- The window closes itself when rain wets the plate, using the battery for the movement. Our guide to rain sensors and automatic closing on rooflights explains how that works.
- Wall switch or remote
- You open and close the window from a wall-mounted control or a handheld remote. A wireless control means no cable to the switch either, which suits a finished room.
- Several windows together
- A run of solar rooflights can be grouped, so one command opens or closes them all. Each unit carries its own cell and battery, so the group does not depend on one point of power.
- Smart control
- Some systems link to a home hub. Our answer on linking electric rooflights to a smart home system covers what is possible and what to check first.
The VELUX INTEGRA range includes solar roof windows that are rain-sensing and controlled by wall switch or remote, which is a useful benchmark when you compare other makes.
Power cuts, and the days the sun hides
A solar rooflight has an advantage in a power cut, because the battery and cell do not depend on the house supply. If the lights go out, the window carries on, within the limits of its charge. A mains unit with no back-up will not move until power returns or you use the manual release.
It also has a limit. After a long run of dark weather, or if a cell is covered by leaves, moss, snow or a layer of dirt, the battery can run low. Most systems warn you, with a flashing light or a change in behaviour, and many keep a reserve to close the window when the battery runs down.
A solar window should still be openable by hand. Most carry a release, a handle or a pole that works with no power at all. Our answer on what happens to an electric rooflight in a power cut goes through the options.
Keep the cell clear. A wipe with a damp cloth from a safe position once or twice a year, and a check after autumn leaf fall, keeps it working as intended. Do not clean from the roof unless you can do it safely, and do not use abrasives on the cell surface.
Solar lanterns, flat rooflights and larger openings
Solar power is most common on roof windows, but it is not limited to them.
A flat rooflight on a kerb, or a lantern with an opening vent, can run on a solar or battery system where the cell is mounted on the roof in an open, unshaded position. This suits an extension over a kitchen in Topsham or Exmouth, where the roof is open to the sky and the ceiling below has just been finished.
The size of the opening matters. A larger or heavier sash needs a stronger motor and draws more power, so the cell and battery have to be matched to the job. A big vent on a lantern that opens many times a day will stretch a small solar cell further than a modest roof window opened a few times a week.
How far the sash opens also affects the energy it uses, and that is set by the window's design. Our answer on how far opening rooflights open covers the usual ranges. Where the maker offers a choice of solar or mains for a bigger unit, we go through the load with you at survey rather than assuming.
Solar power and heat in the room
People sometimes confuse a solar powered window with solar gain, and the two are quite separate. The cell only runs the motor. It does nothing to warm or cool the room, and it does not change the glass.
What a powered window can do is help you control the heat. A rooflight that opens at the push of a button, and closes itself when it rains, is far more likely to be used for purge ventilation on a warm afternoon than one that needs a pole and a stretch. Warm air collects at the highest point of a room, and letting it out through the roof is among the most effective ways of cooling a loft bedroom or a kitchen extension. Our guide to opening rooflights for ventilation and cooling covers how that works.
The glass is a separate decision. If the room overheats in summer, solar control glazing or a blind will do more than any opening vent, and the glass choice is set at specification stage. A solar cell mounted on the outside of the frame also does not shade the glass in any useful way, because it is small by design.
It is worth keeping the two ideas apart when you compare quotes, so that a window sold for its solar cell is judged on what it does, which is run the motor without a cable.
Getting it right from survey to finish
At survey we check the things that decide whether solar suits your roof: which way the pitch faces, what shades it through the day, how often you will use the window, and how the battery will be reached. We also look at the ceiling underneath and how any alternative power route would cross it.
Solar does not take the work out of the job. A new opening rooflight is building work: the installation has to meet current Building Regulations, the roof structure is trimmed around the opening, and where a notification is needed, we send it to Building Control. Solar rooflights are not exempt just because they have no mains supply.
Written into your quote, with the finish schedule, are the product, the control, the position of the battery and the lining and finish of the opening. The work carries a 10-year workmanship guarantee, while the cell, battery and motor are covered by the maker's own warranty. If you want a first look at the options for your house, see our electric and opening rooflight installation page, or ask for a quote and tell us the room and the roof.