Solar powered rooflights without a mains run
A solar powered rooflight carries its own photovoltaic panel, its own rechargeable battery and its own control electronics, all built into the sash and frame. Nothing connects it to the house. It arrives as a sealed unit, it is fitted and flashed exactly like a manual roof window, and it opens on a remote from the day the scaffold comes down. For a great many Gloucestershire houses that is the difference between having an opening rooflight and not having one, because chasing a cable through a finished Cotswold stone slate roof or a plastered vaulted ceiling is a substantial piece of work in its own right. This page covers what is inside a solar unit, what it will and will not do, and where it is the right specification rather than a fallback. The full range of powered options is on the electric rooflight installation page.
What is inside a solar unit
Four components, all factory fitted. A photovoltaic panel, usually a strip 300 to 400mm long set into the top flashing above the glass or into the head of the sash. A rechargeable battery, on current ranges a lithium iron phosphate or nickel metal hydride pack of modest capacity. A charge controller that manages the panel output into the battery. And the same chain actuator, control board and radio receiver that a mains unit uses.
The actuator is not a smaller or weaker motor. It is the same mechanism doing the same job at the same 24V or 12V, drawing power from a battery rather than a transformer. Everything on the actuators page applies unchanged. The difference is entirely in where the electricity comes from.
How much energy an opening cycle actually uses
This is the number that makes solar viable, and it is smaller than people assume. An actuator drawing around one amp at 24V for twenty seconds uses roughly 480 joules, which is about 0.13 watt hours. Opening and closing is double that.
Ten full cycles a day therefore comes to under three watt hours. A panel of a few hundred square centimetres, even at British latitudes under overcast cloud, produces enough to cover that many times over across a day. The energy budget is comfortable; what has to be managed is the mismatch between when the sun shines and when the window is used, which is what the battery is for.
The battery, and what carries the unit through winter
Battery capacity on a typical solar roof window is in the region of 10 to 20 watt hours, which is a few days of ordinary use with no charging at all. That reserve is sized for a run of dark, still, overcast December days, which Gloucestershire supplies reliably.
The controller manages the reserve rather than spending it freely. As the state of charge drops it reduces sensor polling, cuts the rain sensor heater duty and, at a low threshold, restricts operation to closing only. That last behaviour is deliberate: the last of the charge is kept for the one movement that matters, which is shutting the window. In practice a unit that gets any daylight at all does not reach that state.
Panel orientation, and the roof slope it is fitted to
The panel is fixed in the plane of the roof, so its orientation is decided by the slope the window goes into and cannot be adjusted. A south facing 40 degree slope in the Cotswolds is close to ideal. A north facing slope in a Stroud valley combe, overshadowed by the hillside behind, is at the other end of the range.
The panel still works on a north slope, because a large part of British daylight is diffuse rather than direct, and a photovoltaic cell under uniform overcast still produces useful current. But the margin is much smaller, and on a north facing unit under a tree canopy or a taller neighbouring building it is worth being honest at survey rather than optimistic. Where the orientation is genuinely poor and the window will be used heavily, a mains unit with a cable run is the better answer.
Shading, and the things that cast it
Photovoltaic output falls off sharply under partial shade, more sharply than the shaded fraction alone would suggest, because the cells are wired in series and the weakest cell limits the string.
The recurring culprits in this county are a mature tree to the south, a chimney stack on the same slope, an adjoining building on a terrace where the neighbour’s roof is higher, and a dormer cheek. A survey looks at all four, and it looks at them with the season in mind: a deciduous tree that casts nothing in February will cast a great deal in July, and the low winter sun that puts a chimney shadow across a panel at eleven in the morning does not do so in June. Leaf litter and moss growth over the panel face do the same thing more gradually.
Solar against mains, side by side
| Solar unit | Mains unit | |
|---|---|---|
| Cable run required | None | Low voltage run to a transformer |
| Best moment to decide | Any time, including after completion | Before the ceiling is boarded |
| Disruption to a finished room | Roof work only | Ceiling opened or a surface run |
| Cycles per day, comfortably | Around 10 to 15 | Unlimited |
| Performance on a shaded north slope | Reduced, assess at survey | Unaffected |
| Electric blind on the same unit | Usually a separate solar blind | Shares the control unit |
| Long term consumable | Battery pack, replaceable | None |
Where solar is genuinely the better specification
Solar is often treated as the option you take when the cable was forgotten. That undersells it. There are situations where it is the correct first choice on the merits.
- A finished Cotswold stone slate roof, where getting a cable to the head of the opening means taking courses back and re-laying them.
- A listed building in central Cheltenham, where reducing the extent of intervention is the point.
- An outbuilding, garage conversion or garden room with limited or no electrical supply of its own.
- A single rooflight in an otherwise finished plastered vaulted ceiling with no accessible void.
- A house where the owner wants one opening unit now and does not want the extension programme opened back up for it.
Where a mains run is worth the trouble instead
The other side of the same assessment. A heavily shaded north slope with no useful daylight. A very large made to size flat roof unit whose actuator draws more than a small panel will support. A bank of several openings that will be operated together many times a day. And any job where the ceiling is open anyway, because at that moment a cable costs almost nothing, and the wiring is described on the first fix wiring page.
The honest position is that on an open ceiling, mains is usually the better long term choice. On a finished one, solar usually is. The building decides more than the preference does.
Charging behaviour through a Gloucestershire year
Useful solar irradiance in this part of England runs from roughly 30 watts per square metre averaged across a December day to well over 200 in June. The panel sees that whole range.
In practice a well oriented unit charges to full within a few hours on any bright day from March to October and coasts through the summer with the battery permanently topped up. November to February is where the reserve does its work, and the pattern is that the battery slowly falls through a dull week and recovers fully in one clear day. River fog in the vale, which can sit until midday through November, delays but does not prevent that recovery. A unit that has been shut all winter and barely used comes into spring with plenty in hand.
Installing a solar unit on a stone slate roof
The fitting is a roof window installation and it follows the same sequence as any other: courses taken back, the opening formed, a trimmer where the rafters are cut, the unit set to the correct height above the covering, the flashing kit dressed in, and the slates re-laid to close the perimeter.
What changes on Cotswold stone slate is the flashing. The slates are thick, laid in diminishing courses, and bedded with a mortar torching, so a standard flashing kit for plain tile does not sit right. The recessed or conservation profile of unit sits lower in the slope and reads correctly on a Painswick or Northleach roof, and there are solar operated versions in those ranges. That work is set out on the conservation rooflight installation page.
The rain sensor on a solar unit
Solar roof windows carry the same conductivity rain sensor as mains units and close on rain in the same way. The difference is the heater behind the sensing face, which runs continuously on a mains unit and is duty cycled on a solar one to protect the battery.
The practical effect is a slightly longer reset after rain on a dark day, not a slower close. On a unit that is routinely left open unattended, which is the whole point of an out of reach installation, the sensor is the component that makes it defensible. Sensor behaviour in detail is on the rain sensors page.
Solar blinds, and running two solar devices on one window
A solar blind is a separate product with its own small panel and its own battery, mounted in the head of the sash or on the frame. It pairs to the same remote and operates independently of the window.
Two panels on one unit is not a problem, because the blind’s energy demand is lower still: a blind motor moves a light roller for a few seconds. What does need thinking about is where the second panel sits and whether the first one shades it. On factory built combinations the geometry is already resolved. On a mixed retrofit it is worth checking. Blind options generally are covered under electric blinds fitted inside the rooflight.
The battery as a long term consumable
The battery is the one part of a solar unit with a finite life. Manufacturers quote service lives in the region of ten years under normal cycling, and the pack is a replaceable component rather than a sealed part of the unit.
That is a genuine consideration on a rooflight four storeys up in a Cheltenham stairwell, because replacing the pack means reaching the sash. It is a small job in itself; the access is the cost. Where a unit is going somewhere genuinely difficult to reach, that fact belongs in the specification conversation alongside everything else, and it is one of the arguments for a mains run where a mains run is achievable.
What a survey establishes before recommending solar
A solar recommendation rests on daylight, and daylight is measured rather than assumed.
- Orientation of the slope, taken as a compass bearing, and the pitch angle.
- Obstructions to the south and south west, with their height and distance, assessed for winter sun angles.
- Tree cover, including deciduous trees in leaf, and any overhanging branch that will drop debris on the panel.
- Expected pattern of use: a bathroom purge vent twice a day is a different load from a stairwell unit cycling all summer.
- Whether an accessible cable route exists at all, and what it would cost in disruption.
- How the unit will be reached in ten years for a battery change.
Where the answers come back marginal, the recommendation is a mains unit if the ceiling can take a cable and a solar unit if it cannot. That is a straightforward call once the survey figures exist, and it is settled before anything is ordered rather than discovered afterwards. The broader comparison between fixed, manual and powered units sits on the fixed, manual and electric compared page.
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