Do Skylights Reduce Lighting Bills?

A rooflight does one simple thing for a lighting bill: it lets daylight reach a work surface, a landing or a bed without a bulb switched on to help it. On a kitchen worktop, a desk or a bathroom mirror, that daylight arrives from directly overhead, which is a stronger, more even source than a side window can usually give a deep room. Where a room previously ran a pendant or a row of downlights through a grey morning, a well-placed rooflight can take that load off the switch for the hours the sky is bright enough.

The saving sits in daytime hours only. Once the sun goes down, a rooflight does nothing for the electricity bill, and in Peterborough's shorter winter days that window narrows a great deal. The gain is real but seasonal: strongest from spring to autumn, and smallest in December and January, when a loft bedroom or landing still needs its lamp on by mid-afternoon.

Two rooflights of the same size can light a room very differently, because the glass itself governs how much daylight reaches the room below.

• Light transmittance. This is the share of visible daylight the glass lets through, and it is the single biggest factor in how bright a room feels under a rooflight. Our guide on light transmittance on skylight glass sets out what the figure means and why two panes that look identical can behave very differently.
• Size and position. A rooflight over the exact spot you read, cook or work at does more for the lighting bill than a larger one placed off to the side. Balancing daylight and heat loss when sizing covers how we size a unit for the daylight a room needs without opening up more heat loss than it is worth.
• Roof pitch and orientation. A steep pitch and a south-facing slope both change how much sky the glass actually sees through the day, and so how many hours it does the lamp's job for you.
• Number and layout of units. One large rooflight and two smaller ones spread along a landing can deliver the same area of glass but very different daylight, because daylight falls off quickly with distance from the opening.

None of this is a promise of a set saving. It is the set of things that decide whether the daylight gain is large or modest in your particular room, and it is why we look at the room itself before we size anything.

Cutting a lighting bill and cutting a heating bill are not always the same job, and the glass that helps one can work against the other. A solar control coating is designed to hold back summer heat gain, and because it works by reflecting part of the solar spectrum, it can also take a small amount of visible light with it. Our page on solar control glass for skylights explains how these coatings work and how they colour the sky you see through the pane.

The practical question worth asking is whether that coating leaves the room looking dim. Will solar control glass dim the room below? answers that directly, and do heat-reflective coatings affect the view through the glass? covers the tint and reflection some coatings add. A related figure, the g-value, describes how much of the sun's heat gets through a pane rather than how bright it looks; our answer on what the g-value of a rooflight is explains why a low g-value and a high light transmittance are not the same target, and why we specify each on its own merits rather than assuming one follows the other.

A rooflight that lets in plenty of daylight can also let in glare, and glare is often what sends a hand back to the light switch, not a lack of brightness. A low winter sun straight overhead, or a bright patch of sky reflecting off a screen or a worktop, can be uncomfortable enough that a blind goes down and a lamp goes on regardless of how much daylight is available. Placing the unit off the exact line of a desk or a hob, and choosing a blind that filters rather than blacks the room out, keeps the daylight working for you instead of against you. Our page on whether blinds make a rooflight warmer covers the same fitting from the heat side, which is worth reading alongside this if you are weighing up a blind for glare control.

This is also where a sightline sketch earns its keep. Every quote we give comes with one, a drawing of the view from the bed, the sofa, the desk or the sink the room is actually used from, showing where the frame sits and how much sky you see from that position. It shows the same daylight path that decides whether a lamp gets switched off at nine in the morning or stays on until lunch, so it is as useful for a lighting question as it is for the view. See the sightline sketch: how we plan the view for how that drawing is put together.

A rooflight sits inside a bigger set of choices about glazing, frame and fitting, all covered on our energy efficient rooflights page. Two panes against three changes both the daylight and the heat loss, which our guide on glazing a rooflight for warmth and clarity sets out plainly. What affects the running figures for a job overall, glazing included, is covered on what affects the cost of a rooflight. None of these pages will hand you a single percentage for a lighting saving, because that figure depends on your room, your roof and how you use the space, but together they show what is actually being traded off when a rooflight is specified.

Does a bigger rooflight always mean less need for a lamp?

Not on its own. A larger unit lets in more daylight only if the glass has good light transmittance and the opening sits over the part of the room you use. A large rooflight with a heavily tinted or coated pane can let in less usable daylight than a smaller one in clear glass placed directly over a desk or worktop.

Do rooflights help on an overcast day?

Yes, though less than under a clear sky. Daylight from an overcast sky is more diffuse and lower in level, so a rooflight still lifts the ambient light in a room, just not by as much as on a bright day. Position and glass choice both still matter on a grey morning, if anything more than on a sunny one.

Which rooms see the biggest change to their lighting use?

Rooms that are used during daylight hours and previously relied on artificial light because they had no window, or only a small one, tend to see the clearest change. A landing with no other natural light, an internal bathroom, or a kitchen extension with a deep floor plan are typical examples, because the daytime lamp use they are replacing was significant to begin with.

Does the direction a roof faces change how much lighting a rooflight saves?

It changes the pattern more than the total. A south-facing slope brings strong, direct light for a shorter, brighter part of the day, while a north-facing slope brings a steadier, more even light for longer. Either can cut daytime lamp use; which one suits a given room depends on what the room is used for and when.