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Founder & Renewable Energy Author, kilowatts.uk
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Unused Roof Space
Unused suitable roof space can represent decades of forgone generation, so system value should be considered over its lifetime.
Unused Roof Space
Unused suitable roof space can represent decades of forgone generation, so system value should be considered over its lifetime.
Unused Roof Space
Unused Roof Space
Unused suitable roof space can mean decades of generation you never get. That is the answer the film is pushing at you. If a face of the roof would have worked and you leave it empty, the loss is not one bright afternoon. It is every year that face could have been making electricity, added up for as long as a system would have sat there.
The surprise is how ordinary it looks
You walk under your own roof every day. It does not announce itself. A spare slope, the flat bit over the extension, a face you stopped seeing because the bins live beneath it. Unused roof space can be a surprise, and not because anyone hid it. You filed it as spare without quite deciding that.
Suitable is the word that does the work. Not every tile is a candidate. Shade, structure, access and what that part of the roof is for can all rule a face out. The film is not telling you to cover every square metre for the look of it. It is asking you to notice the space that would have generated, and that you have left alone.
Once you see it, the question changes. It stops being whether you have room. It becomes what it costs, in energy, to keep treating that room as nothing.
Count the repetition, not a good week
A roof does not generate in a highlight reel. On screen the day speeds up: the sun moving, cloud passing, the seasons turning, a counter climbing. Output on a suitable face is ordinary and repetitive. Some hours are poor. Some days are poor. The years still add up, because the poor hours are not the whole story and the good ones come back.
That is why one hot afternoon is a bad way to judge empty space. You remember the day the garden was unbearable and forget the long run of usable daylight either side of it. Over the life of a system, those ordinary days are the generation. Leave the space empty and you do not lose a demonstration. You lose the repetition.
Unused suitable roof space can represent decades of forgone generation. Forgone means it was available and not taken. The roof was already there. The daylight was going to arrive anyway. A system that was never fitted cannot hand you that electricity later.
What you lose if you leave it
Sit with the film’s first question on your own house. What is the potential energy lost if you do not use this space?
You lose the electricity that face would have made, year after year, for as long as a system on it would have run. You do not recover it by being careful with the lights. The energy that was never generated is simply absent. If the rest of the roof is already working, this is the extra the unused face would have added. If nothing is up there yet, it is the whole of that missing run.
This is not a fine, and it is not a figure I can invent for you. It is generation that does not exist because the space stayed empty. It shows up as electricity you still have to buy, or as electricity you never had spare. Which of those matters more depends on the house. The film does not need the neighbour’s roof to make the point. It needs you to treat a suitable empty face as a stream of energy currently set to zero. An empty roof looks like nothing happening, and nothing happening feels free of consequence. It is not neutral. Neutral would be a face that could not generate. A suitable face left unused is a decision, even if you never sat down and made it.
What you gain if you use it
Turn the same roof the other way. What is the potential energy gained if you do use this space?
You gain what that face can actually produce across a messy British year, not on a brochure day. Cloud will move. Output will dip and recover. In the film the generation keeps ticking through that movement rather than waiting for a perfect hour. Imperfect days still count, and they count again next year.
Gain is not “a bigger system because bigger looks serious.” It is the energy that particular unused space can add, on top of what you already have, or instead of having nothing. A modest suitable face can still be worth taking seriously if the alternative is decades of zero from that face. A large face you cannot use well is a different conversation. Suitability comes first. The gain is whatever that suitable area would have produced over the life of the system. Use the space and the count starts from the day the system is working, not from the day you first noticed the roof. Years you have already left empty are gone. The gain is what is still ahead of you.
How many years are still available
The third question is the one that stops this being a conversation about a single install day. How many years of generation can you get from this unused roof space?
A short film cannot put a number on your house. How long the equipment lasts, what the roof will bear, and how long you will actually be there are all yours to establish properly. What the film wants is the habit of counting in years. A system is a run of generation. Unused space is the same run, currently missing.
If you will only live with the decision briefly, the forfeited years are fewer. If the roof will be yours for a long stretch, empty suitable space is a long stretch of electricity you arranged not to have. That is the surprise, once you sit with it. The space does not look like decades. It looks like tiles. The decades are in the repetition the film speeds up so you can see it. The film’s phrase “free energy” is a provocation, not a price. Daylight on a roof you already own does not invoice you. The kit that captures it does cost money, and nothing here is a quote. The useful thought is narrower. Once a suitable system is in, generation from that previously unused face keeps arriving for years. Every year you leave the face empty is a year it contributes nothing.
Look before you shrug
You do not need a system down the street, or a guess from the pavement, to start. Walk the house. Which faces get sun, and which are only empty out of habit? Which bits are genuinely unsuitable, and which have you filed under “not now” for so long that “not now” became the design?
Then hold the three questions without rushing to a product. Energy lost if this space stays unused. Energy gained if it is used. How many years of that gain are still available to you. If you cannot answer them even roughly, you are not ready to shrug the roof off, and you are not ready to cover it either.
The film is a design lesson, not a push. Unused suitable roof space surprises people because the loss is quiet and long. Notice the space that would actually work, then judge it over the years it could keep generating — not over how the house looks from the road, and not over a single good week.
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FAQ
Need Help? RoboMo's Got Answers
Why does unused suitable roof space matter?
Suitable roof that is left unused can mean decades of daylight generation you never capture. The useful question is not how the house looks from the street, but what that space could produce over the life of a system, set against how and when the home actually uses electricity. Value should be judged over the system’s lifetime, not from a single sunny afternoon.
Can two identical houses need different solar and storage designs?
Yes. Two homes can look the same and still use electricity at different times of day. A house that uses most of its power while the sun is up can consume more generation directly. A house that is quiet by day and busy in the evening needs a different mix of panels, storage or timed charging, even if the roof and the installer quote look similar.
How can I tell which pattern my home has?
Do not guess from kerb appeal or from a neighbour’s system. In Britain, half-hourly smart meter data is the honest starting point: when the house draws power, and how that lines up with daylight. Pull a year of import data if you can, and mark when cooking, heating, laundry and any EV charging actually happen.
Does an evening-heavy house need a battery more than a daytime-heavy house?
Usually yes, because it cannot use as much generation while the panels are producing. Solar makes electricity in daylight. If demand spikes after sunset, surplus midday energy is either stored, exported, or effectively bought back later. A battery is a way to move daytime generation to the hours that particular house uses power, not a fashion extra.
Does adding an electric vehicle mean the roof has to get bigger?
Not necessarily. An EV often changes storage, or when you charge, more than it changes the roof. An evening charge is the same rule as any other after-dark load: the array may stay the same size while the battery, or a decision to charge off-peak, is what changes. Design should follow measured demand, not roof size alone.
What should I check before anyone sizes a system?
Look at a year of half-hourly import data rather than one bright day. Note whether cooking, heating, laundry and EV charging sit in daylight or after dark. Ask what happens to surplus midday generation — used in the house, stored, or exported — and what the house still buys after sunset. Treat solar, battery and timed charging as one design, not three separate products.
Is there a fixed number of years of free energy from unused roof space?
No. The film poses that as a design question, not as a guaranteed figure. How much generation you actually keep depends on roof suitability, shading, system size, and whether your demand lines up with daylight. Capital cost, export arrangements and future tariffs are separate from generation itself, so nobody can honestly quote a universal number of free years from the look of a roof.
Should a survey start with the roof or with how we use electricity?
It should start with the load profile. Roof space still matters — unused suitable area is potential generation — but the right mix of solar and storage depends on when the house draws power. A useful survey looks at your half-hourly data first, then designs around that demand rather than around how the house looks from the street.
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