Alectrona
Commercial guideA plain rule of thumb gets you a first sense of scale, but the usable area is always smaller than the roof, and the firm figure comes from an on-site survey.
OrientationThis is a plain-English guide to help you size the opportunity, not formal engineering advice. The rules of thumb here are for orientation only; we confirm the usable area and the right system size for your specific site with a measured survey.
One of the first questions a finance or facilities director asks is whether the roof is big enough to make commercial solar worth it. It is a fair question, and a rough answer is easy to give: panel area scales roughly with the kWp you install, so a larger roof carries a larger array. That gives you an order-of-magnitude read before anyone visits the site.
The honest caveat is that the gross roof area and the usable array area are rarely the same number. Rooflights, plant, walkways, parapet set-backs, shading and the way the roof faces all take a bite out of what you can actually cover. This guide gives you the rules of thumb to size the opportunity yourself, then explains what trims the figure down and why the real number needs a measured survey.
For planning purposes, a useful way to think about it is roof area per kWp installed. As an indicative rule of thumb, each kWp of modern commercial panels takes up a fairly predictable patch of roof once you allow for the spacing between rows and the access gaps a real layout needs. That is array footprint, not the bare panel size, because a working roof needs maintenance walkways and row-to-row clearance that a datasheet figure ignores.
Run that the other way and it gives you a quick first sieve. A roof with a few thousand square metres of clear deck is comfortably in commercial-array territory; a smaller unit roof may still carry a worthwhile system once the load is modelled. Treat any number you reach this way as orientation only. It tells you whether the conversation is worth having, not what your system will actually be.
Mapping the rule of thumb onto the buildings we see most often gives a feel for scale. These are indicative for orientation, not quotes, and they assume a reasonably clear roof before the deductions below:
The pattern that matters is the one underneath the numbers: usable area, not gross area, sets the ceiling, and the load underneath the roof sets the system you should actually build. That sizing question is set out across our system sizes pages, which explain why the array is sized to your daytime base load. What changes at each scale, once the capacity is settled, is covered across our system sizes pages.
The gap between gross roof and usable array is where most first estimates go wrong. The common deductions are:
Each of these is site-specific. Two roofs of identical area can carry very different arrays once you account for what is actually on them and how they sit.
Because the deductions are particular to your building, a desktop estimate from a satellite image will only ever be approximate. The firm number comes from an on-site survey. Our process starts with an in-house drone survey that builds a measured 3D model of the roof, so the design accounts for the real array area, the rooflights, the plant, the shading and any structural span limits, rather than a guess from above.
That measured roof then feeds a generation model run against your own half-hourly consumption data, which sizes the system to your load rather than to the bare roof area. Because these are systems over 50 kWp, they sit outside the domestic MCS scheme, and the assurance comes from the commercial engineering stack instead: a measured survey, a modelled design and a properly contracted, commissioned install.
That is the reason a single fixed rule of thumb can only ever be indicative. The roof area a kilowatt-peak occupies is set by the efficiency of the module you put on it, not by the kWp figure itself. A higher-efficiency, higher-wattage panel packs more rated power into the same physical sheet, so a roof laid with a current high-output module reaches a given kWp across noticeably less area than the same roof laid with an older or lower-class panel. When you scale a roof area against "roughly so many square metres per kWp", you are implicitly assuming a module efficiency, and the real answer moves as that assumption moves.
This matters most on a constrained roof where the usable area is the binding limit rather than the budget. On a tight unit roof, specifying a higher-efficiency module is often what lets the array hit a worthwhile size at all, because it lifts the kWp you can fit into the area that survives the deductions. On a large, clear shed roof the opposite can be true, where the area is not the constraint and a sensibly priced module at a slightly lower efficiency carries the system perfectly well. The module class is therefore a design choice tied to your specific roof, which is one reason we select panels per site rather than from a fixed list. The commercial panel selection guide sets out how that choice is made, and our system sizes pages cover the panel-count side of the same arithmetic.
A flat or near-flat roof, the kind found on most warehouses and distribution sheds, does not simply take panels laid flush. The modules are mounted on a frame at a tilt, and once you tilt a row it casts a shadow behind it, so the next row has to sit far enough back to clear that shadow through the working part of the day. That row-to-row gap is real roof area that carries no panel, and it is governed by the ground-coverage ratio the design adopts: the proportion of the roof actually occupied by modules once the inter-row spacing is set.
The practical consequence is that a south-facing tilted layout, which generates the most per panel, also spreads the array out and uses more roof per kWp because of those clearance gaps. An east-west layout, where modules sit back to back in two opposing slopes, packs far more capacity onto the same deck because the rows shade each other far less, at the cost of a little yield per panel. Neither is automatically right. The choice turns on whether your roof area or your load is the binding constraint, and on how the building actually uses electricity through the day. The bifacial and flat-roof layout guide goes into that trade-off, and the layout is one of the things the measured survey settles before any density figure is fixed.
On these buildings the structure is frequently the real ceiling rather than the available area. A solar array adds a sustained dead load across the roof, and it changes how the roof behaves under wind and snow. The roof has to carry that combined load with the margin the structural design code requires. Wind and snow actions are assessed under the relevant Eurocode parts (BS EN 1991, the actions on structures suite), and the steelwork or the existing frame is checked against its own design standard, with older portal frames designed to BS 5950 before the Eurocodes superseded it. None of those checks can be read off a satellite image.
Where the assessment shows spare capacity, the array can use the full usable area. Where it does not, the outcome is one of three: a lighter mounting and module specification to bring the load down, localised strengthening of the frame, or simply a smaller array that the structure carries comfortably. A ballasted system on a flat roof, which holds the frame down with weight rather than fixings, adds the most load and is the most likely to run into a capacity limit, whereas a mechanically fixed layout can be lighter. This is why the structural roof survey sits alongside the area measurement, and why a roof that looks big enough can still support a smaller system than the deductions alone would suggest. It is also a safety question as much as an engineering one, since the loaded roof has to remain sound for the people working on it, which the CDM 2015 guide covers.
Not every project takes the full usable area in one go, and there are good reasons to cover a roof in stages. If your current load only justifies part of the available area, building to the load now and leaving headroom for a later phase keeps the capital matched to the saving rather than oversizing for export that earns little. A roof nearing the end of its covering's life is another case: it is rarely sensible to lay a twenty-five-year array over a membrane with a few years left, so the recovering work and the solar phase are sequenced together. Future plans matter too, where a building expecting new electric process load, refrigeration or vehicle charging may justify holding roof area in reserve for a second phase that the larger load will support.
The grid connection can also shape the phasing. A system over 50 kWp connects through an ENA Engineering Recommendation G99 application, and the capacity the network operator (Northern Powergrid across Yorkshire and northern Lincolnshire) is willing to accept can influence how much you build at once and on what timetable. A first phase sized to an accepted connection, with a route to a later uplift, is sometimes the cleaner path than holding the whole project for a larger connection offer. We map the realistic phases against your usable area, your load and your connection position as part of the feasibility study, so the staging follows the building rather than a default to fill every square metre. The half-hourly load data behind that decision is the same input described in the half-hourly metering guide.
Our own insured pilot flies your roof and captures the real geometry and shading, so the design starts from your building instead of a satellite guess.
Booked to suit your operating hours
We model the array in bankable-grade software, size it around your daytime load, and set out generation, savings and payback across three funding routes.
Modelled, not promised
Designed and installed to BS 7671, commissioned to IEC 62446-1, connected under G99 and run under CDM 2015. Alectrona is typically the Principal Contractor.
Outside MCS, assured by the non-MCS stack
A 12-month defects period backed by an Insurance-Backed Guarantee, then ongoing operations and maintenance so the asset keeps earning for its full working life.
Kept performing, year on year
Start from your usable roof area in square metres and scale it against the patch of roof each kWp occupies to get a ballpark figure. Usable area is the key word: subtract rooflights, plant, walkways, shaded zones and any parapet set-backs first, because the gross roof is always larger than what you can actually cover.
Treat the result as orientation only. It tells you whether commercial solar is worth exploring, not what your system will be. The firm figure comes from a measured survey.
More than a first estimate usually allows, and the figure belongs to your building rather than to a percentage borrowed from another roof. Three things dominate the deduction. Rooflights and smoke vents cannot be built over and carry a clearance margin around them, so a deck laid out in regular rooflight bands loses both the openings and the strips beside them. Plant is the second: air handling units, extraction stacks, condensers, ducting and cable trays each take their own footprint, and they also take the access route that keeps them serviceable, because a unit an engineer cannot reach is a unit that eventually fails. Shading is the third and the hardest to see on a plan, since a parapet, a taller adjoining bay, a flue or a neighbouring building throws a shadow that moves through the day and across the seasons.
Shading also behaves differently from the other two. A rooflight simply removes area, whereas a shaded module can drag on the whole string it sits in, so a small obstruction in the wrong place costs more output than its own footprint suggests. That is a design problem as much as an area one, and it is usually answered by putting the affected area on its own string or on optimisers so the rest of the array is unaffected, or by leaving that patch of roof clear where the annual loss does not justify the panels.
Which is why we measure rather than assume. The in-house drone survey builds a 3D model of the roof, so the rooflights, the plant and the parapet set-backs are dimensioned from the real deck, and the shading is modelled through the year rather than eyeballed from a photograph. What survives those deductions is the usable area, and it routinely sits well below the gross roof.
It depends on the building. Flat and shallow-pitch roofs, common on sheds and warehouses, give long uninterrupted runs and are usually the simplest to lay out. Pitched roofs can work well too, but the orientation matters: a slope facing the wrong way generates less per panel, so part of the deck may not earn its place.
Either way the survey reads the real pitch, orientation and obstructions before a layout is drawn.
A satellite view shows the roof outline but not the things that decide the array: the exact rooflight positions, the plant and ducting, the shading through the day, the roof covering and the structural capacity. Those deductions are particular to your building and routinely change the usable area.
Our in-house drone survey captures the roof in a measured 3D model, which is what turns an order-of-magnitude estimate into a firm design.
Capital cost is survey-led and tied to your specific roof, so a larger usable area generally means a larger array and a larger figure, but the relationship is not flat: row spacing, module class, the mounting method and any structural strengthening all move the cost per kWp. There is no honest per-square-metre headline. A phased install spreads the capital across two stages rather than reducing the total. We confirm the figure from a measured survey, and the commercial solar cost guide explains what drives it.
The measured area itself is quick: an in-house drone survey captures the roof in a single site visit and builds the 3D model from there. Turning that into a firm usable-area figure and a sized design takes longer, because it waits on the structural assessment and on modelling generation against your half-hourly consumption data. The grid-connection position, handled through a G99 application, runs on the network operator's own timetable and can be the longest single step. We give you the realistic programme for your site once the survey is booked.
Last updated August 2026
A guide can only take you so far. The figure you get is modelled from your own half-hourly load and a system sized from the on-site drone survey. No obligation, and systems this size sit outside the domestic MCS scheme, so the assurance is the engineering stack.