Be Green: sizing renewables on a constrained roof

Be Green is the stage where the roof plan decides the answer. By the time you reach it you have already taken the fabric savings at Be Lean and the supply savings at Be Clean, and what remains is a gap to close with on-site renewable generation. On most urban schemes that means photovoltaics, and the amount of photovoltaics you can install is set by how much usable roof survives after plant, access, greening and edge setbacks have taken their share.

Energy assessments fail at this stage for a predictable reason. The array area is taken from the gross roof area rather than the net, the figure is carried through the carbon calculation, and the roof plan drawn six months later shows that half of it was never available.

What the Be Green stage has to show

The energy hierarchy in the London Plan runs Be Lean, Be Clean, Be Green and Be Seen, under Policy SI 2 on minimising greenhouse gas emissions. Major development is expected to achieve a stated minimum reduction in regulated carbon emissions on site against the Building Regulations baseline, with a minimum proportion of that coming from fabric and passive measures at the Be Lean stage, and any residual shortfall dealt with through an offset payment.

For the Be Green stage the assessment has to state the technology proposed, the installed capacity, the predicted annual generation, the carbon saving attributed to it, and the physical basis for all of that. Outside London, local plan policies vary, and many authorities now set their own on-site reduction requirements, so check the adopted policy and any supplementary guidance rather than assuming the London approach applies.

What makes an assessment credible is the link between the carbon number and the drawing. An Energy Statement that reports 180 kWp of photovoltaics without a roof layout showing where the modules sit will be queried, and on a referable scheme it will be queried by the Greater London Authority as well as by the borough.

Competing demands on a flat roof

A flat roof on a residential block has to accommodate more than most design teams allow for at planning stage.

• Air source heat pumps or chillers, with acoustic enclosures and clearance for airflow
• Mechanical ventilation plant, risers and flue terminations
• Lift overrun and stair access, which also cast shade
• Biodiverse or green roof area required by an urban greening policy
• Communal amenity space and play space where the scheme is expected to provide it
• Attenuation where surface water cannot be dealt with at ground level
• Edge setbacks for wind uplift and fall protection, plus maintenance access routes between everything else

Those demands are not negotiable in the same way. Plant is dimensioned by the mechanical design and cannot be shrunk by argument. Greening is usually policy-driven and quantified. Amenity space is policy-driven and located. Photovoltaics are the flexible item, which is why they are the item that gets squeezed.

The sensible response is to settle the plant footprint before the energy assessment is finalised, and to treat the competition for roof space between plant, solar panels and a green roof as a design exercise rather than an arithmetic one. Mounting modules above a biodiverse substrate on raised frames recovers area, and modules sitting over a green roof run slightly cooler and therefore slightly more efficiently.

Calculating the available array area honestly

Work from gross to net, show the deductions, and put the result on a drawing.

The two deductions that are most often missed are the edge setback and the inter-row spacing. Wind loading on a flat roof is highest at the perimeter and corners, so ballasted systems are kept back from the edge, and the setback is a function of building height and exposure rather than a fixed dimension.

Inter-row spacing is a larger loss. A south-facing array at a 30 degree tilt needs rows spaced far enough apart that the front row does not shade the one behind at low winter sun angles, which typically means a spacing of two to three times the vertical height of the module for English latitudes. The practical effect is that only a fraction of the net area carries modules. East-west arrays at a shallow tilt of 10 to 15 degrees avoid most of that loss because adjacent rows shade each other far less, and they fit substantially more installed capacity onto the same roof.

For current mainstream modules, around 0.2 kWp per square metre of module area is a reasonable planning stage assumption. Use the actual module datasheet once there is one.

Orientation, tilt and the shading losses

South-facing at a tilt of roughly 30 to 40 degrees gives the highest yield per kWp installed in England. East-west at a shallow tilt gives a lower yield per kWp and a higher yield per square metre of roof, together with a flatter daily generation profile that matches building demand better. On a constrained roof, the second option usually wins on total annual generation and on carbon saved.

Shading has to be assessed from the actual geometry. Lift overruns, stair cores, parapets, flue terminations and neighbouring buildings all cast shade, and a module with one shaded cell string loses more than its proportional share unless the array is configured with that in mind. On schemes with a single tall element, the shadow from that element across the lower roof at low sun angles can remove a third of the apparent array area from useful service.

Annual yield should come from a location-specific calculation rather than a rule of thumb, though the result for an unshaded south-facing array in England usually lands in the region of 850 to 1,000 kWh per kWp per year. Quote the tool and the assumptions used.

An illustrative example of method. A roof with 1,400m² gross area loses 300m² to plant and clearance, 200m² to a biodiverse roof required by policy, 120m² to access and fall zones, and 180m² to edge setbacks, leaving 600m² net. At a shallow east-west tilt with modest spacing, around 450m² of module area fits, giving roughly 90 kWp. The same roof assessed from the gross area at 0.2 kWp per square metre would have claimed 280 kWp. That gap is the gap that gets found at technical design stage.

Where the on-site target cannot be met

Say so early and say it with numbers. A scheme that cannot reach the on-site reduction target because the roof is small relative to the floor area is a normal outcome on a tall building, and the policy framework anticipates it. What is not acceptable is an assessment that claims a target is met using an array that cannot be built.

Before conceding a shortfall, test the measures that are usually left out. Photovoltaics on south and west facades, on balcony balustrades and on canopy structures over car parking all contribute. Reducing peak electrical demand through the heating strategy changes the picture too, and the interaction between peak electrical demand and all-electric heating often matters more to the scheme's carbon performance than the last 20 kWp of array.

Where a shortfall remains, quantify it in tonnes of carbon dioxide over the relevant period and set out the offset proposal. A Sustainability Statement should carry the wider policy case, and on schemes where embodied carbon is also assessed, a Whole Life Carbon Assessment should use the same quantities and the same drawings as the energy assessment.

Carbon offset payments and how they are secured

The offset mechanism converts the residual regulated carbon shortfall into a cash payment to the authority's carbon offset fund, calculated as a nominal price per tonne applied over a defined period. The Mayor's guidance sets out a recommended approach and price for London, and boroughs adopt their own rates in local plan policy, so check the current published figure for the authority you are dealing with rather than carrying one forward from a previous scheme.

The payment is secured through a section 106 agreement, which means the calculation has to be settled before the agreement is completed. Three practical points follow. The figure depends on the Be Lean and Be Clean outcomes, so a late change to the fabric specification changes the payment. The agreement usually requires the calculation to be revisited at the as-built stage under the Be Seen reporting requirement. And where the on-site generation claimed at planning is not installed, the authority has a mechanism to recover the difference.

Drawings that support the figures

Four drawings make the Be Green case defensible. A roof plan at a measurable scale showing the plant zone, greening, access routes, setbacks and the module layout as actually proposed. A roof area schedule matching the deductions in the assessment. A section through the array showing tilt, row spacing and the shading geometry at a low winter sun angle. And elevations where modules or their support frames are visible, because on a scheme in a conservation area the visual case is decided on the elevation rather than on the plan.

Then keep the energy assessment and the drawings in step through technical design. The roof is the part of the building where every discipline wants the same square metre, and the array is the item with the least institutional protection.

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