Solar On A City Roof Is Limited By The Cable

The obstacle to rooftop generation in dense cities is rarely the roof. It is the capacity of the low-voltage network the building is connected to.

Distribution networks were built to deliver, not receive

The cables and transformers serving a street were sized to carry power one way, from a substation to the homes and businesses along it.

Generation reverses that flow on sunny afternoons, and if enough properties export at once the voltage on the local network rises beyond its permitted band.

Network operators therefore assess each connection, and where the local circuit is already near its limit they restrict export rather than reinforce the cable.

Reinforcement is a street works project

Increasing capacity means a larger transformer, a new cable or both, which involves excavation along the street and a programme measured in months.

The cost is significant and the trigger is whichever connection request tips the circuit over, so one applicant can face a bill created by everyone before them.

This produces a queue effect where early applicants connect easily and later ones are quoted figures that make their project unviable.

Timing mismatch reduces the value on site

An office generates most in the middle of the day and uses most at the same time, which makes rooftop generation straightforwardly useful.

A residential block generates at midday and consumes in the evening, so much of the output must be exported at exactly the moment the network least wants it.

Storage aligns the two, but a battery needs a fire-rated room, ventilation and access, all of which are scarce in an existing building.

Roofs are already spoken for

Plant, lift overruns, terraces, green roofs and access routes occupy much of an urban roof, and each was allocated before generation was considered.

Panels also need spacing to avoid shading one another and setting back from edges for safe maintenance, which reduces usable area further.

The result is that a roof of a given size supports considerably less generation than its plan area suggests.

Shared systems complicate the metering

In a building with many occupiers, generation on a common roof has to be allocated to individual meters or sold to the common supply.

The regulatory arrangements for doing that vary and are often designed around single-occupier premises, leaving apartment blocks with awkward workarounds.

Where the simplest route is to power only the landlord's supply, the panels serve corridor lighting and lifts while the homes below buy from the grid.