At the widest point of the Greater Thames estuary, 12 miles north of the Kent coast and 12 miles south of Essex, lies the London Array – the largest operational offshore wind farm in the world. Completed in 2013, after 10 years of planning and construction, it covers an area of 40 square miles – roughly the same size of Bristol – and comprises 175 individual turbines laid out in neat rows like an enormous nursery flower bed.

It's a mature technology, and it’s a very effective way of installing new power on to the grid Jonathan Duffy, London Array general manager

“Standing on a boat in the middle of the wind farm surrounded by these machines is awesome,” says Jonathan Duffy, the farm’s general manager. “Knowing the turbines around you are generating electricity for more than half a million homes from the breeze passing through is a great feeling.”

While the UK ranks comparatively low in the European renewable energy rankings, in wind power it is flourishing, having overtaken France as the sixth largest global generator of wind power, and boasting the largest offshore capacity in the world.

Offshore wind is not only clean and increasingly cheap, it is also among the most popular energy sources in Britain, exploiting the wealth of a well-buffeted coastline without impacting on local landscapes. “When you compare it to other technologies, it’s a fairly sure bet,” says Duffy. “It’s a mature technology and it’s a very effective way of installing new power on to the grid.”

Much of the UK’s offshore territory belongs to the Crown Estate, which manages and leases the seabed on which UK offshore wind farms are built. The London Array originally leased a second area in the hope of doubling the size of the farm, but this second phase was cancelled due to concerns about the welfare of the red-throated diver, a seabird that spends winter in the area.

One of the London Array’s two substations. Photograph: Pedro Alvarez/The Observer

Each turbine begins 30 metres beneath the seabed, rises through about 20 metres of water, with the turbine hub a further 80 metres above the surface. From top to toe, they are taller than the London Eye, or the spire of Salisbury Cathedral, dwarfing the turbines seen on land. Three blades, each of them 58 metres long, rotate clockwise, covering a “swept area” of more than 11,000 square metres, and operate in wind speeds ranging from 7mph to 55mph. The turbine heads are motorised, turning when necessary to face the wind, and transmit their generated power – up to 630 megawatts – through four thick undersea cables, via two substations, to the coast of Kent, where it is distributed to London and the south‑east.

Spinning languidly above the sea, the turbines have a look of serene independence. In fact, they stand in choppy, tidal waters and require constant vigilance and maintenance to sustain their power output. Every day during the summer, 60 labourers spend 12-hour shifts travelling between the turbines, climbing their vertical ladders, tuning, lubricating and realigning their complex machinery.

Duffy hopes to constantly improve the plant’s efficiency, and last December, the plant broke its own output record, generating 369,000 megawatt hours (MWh) of electricity in one month. “We have very good reliability,” says Duffy. “The turbines are designed for 20 years, but we may be able to revisit our calculations and extend their lives.”

By 2036, larger and cheaper installations in British waters will probably have matched the output of the London Array but, with luck, those 525 blades will spin for many years more, harnessing the turbulent British weather for the common good.