World's largest wind farm, London Array, brought fully online

Interesting, but... 1. how many hours is it expected to work at full capacity? 2. what happens when there's no or not enough wind? Do we have to use coal plants to keep everything running?

Wind turbine gearbox reliability is very low. I live near many wind turbines that are in Iowa and there are are always a large percentage not turning because they need mechanical attention of some sort. This English water based wind farm will be no different except that sharks will be waiting below for a tasty meal of wind turbine gearbox fixer and all the birds that meet their demise. Hard to believe that we can't just tap the geothermal energy that is essentially inexhaustible and waiting to be tapped. Large upfront cost but won't kill millions of birds and won't need a fleet of boats to maintain. Geothermal is a way better answer to any energy question.

It would be nice if the article gave yearly TWh, ave MW and capacity factor but we have to calculate it from expected CO2 saved
900,000 tonnes / yr divided by 8,760 gives 102.7 tonnes ie 226,450 lbs /hr of CO2 saved.
A quick search says UK ave CO2 /kWh is 0.527 Kg /kWh ie 1.16 lbs /kWh
So Averaged power is 226,450 / 1.16 ie 195.2 MW Yearly production is 1,709 TWh Capacity factor is 195/630 ~= 31%

The name plate capacity for a wind park has to be multiplied with a capacity factor, usually something from 0.3 to 0.5 to get real-world numbers. Maybe it is close to 0.5, this being off shore. Not bad!
"Gearboxes": While some newcomer manufacturers have had problems with gearboxes, some of the established brands like Enercon don't even have gearboxes, but run a direct drive system. These systems used to be in need of rare-earth magnets and were problematic for that reason, but there are some very good efforts underway (and succeeding) to build high-efficiency direct drive generators that don't even need such magnets.
Keep working. Don't drop the ball!
For the storage problem, look at synthetic methane. Gas distribution and storage networks are already in place in industrialized nations. The natural gas grids in cities and its storage tanks can serve as de-facto batteries. There's a lot of work being done already in the E.U. and China, and a bit in the U.S.

"It would be nice if the article gave yearly TWh"
It would, but there's a reason it doesn't. It's a new farm and we don't know. Sure, you can do a back-of-the-envelope calc derived from the estimated CO2 savings, but you've missed a key part of that quote: "The world's largest operational offshore wind farm will be capable of generating enough energy to..."
So the CO2 savings would seem to be based on the farm's capacity, not on its expected output.

There towers are tall and the average wind speed for the site is 9.2 m/s. The rated power of 3.6 mw is around 13 m/s and above with shut down at 25 m/s.
the rated power at 9.2 m/s is slightly over 2mw. with out looking at a wind profile its hard to say how much power these will produce but my estimate would be 2mw X 24hr X 365 or about 17.5 GWh. Assuming 10 cents per kwh (yes high I know) that would be 1.75 million dollars us? or 100k of power per turbine. The expected length of operation is 20 years.
So each turbine will generated about 2 million dollars worth of electricity.
I used 10 cents per KWh because I expect carbon taxes and other factors will increase energy cost. Really its tough to say what the power will be sold for, but I suspect the total cost of the turbines and towers would be less then 2million dollars (us).
How much is wholesale electricity going for in the UK?

Buellrider, there are many new WT's that are gearless.

Bad gearboxes? Go to China for better? (and rare earth magnets?) Capacity factor is 31% of a perpetual and infinite supply? Millions of Li or nano carbon super capacitor vehicles on the roads very soon, will ballast the intermittentcy ? Intermittency can over-lap - Wind, Solar, Wave, buffer against peak demand/off peak demand. Thorium LFTR styled reactor technologies from China about to "Alter Global Energy Maps Forever", may leave a wide open window for Solar, Wind, Wave, Hydro, Tidal, Geothermal. Biological, domestic local power generation? Anaerobic sewage digestion produces 'easy to store' Methane that can be burned to provide peak electric demand and power when the wind doesn't blow, the sun doesn't shine? Ironprior, Ontario, Dairy Farmer does this now! Meanwhile America Factory Farms waste animal shit, contaminate the land, need to be mandated to Anaerobic digestion electric generation? Many smaller 21st century Chinese (debut: 2017) Thorium technology generator plants: over the 20th century restrictions to single huge central generation enriched Uranium, plutonium and radio-active wastes producing systems? Remember: Saudis pump salt water to float remaining oil - and build enriched Uranium reactors from America and Solar Collector Arrays in the deserts as they have truly discovered that Oil is not eternally sourced from a pipe up &Allah's ass after all. Iran, coveted target for U.S. Oil moguls is in fact China's gasoline tank, and not really up for grabs as Israel would have you believe, and Iraq's oil already on the world market at world prices, and still out of reach for the average peon in U.S.A. Canada's Tar Sands well known for generations there and thought to be economically not feasible, are "mined" for oil today - not because there is a surplus, not because it is the preferred 'light sweet crude' - it is not, but because we face the end of the 'Cheap Oil Era' in the world today. Gasoline engines commonly (and foolishly) used by Americans today are in fact hardly 25% efficient from a fixed and expensive quantity of gasoline, while Wind Turbines are near 30 % efficient from an infinite and free amount of eternal, perpetual Wind? Solar Cells the same? Geothermal too? And as for Humanure and other Factory Farm manure, as long as we eat, there will be a constant(perpetual, infinite) supply ready for methane making, storing, and leaving a Top Soil Building fertilizer behind. Coal? finite, and hardly efficient? Seeking improvements? C02 capture and use ? My Question: where are the potentially energy saving super insulations that made the Moon Shot possible? We need them now as energy prices escalate, just to survive as a nation?

Buellrider Geothermal consumes massive amounts of clean water, that we just do not have anymore.

Bruce Miller, Please review & edit before sending. Aim for better, more grammatical sentences! Super insulation materials have slowly evolved and are now becoming useful! Aerogels, lighter than cigarette smoke have tremendous insulating values. Specifically there is an emerging class that are made by resin coating an underlying fragile aerogel which is then baked or freeze-dried to extract volatiles. They have insulation values in the range of R20 per 1/8th inch! Some have been used for clothing but were found to be too much insulation. The big deal is that a material can formed into a jacket is sturdy enough to be handled. Past aerogels were as light as cigarette smoke and about as durable as smoke. The remaining big question cost per square foot for a given application? The next Really Big Question is how to convince American policy makers and the public to energetically go after low-hanging fruit energy savers. Everyone can save tremendous amounts of energy by improving home insulation efficiency. This would be a great application for a blow in aerogel puff ball with maybe R60 for a standard stud wall. This would be a huge improvement over R19 glass or cellulose. Again, however, at what pricetag?