Wind Power: How Much Is Too Much?

How much wind power is too much? The answer depends on two other questions: First, how much have you got? We'll take it. The demand for clean power is that great.
The Sunday Times (as in the London Times) reports that Great Britain is thinking big:

John Hutton, the energy secretary, will this week announce plans to build enough turbines to generate nearly half Britain’s current electricity consumption. He will open the whole of Britain’s continental shelf to development, apart from areas vital for shipping and fishing.

The opinion that Bluewater Wind should build 200 turbines instead of 150 currently being negotiated seems rather modest by comparison. Which brings us to the next question: How much can the grid manage?
Because wind power is intermittent, the grid needs to be able to smooth out the overall supply to match the load. The Public Service Commission and other state agencies ordered Delmarva Power to negotiate a backup natural gas power plant to deliver electricity to the grid when the wind isn't blowing.
The proposed wind farm is intended to provide a Delaware-based source of electricity that can keep our rates stable for 25 years. But the wind farm will be connected to a grid that is much bigger than Delaware. The PJM grid serves 13 states and the District of Columbia. Some observers, including Willett Kempton and Jeremy Firestone, think the backup facility isn't needed, and that existing power plants can pick up the slack when the winds die down. A recent study in Europe looked at this question and concluded that dedicated backup power isn't needed:

The production of wind power varies and is harder to forecast than the fluctuations in electricity demand. Adding large quantities of wind power to power systems is therefore challenging. The power system impacts of wind power were studied in international collaboration coordinated by VTT Technical Research Centre of Finland. The results indicate that the frequently stated claim of wind power requiring an equal amount of reserve power for back-up is not correct. A substantial adjustment tolerance is already built in to our power network, and the impacts of wind power fluctuations can be
further balanced through a variety of measures.

The English version of the VTT working paper can be found here.
Other sources of electricity create variability. Power plants of all sorts go online and offline all the time. The North American power grid is designed to manage power fluctuations and match supply to demand in real time. Given a large enough grid, the variability of wind power can be accommodated. Slack winds on the Great Lakes can be offset by strong winds on the Atlantic Coast. According to the VTT, a well managed grid can should be able to efficiently handle much more wind power than is currently being produced:

With wind power penetrations amounting to 10—20% of the gross electricity demand, the additional costs (per MWh of wind power) arising from the balancing of wind power fluctuations are estimated to range between 1—4 EUR/MWh. This is less than 10% of the long-term market value of electricity.

This by the way is far less than Delmarva Power's estimate of the cost of backup power. Professors Kempton and Firestone argue that a backup power source is not needed, and that the PSC and other agencies could decide to go ahead with wind power while giving the backup power issue more study.
I don't know how Great Britain might manage with half of its electricity coming from wind power. It's never been done. The British Isles may be able to balance its loads given a grid management system of sufficient sophistication. It certainly is an ambitious goal.