MADRID (Reuters) -Europe's largest utility Iberdrola posted on Wednesday a 50% increase in net profit for the first nine months of the year, in a sign that its focus on Britain and the United States is paying off.

This is part of a broader shift toward building and upgrading power grids while taking a more selective approach toward renewables, with a preference for offshore wind projects.

Net profit for the period was 5.47 billion euros ($5.91 billion), the Spanish company said. This compares with 3.64 billion euros a year earlier.

"Our continued major investment drive is delivering new renewables projects and regulated assets, and the contribution to results of our increased offshore wind capacity underlines the reason why investment in this technology was a key pillar in our strategic plan," Executive Chairman Ignacio Sanchez Galan said in a statement.

Core earnings - before interest, tax, depreciation and amortisation - rose 23% to 13.27 billion euros.

The Spanish company, which has improved the outlook for the year in the previous two quarters, credited its operations in Britain and the U.S. as the main drivers of its performance.

It expects net profit to increase by 14% this year, in line with the growth in interim dividend set at 0.23 euros per share.

($1 = 0.9255 euros)

(Reporting by Pietro Lombard; Editing by David Latona)