But some environmental groups aren’t too worried that placing dozens of these turbines on the sea bottom might have a negative effect on marine life. “If we want to fuel the world, it is not a zero-impact thing,” said Stephan Singer, director of global energy policy at WWF International in Brussels. “Everything has an impact.”

Marine Current Turbines, a small company based in Bristol, England, sold out to Siemens, the German industrial giant, in 2012. Siemens has the global reach and financial power needed for the next phase, which will cost tens of millions of dollars or more. In Bristol, Siemens is working on turbines almost double the size of SeaGen that the company plans to install in a group of five off the Welsh coast.

Achim Woerner, the head of Siemens’s marine energy business, said potential customers like utility companies want to see a group of machines running smoothly before they buy. “Ocean power is 15 years behind offshore wind,” he said in an interview.

Being only that far behind might be encouraging. Over the past three decades, offshore wind has evolved from a fledgling industry to one that is now building installations that cost billions of dollars and are comparable in generating capacity to conventional power stations.

On the other hand, the offshore wind industry is still expensive and dependent on subsidies, leaving it vulnerable to investor skepticism and government cutbacks. SSE, a big British utility, said last week that it was curbing its investment in four large wind projects off Britain because they did not meet its criteria for financial returns.

Although developers say the costs can be brought down sharply, tidal power is even more expensive than offshore wind and so would require even higher subsidies. Britain may be blessed with more than its share of tidal rips, but guarantees that developers will be paid around £300 per megawatt hour — about seven times the current wholesale price for power — are also a big part of its attraction.

Siemens forecasts that tidal power could eventually account for as much as 4 percent of world generating capacity, and Canada, China and France, along with Britain, are encouraging experiments with such energy.