Not Op, but I suspect the use of "efficient" here is used more in terms of "usefulness of output" rather than pure turbine efficiency. For example, you might h

Not Op, but I suspect the use of "efficient" here is used more in terms of "usefulness of output" rather than pure turbine efficiency. For example, you might have a VERY efficient turbine, but if they're constrained to be small (and therefore low-output), you then need to have a squillion of them, which is not an efficient use of money or effort expended, in terms of output power total.