Who cares? Is efficiency a better measure of performance of your system than your rate of return in your investment? However, if you must know we have a 500 Watt (very small) test screw with a maximum efficiency of 75%. Larger screws get more efficient largely due to efficiency increases of the larger gear motors, with bigger system efficiencies in the high 80’s.
But again, who cares. If you run into someone who’s “been in the hydro business their whole life” and they tell you other turbines are more efficient, that can be correct, many traditional turbine types can have efficiencies in the high 90% range. That’s great, but can they operate at lower heads and what are their efficiencies at less than ideal flow and head conditions? More important yet, what’s the return on investment if I use them? If everything else were equal, efficiency would be the basis for deciding on which turbine to use, but everything else isn’t equal. When making a decision to invest it is best to compare different systems for an installation based on IRR of the competing systems because in a sophisticated enough IRR (Internal Rate of Return) analysis it would factor in efficiency but also costs, maintenance, reliability and performance when operating under non ideal flow or head conditions.
It’s hard to factor environmental and fish friendliness effects into an IRR calculation all the time, but surely these have to be considered as well. Whichever has the best (highest) IRR is the best system for you to install.