Each turbine is optimised to specific site requirements of head and flow. The lack of belt drive and or gearbox, coupled to high efficiency elliptical bucket designs means that excellent efficiency is possible even on very small machines.
If you read literature on micro hydro you will see that 50% overall efficiency is often suggested for schemes under 10kW. We have schemes under 10kW delivering ~75% water to wire efficiency. In addition, low standing losses mean that turbines can run down to very low part load… below 5% of design flow on multi jet machines. The actual efficiency achieved depends on the size of runner, number and size of jets and speed and efficiency of generator (small single phase generators having lower efficiency).
Small grid connected machines of 5kW on 50m head will achieve~ 75% water to wire. A small 4 jet machine with 8 pole generator on low head will be around 70% water to wire. A 20kW scheme on 70m head with 3 phase connection may achieve near 80%. Larger schemes can achieve over 80%