Efficiency is the general measure
of cost effectiveness in the sense of
the cost-benefit ratio. 


  • Locational advantage (longitudinal gradients, site development, geology, natural hazards, etc.)
  • Performance / economies of scale (i.e. production costs per kW reduce as production volume increases)
  • Transverse gradient (head)
  • Rate of utilisation (is incorporated proportionally in the energy generation costs)
  • Capital costs parameter (interest rate plus payback period)



P = Q x H x 7.2

(P) Power in kW
(Q) Flow in m3/s
(H) Head in m
(7.2) Estimated efficiency


Stimpfach - Germany: electricity for 61 households of 4 persons, environmentally friendly and generated fully automatically.

So that your sums add up!

Hydropower plants based on a screw are considered small hydro. But don't be fooled by the term "small". This technology is used to supply 13 million households with electricity across Europe, which is sparing the environment a burden of 29 million tonnes of CO2.

More power with the right technology
Systems from the Rehart-Power series can operate even with a head of just 1 m and a capacity of 500 l/s and then continue to work economically when anywhere else, the switch would long have been in the "OFF" position.

Many systems capitulate in lower water levels in hot summers and also fail to cope with flooding in the winter/spring. Cost-effectiveness then drops rapidly.

Something you can really count on
The hydropower screws already installed by Rehart operate for approximately 8,600 hours per year, meaning the systems are also baseload-compatible. Long maintenance intervals are good for operating results and the system even has no problems with dry running.

Good for cost effectiveness
The system works fully automatically and requires very little maintenance, which keeps operating costs low. Its service life is significantly higher compared to other means of generating electricity. Cost effectiveness also means that, taking into account the investment, maintenance and annual yield, a corresponding return and a short payback period can be achieved.

Rule of thumb for efficiency
Efficiency is specified as total efficiency of the system. It encompasses the performance of the screw, gearbox and generator. In our plants, it is approximately 0.75. In conjunction with acceleration due to gravity g (9.81 m/s2), this gives the factor 7.2 for reliable forecast values.

Real efficiency
The real efficiency of the Rehart-Power systems is often greater than the rule of thumb. As is the case with the reference plant in Stimpfach. 
Result of the real figures 2.55 x 2.3 x 7.5 = 44 kW

Case studies

  • Hydropower screw SH, Hausen - Germany, with greater yield
  • Hydropower screw SH in Kirchberg - Germany: yield is above the rule of thumb
  • Hydropower screw CR, Bischofsmais - Germany, also with a greater yield