An assembly for generating electricity that is transportable as a unit from a manufacturing site to an end-use location in which the assembly is positioned in or beside a body of water as a free-standing unit and includes (a) a chamber having a base, a cylindrical side wall extending from the base, a water inlet in the side wall, and a water outlet in the base, with the side wall having a cylindrical internal surface, and with the inlet being formed so that in use water flows in a spiral pathway within the chamber around a central shaft and downwardly to the outlet in the base of the chamber, and with the outlet being formed with a rifled internal surface, i.e. a surface with a series of spiral ridges in the internal surface, to improve water flow through the outlet (17,) (b) a rotor unit having a shaft and a rotor mounted to the shaft located in and rotatable in the chamber in response to water in use flowing through the chamber, with the rotor including a plurality of vanes extending radially outwardly from the shaft towards the side wall, with the vanes being curved vanes that sweep rearwardly from inner edges that are connected to the shaft to outer edges, with the inlet and the vanes being positioned relative to each other so that in use at least a part of water flowing into the chamber from the inlet flows directly towards the vanes, (c) an electrical generator or more than one electrical generator coupled to the rotor unit for generating electricity in response to rotation of the rotor, and (d) a support structure including a plurality of support members in the form of legs that, in use of the assembly, extend downwardly from the base and are adapted to locate the chamber, the rotor unit, and the electrical generator in or beside the body of water as a free-standing unit, wherein the (i) the inlet angle (ii) the dimensions of the inlet, the outlet and the chamber, (iii) the positions of the inlet and the outlet, and (iv) the positions of the inlet and the vanes relative to each other are selected so that a vortex forms within the chamber when in use the assembly is in or beside the body of water as a free-standing unit and there is a flow of water through the chamber from the inlet to the outlet. (a) a chamber having a base, a cylindrical side wall extending from the base, a water inlet in the side wall, and a water outlet in the base, with the side wall having a cylindrical internal surface, and with the inlet being formed so that in use water flows in a spiral pathway within the chamber around a central shaft and downwardly to the outlet in the base of the chamber, and with the outlet being formed with a rifled internal surface, i.e. a surface with a series of spiral ridges in the internal surface, to improve water flow through the outlet (17,) (b) a rotor unit having a shaft and a rotor mounted to the shaft located in and rotatable in the chamber in response to water in use flowing through the chamber, with the rotor including a plurality of vanes extending radially outwardly from the shaft towards the side wall, with the vanes being curved vanes that sweep rearwardly from inner edges that are connected to the shaft to outer edges, with the inlet and the vanes being positioned relative to each other so that in use at least a part of water flowing into the chamber from the inlet flows directly towards the vanes, (c) an electrical generator or more than one electrical generator coupled to the rotor unit for generating electricity in response to rotation of the rotor, and (d) a support structure including a plurality of support members in the form of legs that, in use of the assembly, extend downwardly from the base and are adapted to locate the chamber, the rotor unit, and the electrical generator in or beside the body of water as a free-standing unit,