An modular apparatus for generating electrical power in a borehole penetrating the earth includes a body comprising first and second mechanical connectors at opposing ends and first and second electrical connectors at the opposing ends, the mechanical connectors and the electrical connectors being configured to connect with the same type of corresponding connectors on another modular apparatus when the modular apparatus and the another modular apparatus are stacked together. The modular apparatus further includes: an electrical generator having a rotor and stator in mechanical communication with the first and second mechanical connectors and in electrical communication with the first and second electrical connectors; and a turbine wheel coupled to or integrated with the rotor and comprising one or more turbine blades that are configured to interact with fluid flowing through the borehole causing the turbine wheel to rotate and the rotor to rotate in order to generate the electrical power.