An electrical machine for an aircraft electrical power system includes a stator having current-carrying coils and flux guiding stator iron defining one or more stator slots that house the current-carrying coils. The electrical machine further includes a rotor having a plurality of permanent magnets configured to interact with the stator to produce a torque. The electrical machine has an active parts mass, mact, which is a cumulated mass of components of the electrical machine that contribute to producing the torque. The electrical machine is configured to produce a peak rated torque, τpeak. The electrical machine has a slot current density, Jslot,peak, when producing the peak rated torque, τpeak. A value of a machine parameter, Λ, defined as: Λ=τpeak/(mact×Jslot,peak) is greater than or equal to 5 μNm3 kg−1A−1.