The performance criteria of electrical aerosol chargers are reviewed in terms of charging effectiveness vs. particle losses. The occasionally subtle differences between the many existing parameters are discussed, compared and evaluated for their ability to characterize a charger completely and unambiguously on the basis of measurements external to the device. It is shown that the presence of particle losses, especially diffusive losses inside the charger, can impose significant constraints on the theoretical interpretation of such black-box measurements, especially when using implicitly loss dependent charging criteria. The influence of particle losses is analyzed in great detail on the basis of typical diffusion loss scenarios, which reveal the uncertainties in determining the true charging efficiency (inside the charger); the uncertainty in epsilon(intr) is +/-14% for 2.5 nm particles and +/-3.6% for 10 nm particles, respectively, while above 20 nm the uncertainty becomes negligible for typical aerosol systems at STP. A general framework for characterizing and quantitatively comparing the performance of different kinds of electrical aerosol chargers is then presented. It pairs three standardized variables for average charge per particle, charging efficiency and particle losses in a diagram, which can be applied to all possible charging scenarios ranging from partially charged to highly charged aerosols. In Part II this framework will be applied to data from the literature as well as new measurements. (C) 2005 Elsevier Ltd. All rights reserved.