This paper is the first part of a two-part series on aggregate load models of residential customers for application in harmonic studies. Part 1 paper builds on the previous work on component-based time-domain models (TDMs) of main types and categories of loads found in residential load sector and presents two aggregate TDMs. The first is based on a detailed modelling of individual loads in the aggregate load mix, where types, numbers and power demands of considered loads are obtained from the available energy consumption statistics, while mean values, ranges and distributions of circuit parameters are determined from the generic TDMs. The second model is based on a much simpler representation of the aggregate load mix, where main load categories and sub-categories are modelled with the corresponding 'single-equivalent TDMs', for which circuit parameters are determined by scaling-up generic TDMs to represent power demand of all loads in the same category/sub-category. Particular attention is paid to the harmonic cancellation effects (diversity factors) between loads in the same (sub)category and loads from different (sub)categories, which are evaluated by probabilistic Monte Carlo approach and validated with available network measurements. In Part 2 paper, the presented TDMs are converted in corresponding frequency-domain models, which are expressed in the form of coupled harmonic admittance matrices.