Abstract Thermal energy storage (TES) systems can be designed in order to maximize their impact on a specific design target, such as reducing indoor temperature diurnal swings. Identifying the foremost design objective(s) is highly important since different design objectives result in distinct optimal designs. This paper compares several design concepts and associated criteria that can be satisfied with passive TES in various applications with a focus on isolated gain spaces like solaria and greenhouses. Potential design targets for thermal mass design strategies are compared along with common metrics used to characterize the performance of TES systems. Different design objectives/parameters are discussed, such as: 1) optimal time lag; 2) optimal decrement factor and transfer admittance; 3) reduction of space heating and cooling energy consumption; 4) minimizing indoor temperature swings; 5) maximizing the room average temperature under passive response; 6) reducing peak temperatures. This review of targets and metrics provides a basis for identifying the most relevant performance variables for solaria and greenhouses. A novel metric is presented for characterizing the time lag between the peak absorbed solar radiation and the peak TES surface temperature: τ [ Q a − T s ] . A methodology and design recommendations for integrating TES into solaria and greenhouses are presented in Part 2.