In the highly crowded and complex environment of the cell, controlling protein folding, localization, abundance and complex formation requires an extensive network of quality control machinery known as the proteostasis system. This system promotes proper folding and represses aberrant behaviour such as misfolding and aggregation. However, the capacity of the cell to maintain protein homeostasis is not fixed. Rather, it is tightly controlled by signaling networks, affected by environmental stresses, and is impaired by pathogenic aggregation-prone proteins as seen in many neurodegenerative diseases. Here, we present a new approach to quantitate the proteostasis capacity of cells in terms of folding and aggregation, by linking proteostasis to protein folding energetics. This approach employs a well-studied model for protein folding as bait for the quality control network to examine how the cell modulates protein folding and aggregation. Examining mutants of different stability of the bait protein barnase, the extent of folding as well as aggregation can be monitored by a fluorescent readout. These sensors demonstrate a decline in the proteostasis capacity of cells upon over-expression of huntingtin or shutdown of chaperone activity via ATP-depletion.