In stand-alone operations, on-site generators must balance and meet demand at any time for second-by-second fluctuations in output and load demand. However, the previous optimization tool for private generator configuration in hospital buildings did not consider demand sufficiency. Herein, this qualitative electricity problem was solved by proposing a new optimization method that considers the balance of power supply and demand in the stand-alone operation of on-site generators during power outages. As a demand sufficiency condition, a power balance simulator obtained available configurations of private generators that can be operated within the standard alternating current (AC) frequency range of 49–51 Hz. We also compared case study results by applying these constraints to the findings of earlier studies. The same case study from an earlier paper reported that the optimal amount of photovoltaic systems installed is approximately the upper limit (set at 600 m2 in this calculation) and the optimal solution. In contrast, the optimization results with additional constraints to keep frequency fluctuations within specified limits yielded an optimal value significantly less than the previous optimization; one case study showed that the optimal installation amount of photovoltaics was 0 m2. However, the key equipment in this study was the emergency diesel generator. The emergency generator compensates for power shortages and balances supply and demand under power outages. The results suggest that case studies with demand-satisfying conditions tend to select equipment configurations that effectively improve the expected power shortage rate.