AbstractThis paper discusses a conformation of a micro co‐generation system (µCGS) for residential use. The evaluation is based on the hot‐water demand observed in three families who live in the same apartment house. Assuming DSS operation of µCGS, we evaluated the required capacity of PEFC, hot‐water tank, and backup boiler to meet a series of daily hot‐water demand observed in three houses. The results are: (1) µCGS with 2‐kW PEFC and 350‐L hot‐water tank are required in family B when µCGS is not equipped with the backup boiler, (2) the additional 5 kW of the backup boiler reduces the required capacity of the hot‐water tank by 50 L, (3) the evaluation based on hourly averaged hot‐water demand patterns would underestimate the required capacity of the hot‐water tank, (4) the required capacity of the hot‐water tank could be reduced by the leveling effect of the hot‐water demand pattern along three families. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 143(2): 30–37, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10080