Flexible District Energy plants providing heating and cooling to cities represent an important part of future smart renewable energy systems. Equipped with large combined heat and power units, heat pumps and thermal energy storage they have the possibility to provide flexibility – but an optimized unit commitment is required. A common conclusion has been that unit commitment based on analytic methods is not useful. However, the market-based operation of District Energy plants often being reduced to participation in one or two electricity markets, simplifies the unit commitment problem and brings analytic unit commitment methods back as potentially attractive methods for District Energy plants. This is demonstrated in this paper by establishing a complex generic District Energy plant which is yet so simplified that a solver-based Mixed Integer Linear Programming method is able to deliver optimal unit commitments. An advanced analytic unit commitment method for district energy plants is proposed and the comparison of the unit commitments made by this method with the optimal solver-based unit commitments shows that the method delivers operation income within 1% of the optimal operation income, which is fully adequate for daily operation planning, yearly budgeting and long-term investment analysis for this generic District Energy plant.