Provided herein are systems and methods directed to using renewable energy sources with hot water heating systems. The system and method provide for interconnecting a single electrical heating element of a hot water heater to first and second electrical power sources. Typically, these first and second electrical power sources include a utility provided electrical power source (e.g., a fossil fuel AC power source) and a renewable energy power source. A controller selectively connects the first or second power sources.

The present invention relates to a trigeneration energy supply system having improved cooling and system use efficiency. The trigeneration energy supply system according to one embodiment of the present invention can comprise: a vacuum pump; a vacuum chamber inside which a vacuum is created by the vacuum pump; a condensed water storage tank positioned higher than the vacuum chamber, and prepared so as to store condensed water formed when steam generated by evaporating water brought inside the vacuum chamber is transferred to the inside of the tank by the vacuum pump; a cooling pipeline arranged to pass through the inside of the vacuum chamber cooled during the water evaporation and prepared to deliver cool air to a cooling load; and a small hydroelectric power generation system for generating electrical power by allowing the condensed water stored in the condensed water storage tank to be poured from at least the height of the condensed water storage tank.