Which energy source is commonly used in hot water systems?

Hot water systems can utilize a variety of energy sources to heat water, making the choice of "all of the above" accurate. Natural gas is frequently used in many residential and commercial hot water systems due to its efficiency and cost-effectiveness. It typically powers gas-fired water heaters, providing quick heating with lower operating costs.

Coal, while less common in residential applications in many areas, can still be used for hot water heating in certain industrial settings or regions where it is readily available and more economical compared to other sources.

Electricity is another prevalent energy source for hot water systems, particularly in areas where natural gas is not readily accessible. Electric water heaters are widely used due to their convenience and ease of installation, although the operating cost can vary depending on electricity prices.

Since hot water systems can effectively utilize any of these energy sources, the answer encompasses all options presented. This diversity in energy sources allows for flexibility in selecting a hot water system based on availability, cost efficiency, and user preference.