Ranked by how much of the hot-water demand is met without purchased energy. Solar thermosiphon leads because it collects and stores with no pump; the batch heater collects as freely but loses more overnight because the store is the collector. Flue and compost coils are free but only while the stove burns or the pile is hot, and the compost coil pre-heats rather than reaching final temperature. An electric or heat-pump heater is the most dependable and the only one where every unit of heat is bought.