Combining energy savings with a streamlined, lightweight configuration, Bosch Thermotechnology Tronic 6100C electric tankless water heaters are suitable for whole-house applications. The 13 1/4-inch-by-20 1/4‑inch units employ modulating elements that help ensure a constant output temperature and help deliver 95% efficiency with no stand-by loss, Bosch says. We found a 27-kw model online for $750. bosch-thermotechnology.com