Active Water Heaters
Active water heaters use solar panels or collectors to trap the sun's heat. Water running through the collectors increases in temperature as it absorbs heat. The warmed water is then pumped to a storage tank. These systems are called "active," as opposed to "passive," because they use electric pumps to move the water from the collectors to the storage tank. (In passive systems water moves naturally because of a temperature difference. Warm water is less dense and therefore lighter than cold water, so it rises.)
Active systems cost far more than a conventional water heater, ranging from $2,000 to $6,000. These systems can often supply up to 70% of a family's annual hot water needs. In the right climate and with the right usage, the monthly savings on hot water costs can add up to a reasonable return on investment. This is because even though you paid more for the system up-front, you save every month on what you would have paid for gas, oil, or electricity to heat 100% of your water.