A 1,000-gallon tank sits in the basement. It stores solar-heated water. It serves several functions. It preheats water for the water heater to bring up to higher temperature if needed.

It also provides storage for water heated by the solar panels on the roof. The water acts as the thermal mass of the house and is one of the best ways for storing heat, and unlike concrete or stone, it's easily portable.

The tank has five inches of foam insulation. And with the ladder leading inside, it can double as a hot tub (just kidding).

There are six 4x8-foot solar-thermal panels on the roof to heat both the house and water it uses.