It is not recommended to plant edibles in black water; building permits may be refused for plans indicating such usage of black water. Where it is not possible to use flush-toilets operating on water, dry solar toilets are recommended. Power Earthships are designed to collect and store their own energy. The majority of electrical energy is harvested from the sun and wind. Photovoltaic panels and wind turbines on or near the Earthship generate DC electricity that is stored in deep-cycle batteries. The batteries are housed in a purpose-built room on the roof. Additional energy can be obtained from gasoline-powered generators or by integrating with the city grid. For Canadian winters the solar cell exposed surface areas needs to be increased by over three times. In an Earthship, a Power Organizing Module (POM) takes a proportion of stored energy from batteries and invert it for AC use. The Power Organizing Module is a prefabricated system provided by Earthship Biotecture that is simply attached to a wall on the interior of the Earthship and wired in a conventional manner. It includes the necessary equipment such as circuit breakers and converters. The energy run through the Power Organizing Module can be used to run any household appliance including washing machines, computers, kitchen appliances, print machines, and vacuums. Ideally, none of the electrical energy in an Earthship is used for heating or cooling. Thermal performance Earthships rely on a balance between the solar heat gain and the ability of the tire walls and subsoil to transport and store heat. They are designed to use the properties of thermal mass and with the intent that the exterior earth-rammed tire walls provide thermal mass that will soak up heat during the day and radiate heat during the night, keeping the interior climate relatively comfortable all day. In addition to the exterior tire walls, some Earthships are sunk into the earth to take advantage of earth-sheltering to reduce temperature fluctuations. Some earthship structures have suffered from heat loss into the ground during the heating season. This may be due to climatic differences between New Mexico where earthships were first built and cloudier, cooler, and wetter climates. Thermal performance problems may also have occurred due to thermal mass being erroneously equated to R-value. The imperial R-value of soil is about 1 per foot. Malcolm Wells, an architect and authority on earth-sheltered design, recommends an imperial R-value 10 insulation