In household and commercial usage, most water heaters are of the tank type. These water heaters consist of tanks in which a given amount of water is kept continuously hot and ready for use. Typical sizes for household water heaters are usually 20 to 40 US gallons (75 to 150 L). These water heaters may run on electricity, natural gas, propane gas, fuel oil, or other energy sources. The most popular in the United States is the natural gas type.
Tank-type water heaters can be made more efficient by installation of additional insulation jackets around the tank, flow valve devices at their inlet and outlet, cycle timers, electronic ignition (in the case of fuel-using models), sealed air intake systems (again in the case of fuel-using models), and pipe insulation. The sealed air-intake system types are sometimes called "band-joist" intake units. "High efficiency" units can convert up to 98% of the energy in the fuel to heating the water. The exhaust gases of combustion are cool and are mechanically ventilated without the need of a chimney.
One analysis by the Florida Solar Energy Center (FSEC) found that those who owned a solar water heater, saved as much as 50-85% annually on household utility bills over the cost of electric water heating. However, solar water heaters are more expensive to buy and install than conventional water heaters: A solar water heater can be from $1,500 to $3,000, whereas a gas water heater can be from $350 to $450, and an electric water heater from $150 to $350.