The DOE computes average annual operating costs of $388 for a storage-tank heater, and $272 for a tankless heater. That's $116 less. If the tankless system costs $2,200 installed, and saves $116 a year, it would take about 19 years to break even on the investment.ost homes have a storage tank for hot water — 50 gallons or more that's regularly reheated and ready to use 24 hours a day. The compact alternative, often billed as the more efficient European system, is a tankless, or on-demand water heater.