Tankless heater deliver a limited gallons of hot water per minute. If you are drawing a tub of water from a tank heater, 10-20 gallons per min. is possible. Tankless rate is half. Taking a shower when anyother hot water use is impossible becuase of limit water heating capacity. Tanks last 10-15 years with minimum service. Tankless require periodic deliming because of limited heat exchange surface. Tankless have complicated flow and temperature control mechanics. Tankless are smaller and more efficient ($300/yr operating) than tanks ($400/yr).