Water Heaters
Tankless vs Tank Water Heater: Cost, Efficiency and Sizing Compared
Tankless heaters avoid standby losses, last longer and never run out, but they cost more to install, are limited by flow rate and often need a bigger gas line or electrical service. Tank heaters are cheaper and simpler and deliver big bursts of hot water, but lose heat around the clock. Heat pump tanks are the most efficient option of all where there is space for them.
How each type works
A storage tank heater keeps 30–80 gallons of water hot all day. A gas burner or electric elements switch on when the tank cools, either from use or from slow heat loss through the insulation (standby loss). It is sized by first-hour rating: how much hot water it can deliver in the busiest hour.
A tankless (on-demand) heater has no reservoir. When a tap opens and flow passes a minimum (often about 0.3–0.5 GPM), a high-output burner or set of elements heats water as it passes through a heat exchanger. It is sized by flow rate and temperature rise: how many gallons per minute it can heat from your coldest inlet temperature.
A third option, the heat pump water heater, is a tank that heats water with a small heat pump and keeps resistance elements as backup. It uses roughly a third to a quarter of the electricity of a standard electric tank.
Side-by-side comparison
| Gas tank | Gas tankless | Electric tank | Electric tankless | Heat pump tank | |
|---|---|---|---|---|---|
| Typical UEF | about 0.58–0.70 (condensing 0.80+) | about 0.81–0.97 | about 0.90–0.95 | about 0.96–0.99 | about 3.0–4.0+ |
| Sized by | First-hour rating | GPM at temp rise | First-hour rating | GPM at temp rise | First-hour rating |
| Runs out of hot water? | Yes, after a long draw | No, but flow is capped | Yes, recovers slowly | No, but flow is capped, especially in cold climates | Yes, recovers slowly in heat-pump mode |
| Typical lifespan | 8–12 years | 15–20+ years | 10–15 years | 15–20 years | 10–15 years |
| Typical installed cost (US) | $1,200–2,500 | $2,500–5,000+ | $1,000–2,200 | $1,500–3,500 plus electrical work | $2,500–5,000 before incentives |
| Main installation hurdle | Venting | Gas line size, new venting | Breaker and wire size | Very large electrical load (often 100–150 A) | Space, air volume, condensate drain |
| Maintenance | Flush sediment, check anode | Descale yearly in hard water, clean inlet filter | Flush, anode, elements | Descale, filter | Clean air filter, flush, anode |
Costs are typical US ranges and vary with region, access and code upgrades. UEF (uniform energy factor) is the DOE efficiency rating on the label; higher is better, and heat pump units exceed 1.0 because they move heat rather than make it.
Running cost and efficiency
For gas, the main efficiency gain from tankless is eliminating standby loss from the tank. A typical gas tank loses enough heat through its walls and flue to keep a pilot or burner cycling a few times a day even when no one uses water. In a household that uses a lot of hot water the saving is a smaller share of the total; in a small household with modest use, standby loss is a bigger share and tankless gains more.
For electric resistance, the efficiency difference between tank and tankless is small, because electric tanks are well insulated and have no flue. The big electric saving comes from a heat pump water heater instead. To put fuel prices side by side, see gas vs electric heating and the heating cost calculator, which works for water heating too if you enter your annual hot-water energy.
Sizing each type
- Tank: add up hot water used in your busiest hour and choose a heater with at least that first-hour rating. Use the water heater sizing calculator.
- Tankless: add up the flow of fixtures running at once and the rise from your coldest inlet temperature, then use BTU/h = 500 × GPM × rise. The tankless size calculator gives gas input and kW.
The climate matters more for tankless. A unit that supplies 5 GPM through a 45 °F rise in Florida may manage only about 3 GPM through a 75 °F rise in Minnesota in February.
Common tankless complaints, and fixes
- Cold-water sandwich: a brief slug of cold water when a tap is reopened shortly after closing, while the burner relights. Some models include a small buffer tank or recirculation to reduce it.
- Minimum flow: a trickle from a single low-flow faucet may not fire the burner. Check the activation flow on the spec sheet.
- Long wait for hot water: tankless does not shorten the time for hot water to travel down the pipe. A recirculation pump addresses that, at some energy cost.
- Scale: in hard water, the heat exchanger needs flushing with a descaling solution, typically yearly. A scaled exchanger loses output and can trigger error codes.
Which should you choose?
- Choose a gas tankless if you have natural gas, a gas line that can be upsized at reasonable cost, moderate simultaneous demand, and you plan to stay long enough to benefit from the longer life.
- Choose a gas tank for the lowest upfront cost, frequent large simultaneous draws such as tub fills, or when replacing like-for-like in an emergency.
- Choose a heat pump tank if you have electric water heating today, a space of several hundred cubic feet that stays above about 40–50 °F, and want the lowest running cost.
- Electric tankless suits point-of-use jobs and warm-climate homes with large electrical services; whole-house units in cold climates often need more amperage than the home has.
Replacing any water heater involves gas, venting, or high-current electrical work and a pressure relief valve. Use a licensed installer and get the required permit.
Frequently asked questions
Does a tankless water heater save money?
Gas tankless units typically use less fuel than gas tanks because there is no standby loss, but the higher installation cost can take many years to recover. The savings are larger in smaller households and over the unit's longer life.
Can a tankless water heater run two showers at once?
A properly sized whole-house gas unit (roughly 150,000–199,000 BTU/h input) usually can in most of the US. In very cold-water regions, check the manufacturer's flow chart at your temperature rise.
Which lasts longer, tankless or tank?
Tankless units often last 15–20 years or more with descaling, compared with about 8–12 years for a typical gas tank, which usually fails when the steel tank corrodes through.
Last reviewed 2026-10-28