Best Tankless Water Heaters in 2027

The Rheem RTEX-24 is an electric tankless unit rated at 24000 W and 5.9 GPM. It has the highest power input among the models that list it. For a buyer, that means more heating capacity without a gas line. It ranks first on our spec score, though its flow is lower than the two gas Rheem models.

Power is the catch. The listing calls for 240V single-phase power, three 40-amp double-pole breakers and #8 AWG copper conductors with ground. It also recommends a minimum 200-amp household panel. Check your panel before you buy. Plan for that electrical work first. Output temperature is set in 1-degree steps from 80°F to 140°F, which covers normal showers and dishwashing.

Size is modest. At 17.8 lb and 3.5 in tall, it is the shortest product on the page, so it needs little wall space. The data lists no UEF for it, so its efficiency rating cannot be compared with the gas models.

This Rheem RTG model runs on natural gas and lists 7 GPM max flow. The listing says that is 7.0 GPM at 35°F temperature rise, with 160,000 BTU/hr output. That suits two to three showers running at once. For a larger family, flow is the spec that decides whether the water stays hot.

Efficiency is its strongest point. Its UEF is 0.82, tied for the highest among the five models that list one. The listing calls it non-condensing and says it saves up to 34% on energy costs versus 50-gallon electric tank heaters. That claim comes from the maker. It also has freeze protection and leak detection.

This is the indoor version, with a direct vent system. It weighs 46.5 lb and stands 24.2 in tall, so it is heavier and taller than the outdoor RTG-70XLN-3. Plan on a qualified installer for gas and venting. The data lists no temperature range for it, though the listing mentions ±1°F control.

The RTG-70XLN-3 is the outdoor version of the same Rheem gas line. The listing says outdoor installation eliminates venting, which can simplify the job. It lists the same 7 GPM max flow and 0.82 UEF as the indoor model. That UEF ties for the highest among models that list one.

It is lighter and shorter than the indoor unit. It weighs 39.25 lb and stands 20.75 in tall. Both weights stay below the MIZUDO's 55.5 lb. That helps when you mount it on an outside wall. The listing mentions a durable metal cabinet, a 15-year heat exchanger warranty and a 5-year parts warranty. Freeze protection is built in, which matters for an outdoor unit in cold weather.

Check the listing before you buy. One line calls it a professional-install indoor unit, which conflicts with the outdoor description above it. Confirm the install location with the seller. Like the other Rheem gas model, it lists no temperature range.

The GE GE20P08BAR does not fit this keyword. It is a storage heater with an 18 gal tank, not a tankless unit. It makes sense only if you want a small point-of-use heater. The listing mentions a single hookup, a small house, a condo or an apartment.

Its strength is easy setup. It plugs into a 120V supply and does not need a hardwired install, which avoids the heavy wiring the RTEX-24 requires. The listing says it uses standard 3/4-in NPT water connections. It can also work inline with another heater to boost long pipe runs or washing machines.

The limits are clear. A tank holds 18 gal, which ties for the highest capacity among the three models that list one. When it runs out, you wait for a refill. It also weighs 54 lb and stands 25.75 in tall. The data lists no flow rate or UEF.

The MIZUDO WCIU-HW199A1G-NG-M0 lists 10.3 GPM, the highest max flow rate among the models that list it. It has 199,000 BTU of heating input. The listing says that supports two to three bathrooms plus kitchen faucets and appliances running together. It starts heating at 0.53 GPM, so it can respond to low-flow fixtures.

The temperature range runs from 95°F to 149°F, a higher top end than the other ranked tankless models. The listing cites ±1°F control, WiFi app control and Alexa support. Freeze protection is built in. It uses a sealed concentric direct-vent system with a 3 × 5 in vent kit.

Size is the trade-off. At 55.5 lb it is the heaviest product on this page, and it stands 24.8 in tall. No UEF is listed, so efficiency cannot be compared. The listing also says it is not recommended above 3,000 feet. Warranty is two years on the unit and five on the heat exchanger.