Once you have decided to go tankless, the next fork in the road is fuel. Electric and gas tankless water heaters solve the same problem with completely different engineering, and the right answer depends far more on your house than on which technology is objectively superior. Neither one wins across the board.
The short version: electric units are cheaper to buy, simpler to mount, and need no venting, but they demand extraordinary electrical service and lose capacity fast in cold climates. Gas units deliver much higher flow rates, work well in northern winters, and cost more to install because of gas piping and combustion venting. Below is what actually separates them in practice.
Capacity in Cold Weather
This is the single biggest differentiator. Electric tankless output is capped by amperage, and amperage is capped by your panel. A 36kW electric unit is roughly the ceiling for residential service, and even that delivers only about 4 GPM at a 60 degree temperature rise.
Gas units face no such wall. Residential models routinely reach 150,000 to 199,000 BTU of input, which translates to 6 to 9 GPM in mild conditions and still holds 4 to 5 GPM through a hard winter rise. A high output condensing model like the Rinnai CX199iN is built for exactly that scenario: multiple bathrooms running while groundwater sits near freezing.
If you live where incoming water drops below 45 degrees in winter and you need two showers at once, gas is usually the practical choice. In warm climates where the rise rarely exceeds 45 degrees, a strong electric unit such as the Rheem RTEX-36 can genuinely serve a whole house.
Installation Cost and Complexity
Electric units win on the unit itself and on venting. There is no exhaust, no combustion air requirement, and no condensate drain, so the heater can sit in a closet, under a sink, or in a space with no exterior wall access. The catch is the panel. A whole house electric model may need three double pole 40 amp breakers and six or more conductor runs of heavy gauge wire. Homes with 100 amp service frequently need a full service upgrade, which is a permit level job.
Gas units invert this. The gas supply usually exists, but a tankless heater consumes three to five times the input of the tank it replaces, so a half inch line often has to become three quarter inch. Condensing models vent through inexpensive PVC, while non condensing models need stainless steel category three venting, which is considerably more expensive per foot. The Rinnai RX199iN illustrates the flexible end of this, with indoor and outdoor configurations that change what venting is required.
Operating Cost
Electric tankless heaters convert nearly all incoming energy to heat, so their efficiency is close to 99 percent. Gas condensing units land around 0.92 to 0.96 on the uniform energy factor scale. On paper electric looks better, but energy price is what determines the bill.
In most of North America, natural gas costs substantially less per delivered BTU than electricity, which is why gas usually wins on monthly cost despite lower nominal efficiency. Where electricity is cheap or gas service does not exist, that reverses. Propane sits in between and fluctuates with delivery pricing. Run your own rates rather than trusting a general claim, because regional spreads are wide enough to flip the conclusion.
Where Each One Clearly Fits
Electric makes obvious sense for point of use duty, apartments, additions, warm climates, and any location where running a vent is impractical. Smaller whole house models like the EcoSmart ECO 18 suit one or two bathroom homes in moderate regions, and they mount almost anywhere.
Gas makes sense for larger households, cold climates, high simultaneous demand, and homes that already have adequate gas piping. It is also the safer bet if you expect household demand to grow. Comparing input ratings, flow curves, and venting types across the water heaters category is the quickest way to narrow candidates once you know which fuel your house can actually support.
Key Takeaways
- Gas tankless units deliver substantially higher flow rates, especially in cold climates.
- Electric units cost less upfront and need no venting, but may require a costly service upgrade.
- Electric efficiency is higher on paper; gas is usually cheaper per month where natural gas is available.
- Condensing gas models vent through PVC; non condensing models need pricier stainless venting.
- Check panel capacity or gas line diameter before choosing, since that constraint often decides for you.
Frequently Asked Questions
Can an electric tankless heater serve a whole house?
In warm and moderate climates, yes, provided the electrical service supports it. In regions where winter groundwater falls below about 45 degrees, whole house electric coverage becomes difficult without an unusually large service.
Do gas tankless heaters work during a power outage?
Most modern units use electronic ignition and a control board, so they will not operate without power. A small battery backup or generator circuit can keep one running, but assume no hot water in an outage unless you plan for it.
What is the difference between condensing and non condensing gas models?
Condensing units add a secondary heat exchanger that extracts heat from exhaust gases, raising efficiency and lowering vent temperature enough to allow PVC venting. They cost more upfront and produce acidic condensate that needs a drain.
Is propane a reasonable alternative to natural gas?
Yes, and many models are available in propane configurations. Performance is comparable, but fuel cost is typically higher and depends on delivery pricing in your area.
