In this guide, I’ll break down performance, sizing, installation realities, long‑term costs, maintenance, climate considerations, safety, and a simple decision path so you can choose confidently.
Why Go Tankless at All?
Tankless water heaters don’t store 40–50 gallons of hot water that constantly loses heat. Instead, they heat water only when you open a tap. The benefits:
- Endless hot water (within the unit’s flow capacity).
- Smaller footprint (often wall‑hung, freeing up floor space).
- Higher efficiency than many storage tanks.
- Longer lifespans when properly maintained.
That said, not all tankless units are created equal, and “gas vs electric” is where performance and practicality truly diverge.
Efficiency vs. Output: What the Numbers Really Mean
You’ll hear two ideas tossed around: efficiency and output. They’re not the same.
Efficiency (How little energy is wasted)
- Electric tankless units are typically 98–99% efficient. Nearly every watt you buy goes into heating water.
- Gas tankless units generally range 80–95% efficiency, with high‑end condensing models at the top of that window.
Output (How much hot water you can make at once)
- Gas tankless can deliver more hot water at higher flow rates, especially in colder climates. Common residential models are rated up to ~199,000 BTU/h.
- Electric tankless whole‑home units usually land around 24–36 kW (≈82,000–123,000 BTU/h). That’s solid for small homes or warm‑water regions but can be limited with multiple simultaneous showers in colder areas.
Bottom line: Electric often wins on efficiency, but gas typically wins on muscle. Your household size, climate, and hot‑water demand decide which matters more.
Sizing in Plain English (and a Simple Formula)
A tankless unit must raise incoming water from its groundwater temperature to your target temperature (often 120°F). The colder your incoming water, the harder the unit works.
A quick way to estimate needed capacity is:
Required BTU/h ≈ Flow (GPM) × Temperature Rise (°F) × 500
- Example: Two showers at once (say 5 GPM total) with a 70°F rise (50°F incoming to 120°F target) requires roughly 5 × 70 × 500 = 175,000 BTU/h of output.
- A strong gas tankless can handle this.
- An electric unit would need to be very large (and very power‑hungry) to keep up.
If you’re in a warm region where the rise is only 40–50°F, electricity can cover more fixtures. In northern climates where the rise can hit 70–80°F, gas is usually the practical choice for whole‑home use.
Installation Realities: Venting, Wiring, and “Hidden” Upgrades
Electric: No Vent, Big Wires
- Pros: No combustion air or flue gas, so no venting required. Units are compact and whisper‑quiet.
- Cons: Whole‑home models often require multiple dedicated 240V circuits (for example, three 40–50A breakers) and heavy‑gauge wire runs. Many homes with 100–150A service must upgrade to 200A or more. That’s a real cost—possibly the biggest line item for electric conversions.
Gas: Venting and Gas Supply
- Pros: High output and better performance in cold‑water conditions; often the best fit for larger families or multi‑bath homes.
- Cons: Requires proper venting (material depends on model type) and sufficient gas line capacity. A tankless can demand up to ~200,000 BTU/h, several times more than a typical tank heater. You might need to upsize the gas line, sometimes the meter, and plan a condensate drain and neutralizer on condensing models.
Real‑world tip: When I’ve installed electric tankless in large houses, we sometimes had to pull multiple new electrical runs across long distances—labor‑intensive and not cheap. On gas jobs, we’ve had to reroute and upsize gas piping to meet the BTU demand. Either path can cost more than you expect, depending on where the unit lives and how your home is laid out.
Upfront Cost vs. Operating Cost
Upfront
- Unit prices (ballpark): Gas tankless often land higher than electric for the equipment alone. But install costs can swing either direction based on venting/gas upgrades (gas) or panel/circuit upgrades (electric).
- Retrofit rule of thumb: Sticking with your home’s existing energy type (gas‑to‑gas, electric‑to‑electric) typically reduces surprises and keeps labor costs in check.
Operating
- Energy rates vary by location. In many regions, natural gas is cheaper per BTU than electricity, which can make gas less expensive to run even if its raw efficiency is lower.
- However, if your electricity is inexpensive or your home uses renewables (solar with net metering, for example), electric can be competitive—sometimes cheaper.
Quick comparison method (rule‑of‑thumb):
- Find your average electricity rate (¢/kWh) and natural gas rate ($/therm).
- Convert to cost per 100,000 BTU delivered at typical efficiencies (e.g., 90% for gas, 99% for electric).
- Compare. The lower number is your likely winner on operating cost.
Lifespan and Maintenance
- Gas tankless commonly last around 20 years with proper care.
- Electric tankless often run around 15 years, but their simpler design can mean fewer moving parts.
Maintenance matters for both:
- Annual descaling/flush is smart in most regions, especially with hard water. Mineral buildup insulates the heat exchanger and kills efficiency.
- Gas‑specific tasks: Burner inspection/cleaning, vent checks, and a yearly performance test.
- Electric‑specific tasks: Fewer combustion‑related checks, but you still want that heat exchanger clean.
Pro tip: Install proper water treatment/filtration. Many manufacturers require it for warranty in hard‑water areas. Even a basic sediment filter and a scale‑reduction system can add years of efficient service.
Noise, Safety, and Reliability
Noise
- Gas models make noise when the burner and fan kick on, plus you’ll hear the exhaust fan.
- Electric models are nearly silent.
If the unit will sit near a bedroom (especially above a primary suite or behind a shared wall), noise can be a deciding factor.
Safety
- Gas introduces combustion. That means you must install and maintain venting correctly and treat gas leaks and carbon monoxide as non‑negotiable safety issues. Install CO detectors and keep vents intact.
- Electric removes combustion risks but adds high‑amperage electrical considerations. Your electrician must size wire, breakers, and service properly.
Reliability (Power Outages, Cold Snaps)
- Many gas tankless still need electricity for ignition and controls. In a power outage, that can stop hot water unless you have a generator or backup.
- Electric tankless obviously require power 100% of the time.
- In cold climates (or during extreme cold events), gas typically maintains capacity better than electric, thanks to its higher output. That margin can be the difference between an actual shower and a lukewarm drizzle when the whole family is home.
Climate and Groundwater Temperature: The Silent Influencer
Your incoming water temperature drives everything:
- Warm regions (incoming 65–75°F): Electric can be an excellent whole‑home solution for 1–2 bathrooms, maybe more if fixtures are low‑flow and simultaneous demand is modest.
- Moderate regions (incoming 55–65°F): Either works, but gas gives you headroom for multiple showers and laundry at once.
- Cold regions (incoming 35–55°F): Gas is usually the practical choice for whole‑home use. Electric may be better suited as point‑of‑use (e.g., for a single bathroom or accessory dwelling unit) or in ultra‑efficient homes with very low flow demands.
Incentives and Local Rules
Utilities and municipalities often offer rebates for high‑efficiency water heaters (both gas and electric). Codes may also nudge you toward condensing efficiency or require specific venting and condensate treatment. Always check local incentives and code requirements before you buy—those dollars can swing the decision.
New Build vs. Retrofit: Plan It Right
New Build (Best‑Case Scenario)
- Design for tankless from the start.
- For gas, spec a gas load calculation and run a line sized for ~200,000 BTU/h to your chosen location.
- For electric, design the service panel capacity and dedicated circuits (and conduit paths) up front.
- Plan the location near major hot‑water loads to reduce wait times and consider a recirculation loop (with controls) to save water and improve comfort.
Retrofit (Most Common)
- The most economical path is usually like‑for‑like energy (gas to gas, electric to electric).
- If you’re switching types, budget for structural changes: venting and piping (gas) or panel upgrades and heavy‑gauge wiring (electric).
Real‑World Scenarios to Guide You
- Two‑person condo in a warm climate (1–1.5 baths):
Electric tankless shines—no venting, quiet operation, and plenty of capacity for modest simultaneous use. - Family of five, 2.5–3.5 baths in a moderate/cold climate:
Gas tankless wins—headroom for morning rushes and laundry days. Add a recirculation pump if your runs are long. - Rural home on propane with frequent outages:
Gas tankless with a generator/backup can be a solid bet. Verify the propane supply and consider a high‑efficiency condensing model to stretch fuel. - Solar‑forward home with a 200–400A service and net metering:
Electric tankless can be attractive if your PV offsets usage. Confirm breaker space and wiring paths before committing.
Step‑by‑Step Decision Path
- Map Your Demand
- Count showers, tubs, and appliances that may run together. Estimate peak GPM (e.g., 2.0–2.5 GPM per shower, 1.5–2.0 GPM for a tub fill, 1.0–1.5 GPM for a dishwasher).
- Find Your Temperature Rise
- Subtract your incoming water temp from the desired setpoint (often 120°F). Your utility’s map or a quick thermometer reading at a cold tap can help.
- Do the Capacity Math
- Use GPM × ΔT × 500 to estimate BTU/h. If you need ~175,000 BTU/h or more, gas is the simpler fit for whole‑home.
- Audit Your Infrastructure
- Gas: Line size, meter capacity, vent path, condensate drain.
- Electric: Service size (200A+ for many whole‑home installs), breaker spaces, wire runs.
- Price the Upgrades
- Get real quotes for venting, gas piping, panel upgrades, and wiring. These line items—not the box on the wall—often decide the winner.
- Compare Energy Costs
- Use your bills to compare $/therm vs ¢/kWh in your area. Factor typical efficiencies (99% electric, 90–95% condensing gas).
- Weigh Noise & Location
- Bedrooms nearby? Electric’s silence might be worth it.
- Consider Outage Behavior
- If outages are common, plan a backup for either type (generator for gas controls or whole‑home backup for electric).
- Plan for Maintenance
- Budget time (or a service plan) for annual flushing and inspections. Add water treatment if hardness is moderate to high.
- Look for Rebates
- Incentives can easily offset part of your installation or upgrade costs.
Pro Tips from the Field
- Recirculation Done Right: A smart, on‑demand recirculation system gives you near‑instant hot water without wasting energy circulating all day. Some gas models include integrated recirc; electric solutions can use external pumps with controls.
- Dial in the Setpoint: 120°F is a sweet spot for comfort and safety in many homes. Higher setpoints increase scald risk and energy consumption.
- Mixing Valves Matter: Low‑flow fixtures can “starve” tankless units if flow dips below the activation threshold. Anti‑scald mixing valves and fixture choices should be coordinated with the heater’s minimum flow specs.
- Think Future‑Proof: Even if you’re not ready for tankless today, pre‑run the infrastructure during a remodel—vent sleeves, spare conduit, or a home‑run gas line branch. You’ll thank yourself later.
- Water Quality First: If your water is hard or full of sediment, treat it. You’ll protect the heat exchanger, preserve efficiency, and keep warranty conversations easy.
Quick Comparison at a Glance
Gas Tankless
- Strengths: High output, better for cold climates and big families, solid for multi‑fixture use.
- Watch‑outs: Venting, combustion safety, potential gas line/meter upsizing, unit noise, annual burner/vent checks.
Electric Tankless
- Strengths: Ultra‑high efficiency, compact, no venting, quiet, simpler maintenance.
- Watch‑outs: Heavy electrical demand (multiple high‑amp circuits), possible panel/service upgrade, limited whole‑home capacity in cold climates.
So…Which Should You Choose?
Here’s the blunt answer:
- If you live in a cold‑water region or regularly run multiple showers/appliances at once, gas tankless is usually the right tool. It delivers the flow and temperature rise without living on the edge of capacity.
- If you’re in a warm climate, have a smaller household, or already have a beefy electrical service (or home solar), electric tankless can be a clean, efficient, and quiet solution with a simpler install.
- When retrofitting, follow your house. If your home already has robust gas infrastructure but a modest panel, go gas. If you have a modern 200–400A panel and a great path for circuits—but no good vent route—go electric.
Whichever path you take, treat the project like the whole‑home system change it is: size it correctly, budget for the right upgrades, plan for maintenance, and protect the heat exchanger with proper water treatment. Do that, and your tankless—gas or electric—will run efficiently, quietly, and reliably for years.
Conclusion
Choosing between gas and electric tankless isn’t about which technology is “better” in a vacuum—it’s about which one fits your home and lifestyle. Electric wins on near‑perfect efficiency, compact size, and no venting. Gas wins on sheer hot‑water output, especially when winter bites and multiple fixtures run at once. Use the sizing math, look honestly at your infrastructure, and pencil out both the installation and operating costs. Plan for annual maintenance and address your water quality from day one.
Do that, and you’ll enjoy exactly what tankless promises: consistent comfort, smart use of space, and energy savings that make sense for your specific situation.
