Understanding the Energy Factor (EF) in Tankless Water Heaters

When shopping for a tankless water heater, one of the most important numbers you will encounter is the Energy Factor (EF). This single metric distills the unit's overall efficiency into a meaningful figure that directly impacts your utility bills and environmental footprint. While the EF rating has been a standard for decades, its calculation, interpretation, and applicability require careful understanding—especially as newer metrics like the Uniform Energy Factor (UEF) have become the official standard. This article provides a comprehensive, in-depth look at the Energy Factor for tankless water heaters, covering how it works, why it matters, what influences it, and how to use it effectively when choosing your next water heating solution.

What Is the Energy Factor (EF)?

The Energy Factor (EF) is a rating developed by the U.S. Department of Energy (DOE) to measure the overall energy efficiency of a water heater. It represents the ratio of useful thermal energy delivered to the household (heat added to the water) divided by the total energy consumed by the appliance over a standard test period. The result is a number between 0 and 1 (or expressed as a percentage). For example, an EF of 0.92 means that 92% of the energy consumed goes into heating water, while the remaining 8% is lost through standby heat loss, combustion inefficiency, or other operational waste.

Importantly, EF is not a measure of instantaneous or peak efficiency. It accounts for real-world losses that occur during a typical day of use, including:

  • Standby losses – heat escaping from the unit when no hot water is being drawn. Traditional tank heaters suffer heavily here; tankless models have minimal standby losses because they lack a stored water volume.
  • Recovery efficiency – how effectively the burner or heating element transfers heat to the water during operation.
  • Cycling losses – energy wasted during start-up and shut-down cycles (more relevant for tank models, but also present in some tankless systems with recirculation loops).

The EF is determined using a standardized DOE test procedure that simulates a typical household's hot water usage pattern over 24 hours. This makes EF a useful metric for comparing different water heater types and models on a level playing field.

How EF Is Calculated: The Formula and Test Protocol

Understanding the calculation method helps you appreciate why tankless water heaters often achieve higher EF ratings than storage tank heaters. The basic formula is:

EF = (Total useful heat output) / (Total energy input)

The DOE test procedure (details available at Energy.gov) involves the following key steps:

  • A set number of hot water draws are taken at specified flow rates and temperatures over a 24-hour period. The draws mimic morning showers, dishwashing, laundry, etc.
  • The water heater is allowed to run in its normal operating mode. For gas units, the energy content of the fuel is measured; for electric units, kilowatt-hours are metered.
  • The total thermal energy delivered to the water (temperature rise × flow × specific heat) is accurately measured.
  • The EF is computed as the ratio of delivered thermal energy to the energy consumed by the appliance (including standby energy, pilot lights for gas units, and other auxiliary power).

For tankless water heaters, standby losses are minimal because there is no stored water to cool down. However, they do have a small amount of thermal mass in the heat exchanger, and some units have a small "false load" to manage condensation or maintain a minimum temperature. The dominant energy consumption occurs only when hot water is flowing. This is why tankless water heaters typically post EF values between 0.82 and 0.98, whereas standard storage tank water heaters rarely exceed 0.67 (gas) or 0.95 (electric heat pump hybrids).

Important Caveat: The Transition from EF to UEF

As of 2015, the DOE replaced EF with the Uniform Energy Factor (UEF) for all water heaters manufactured after April 2015. The UEF uses a similar concept but adjusts the test procedure to better reflect real-world usage across different unit sizes and types. New water heater labels now show UEF instead of EF. However, many consumers still reference EF, and existing units with EF ratings are still in service. For tankless water heaters, the UEF values are essentially comparable to EF values but may be slightly lower due to the updated test procedure that accounts for more realistic draw patterns (see DOE heat pump water heater page for UEF details).

When comparing older EF ratings to newer UEF ratings, you should expect that a 0.92 EF model will have a UEF close to 0.90 or 0.91. Always check the most recent label on the unit you are considering.

Why EF (or UEF) Matters for Tankless Water Heaters

The Energy Factor directly influences operating costs. A higher EF means that more of every dollar you spend on gas or electricity actually heats your water. Over the 15–20 year life of a tankless water heater, even a 5% difference in EF can translate into hundreds of dollars in savings. Additionally, higher efficiency reduces greenhouse gas emissions—especially if your electricity comes from fossil fuels, or if you are using natural gas.

However, EF is not the only factor. Tankless water heaters also have a flow rate limit (gallons per minute, or GPM) that determines how much hot water they can deliver simultaneously. A unit with an excellent EF but insufficient GPM for your household will cause frustration. Therefore, efficiency must be balanced with capacity.

Typical EF Ranges for Tankless Models

Here are representative EF (or UEF) values for common tankless categories:

  • Electric tankless – EF 0.97 to 0.99. Electric heaters have near-100% conversion of electricity to heat, so their EF is very high. However, electricity often costs more per BTU than gas.
  • Gas (natural gas or propane) condensing tankless – EF 0.92 to 0.98. These units capture latent heat from flue gases, achieving very high efficiency.
  • Gas non-condensing tankless – EF 0.82 to 0.86. They vent hot exhaust directly, losing significant heat. They are cheaper upfront but less efficient.
  • Outdoor vs. indoor installation – Outdoor units may have slightly lower EF because they are exposed to weather, but the difference is small.

Compare these to storage tank water heaters: gas storage EF 0.59–0.67, electric resistance storage EF 0.90–0.95, and heat pump water heaters EF 2.0–3.5 (coefficient of performance, not EF). Clearly, tankless units shine in the gas category, and electric tankless narrowly beat electric storage.

Factors That Influence the EF of a Tankless Water Heater

Manufacturers test units under ideal laboratory conditions. Real-world EF depends on many variables. Understanding these will help you choose the right unit and use it optimally.

1. Inlet Water Temperature

Groundwater temperature varies by region and season. Cold incoming water requires more energy to heat, reducing the unit's ability to maintain a high flow rate at the desired temperature. This does not change the EF per se, but it affects the practical energy usage for a given hot water demand. In northern climates, a larger unit may be needed to avoid overwhelming the heater.

2. Set Temperature

Higher thermostat settings increase the temperature rise needed, thus more energy consumed per gallon. While EF remains constant in the test protocol (using standardized draw temperatures), your actual energy consumption grows if you set the water heater to 140°F instead of 120°F. Lowering the set point is a simple way to improve real-world efficiency.

3. Installation Configuration

Long hot water pipes lose heat to the surrounding air. Insulating pipes reduces standby losses in the distribution system. Recirculation loops, if not properly controlled, can dramatically increase energy use because they force the tankless unit to fire frequently to maintain hot water in the loop. Some condensing tankless units have internal recirculation pumps with temperature sensors that minimize this penalty.

4. Maintenance

Mineral buildup (scale) on the heat exchanger reduces heat transfer efficiency. In gas units, soot accumulation can impair combustion. Annual descaling and cleaning help maintain the rated EF. Failure to maintain can degrade EF by 5–15% over time (NRDC maintenance tips).

5. Altitude

At higher elevations, natural gas contains less oxygen per cubic foot, reducing burner output. Tankless manufacturers often derate the BTU capacity at altitude, which can slightly affect efficiency because the unit may run longer to meet demand. The EF rating is measured at sea level; subtract 2–4% per 1000 feet of elevation for gas models.

6. Flow Rate

A tankless water heater operates at maximum efficiency when the flow rate is in the middle of its range—neither too low (where the unit may short-cycle) nor too high (where efficiency declines due to excessive firing rate). If you only use a trickle of hot water, some models may not even activate, but those that do will likely run at low fire, which can be less efficient. Matching the unit size to your typical demand is critical.

Comparing Tankless EF with Other Water Heater Types

Gas storage tank (conventional): EF 0.59–0.67. These are the least efficient. The bulk of loss is standby heat loss through the tank walls and flue. Upgrading to a tankless can almost double the efficiency.

Electric resistance storage tank: EF 0.90–0.95. The EF is high because electric elements convert nearly all electricity to heat. However, the cost per kWh is often higher than gas per BTU, and the tank still loses heat to the surroundings. Tankless electric units can achieve EF 0.97–0.99, but they require a very high electrical load (up to 28 kW), which may necessitate a panel upgrade.

Heat pump water heater (hybrid): These have an EF typically reported as 2.0–3.5 (coefficient of performance, not the same scale). They are far more efficient than any resistance or gas option for most climates, but they cost more and require a conditioned space. They are not tankless; they use a large storage tank.

Solar water heater: EF is not directly applicable because they rely on solar energy. Instead, the Solar Energy Factor (SEF) is used.

For most households replacing a gas storage tank, a condensing gas tankless water heater offers the best balance of efficiency, upfront cost, and operational freedom.

Beyond EF: Other Efficiency Metrics You Should Know

While EF (or UEF) is the headline number, modern tankless water heaters also come with additional ratings:

  • Uniform Energy Factor (UEF) – As noted, this is the current standard. Look for UEF on the EnergyGuide label.
  • First Hour Rating (FHR) – Not applicable to tankless because they don’t have a storage tank. For tankless, the key is Maximum Flow Rate at a given temperature rise.
  • Thermal Efficiency (η_th) – Some manufacturers list this separately. It is the steady-state efficiency while firing—typically 0.94–0.98 for condensing units. It does not include standby losses.
  • Energy Star Certification – To earn the Energy Star label, a gas tankless water heater must have a UEF of at least 0.90 (varies by size). Electric tankless must have a UEF of at least 0.93. Energy Star models are verified by third-party testing (Energy Star tankless water heaters).

How to Use EF/UEF When Choosing a Tankless Water Heater

Start by determining your required flow rate (GPM) at the desired temperature rise (groundwater temp to outlet temp). A typical shower uses 2.0 GPM; a kitchen sink 1.5 GPM; a washing machine 2.0 GPM. Add up the max simultaneous demand. Choose a model that meets or exceeds that flow rate. Then, among units of that capacity, compare UEF ratings. The highest UEF will give the lowest operating cost, but factor in the unit's price, installation complexity, and warranty.

Do not sacrifice capacity for a slightly higher UEF. An undersized unit that struggles to deliver hot water will be used inefficiently (low flow, long run times) and frustrate you. Similarly, avoid grossly oversized units; they cost more and may short-cycle if flow rates are too low, slightly reducing efficiency.

Installation Considerations That Affect Real-World EF

Installing a tankless water heater is not a simple swap. The following factors can make or break your actual energy savings:

Gas Line Sizing

Gas tankless units typically require a larger gas supply line (3/4" or 1") and higher BTU input (150,000–199,000 BTU/hr) than a storage tank heater (30,000–50,000 BTU/hr). If your existing gas line is undersized, you may need to upgrade it. An undersized line causes low gas pressure, leading to incomplete combustion and poorer efficiency. Always have a licensed plumber or gas fitter perform a load calculation.

Venting

Non-condensing units require stainless steel venting (Category III), while condensing units can use PVC or CPVC because the exhaust is cooler. Using the wrong vent material can lead to corrosion and safety hazards. Proper venting also ensures the unit operates at its rated efficiency by maintaining correct draft.

Electrical Requirements

Even gas tankless heaters need electricity (120V, 3 amps or so) for the electronic controls, fan, and ignition. In a power outage, a gas tankless will not produce hot water unless you have a backup generator. This is a disadvantage compared to a conventional gas storage tank (which may have a standing pilot and produce hot water without power, albeit inefficiently).

Water Quality and Scale Prevention

If you have hard water (calcium > 7 grains per gallon), install a whole-house water softener or a scale inhibitor before the tankless unit. Scale buildup drastically reduces heat transfer efficiency and can void the warranty. Annual descaling using vinegar or a commercial descaler is still recommended.

Maintenance to Preserve High EF

Unlike storage tank water heaters, tankless units require regular servicing to maintain their efficiency. Neglecting maintenance is the number one reason for premature failure and efficiency decline.

  • Annual descaling – For gas units, descale the heat exchanger to remove mineral deposits. This restores heat transfer efficiency and can recover 5–10% in lost performance.
  • Check and clean the air filter – Most indoor units have a cleanable air filter. A dirty filter restricts airflow, reducing combustion efficiency.
  • Inspect the burner and flame sensor – Soot or dust can cause uneven burning and wasted fuel. A technician can visually inspect and clean if needed.
  • Flush the system – For electric tankless, flushing the heating elements prevents scale buildup. For gas, flushing the entire water side (heat exchanger) is critical.
  • Test the condensate drain – Condensing units produce acidic condensate that must be neutralized and drained. A blocked drain can cause the unit to shut down.

Following the manufacturer's maintenance schedule ensures your tankless water heater retains its factory EF rating year after year. Some brands offer extended warranties if you use authorized service providers.

Common Misconceptions About EF and Tankless Efficiency

“A higher EF always saves money.” Not necessarily if the unit is oversized or if you pay a high premium for the highest EF model. The difference between EF 0.92 and 0.98 is about 6% energy savings. If the more efficient model costs $500 more, the payback may be 10+ years depending on usage. Do the math.

“Tankless water heaters are always more efficient than storage tanks.” For gas, yes—very much so. For electric, the difference is small (EF 0.97 vs 0.92). The real advantage of electric tankless is space savings and endless hot water, not necessarily energy savings.

“You can use a tiny tankless unit if you have low-flow fixtures.” Not entirely. The unit must still provide a minimum flow rate (usually 0.5 GPM) to activate. Very low flow (like a trickle from a faucet) may not trigger the heater, so you get cold water. A small recirculation pump or a point-of-use tankless might be needed.

Conclusion

The Energy Factor (EF) remains a valuable concept for understanding tankless water heater efficiency, even as the industry has moved to the Uniform Energy Factor (UEF). A higher EF directly translates into lower operating costs and reduced environmental impact. However, EF is just one piece of the puzzle. Real-world efficiency depends on proper sizing, installation, maintenance, and usage patterns. When evaluating a tankless water heater, consider the UEF rating alongside flow capacity, fuel type, upfront investment, and installation requirements. By taking a comprehensive approach, you can choose a water heating solution that delivers both energy savings and reliable comfort for years to come.

For further reading, refer to the U.S. Department of Energy's Water Heating Guide and Energy Star Tankless Water Heaters for certified models. If you have hard water, check the Water Quality Association's scale resources.