Key Summary

The Accelera 220 E and 300 E are high-efficiency heat pump water heaters that share the same operating temperature range, noise level, airflow, booster heater, and electrical requirements. The big differences are storage capacity, first-hour rating, and efficiency, which make the 220 E a better fit for smaller homes with moderate hot water use and the 300 E better for larger households or frequent back‑to‑back draws. Choosing the right one means matching tank size and performance to your household size, hot water patterns, and installation constraints rather than assuming bigger is always better.

TL;DR

• The Accelera 220 E has a 58-gallon tank, UEF 3.115, and a first-hour rating of 50.3 gallons; it suits 1–3 people with moderate hot water use.
• The Accelera 300 E has a 79.8-gallon tank, UEF 3.609, and a first-hour rating of 74.2 gallons; it suits 3–5+ people or high-demand homes.
• Both units share the same 42°F–108°F operating air temperature range, 60 dB sound power level, 324 CFM airflow, 1500W booster heater, and 15-amp 240-volt electrical requirements.
• DOE estimates list yearly usage at 1,040 kWh for the 220 E and 1,289 kWh for the 300 E, with estimated costs of $169 vs. $155 based on standard rate assumptions and regional averages.
• Pick the 220 E when space is tight, hot water demand is modest, and up-front cost is a priority; choose the 300 E when you regularly run multiple showers, laundry, and dishwashing close together.
• For many typical families of 3–4, the 300 E offers more comfort margin and better efficiency, but the 220 E can be the smarter choice in smaller, efficient homes or mild climates.

Product Introduction

If you are browsing an e-commerce site for a heat pump water heater, you will often see both the Stiebel Eltron Accelera 220 E and 300 E listed side by side. On paper they look almost identical—same brand, same technology, same electrical and noise specs—but the differences in tank size, efficiency, and first-hour rating have big implications for comfort and operating cost. The goal of this guide is to make those trade-offs crystal clear so you can confidently select the model that matches your home’s hot water needs, without guesswork.

Accelera 220 E vs 300 E: Quick Specs Comparison

Before diving into detailed analysis, it helps to see how the Accelera 220 E and 300 E compare on the core numbers. These are the specs that matter most for homeowners and light-commercial users deciding between the two models.

• Capacity: Accelera 220 E = 58 gallons; Accelera 300 E = 79.8 gallons.
• Uniform Energy Factor (UEF): Accelera 220 E = 3.115; Accelera 300 E = 3.609.
• First-hour rating (FHR): Accelera 220 E = 50.3 gallons; Accelera 300 E = 74.2 gallons.
• Operating air temperature range: Both 42°F to 108°F.
• Sound power level: Both 60 dB, with a sound pressure level of about 52 dB at 3.3 feet.
• Airflow: Both 324 CFM, affecting how much air they move through the space.
• Booster heater: Both include a 1500W electric booster heating element.
• Electrical: Both are 220–240 V, 60 Hz with a 15-amp circuit breaker requirement.
• DOE estimated yearly energy use: 220 E = 1,040 kWh; 300 E = 1,289 kWh.
• DOE estimated yearly energy cost: 220 E = $169; 300 E = $155 (based on national average rates and testing assumptions).

The core trade-off is straightforward: the Accelera 300 E stores and delivers more hot water and is more efficient on paper, but it is larger, heavier, and typically costs more up front. The Accelera 220 E is more compact and uses less electricity in absolute terms but has less hot water available for back‑to‑back draws.

Capacity and First-Hour Rating: How Much Hot Water You Actually Get

Tank capacity and first-hour rating are the most important numbers for day-to-day comfort. They determine whether you can run two showers and the dishwasher at the same time without running out of hot water—or whether someone ends up with a cold rinse.

Tank size: 58 gallons vs 79.8 gallons

The Accelera 220 E has a 58-gallon tank, while the Accelera 300 E offers 79.8 gallons. That roughly 22-gallon difference is significant. Once the stored hot water is drawn down, both units rely mainly on their heat pumps (and occasionally the booster heater) to reheat incoming cold water, which is slower than a traditional gas or standard electric resistance heater.

In practical terms, the larger 79.8-gallon tank gives the 300 E much more buffer during peak usage periods—morning showers, evening baths, or times when laundry and dishwashing overlap. For homes that rarely push the limits, the smaller 58-gallon tank of the 220 E may still be perfectly adequate, especially if occupants tend to stagger hot water use.

First-hour rating: why 50.3 gallons vs 74.2 gallons matters

First-hour rating (FHR) is the best single number to compare how much hot water you can get from a fully heated tank in the first hour of use. It combines storage capacity and recovery rate. The Accelera 220 E has an FHR of 50.3 gallons, while the Accelera 300 E reaches 74.2 gallons.

That 24-gallon gap is roughly the size of an extra medium-length shower. In a scenario where three people want morning showers back-to-back, the 300 E is better equipped to keep up. The 220 E is more likely to require some spacing between uses or the occasional engagement of the booster element to maintain comfortable temperatures.

• 1–2 person home with staggered use: Both models can work; the 220 E is usually sufficient.
• 3–4 person home with overlapping showers and laundry: The 300 E provides a more comfortable margin and reduces the risk of running out.
• 5+ people or light-commercial usage (small office, studio, rental): The 300 E’s higher FHR is generally the safer choice.

If you have ever had to race family members to the shower to avoid lukewarm water, prioritizing first-hour rating and tank size is more important than splitting hairs over small efficiency differences.

Efficiency and Operating Cost: UEF 3.115 vs 3.609

Both Accelera models are highly efficient compared with standard electric resistance water heaters, but they do differ in their official Uniform Energy Factor (UEF). UEF is a standardized measure that reflects how efficiently a water heater converts electricity into hot water under prescribed test conditions.

What UEF means in real life

The Accelera 220 E is rated at a UEF of 3.115, while the Accelera 300 E has a higher UEF of 3.609. That means, under the test procedure, the 300 E delivers more hot water per unit of electricity consumed than the 220 E. Put another way, for the same amount of delivered hot water, the 300 E should use less electricity.

However, UEF is not the only factor that drives your bill. The total energy your water heater uses depends on:

• How much hot water you use over the year (showers, baths, laundry, dishwashing).
• Incoming water temperature (colder climates require more heating).
• Setpoint temperature on the water heater.
• Where the unit is installed (conditioned vs unconditioned space).
• Local electricity rates, including time-of-use pricing if applicable.

In climates where the heat pump can operate most of the year without relying heavily on the booster element, the higher UEF of the 300 E can translate into noticeable electricity savings for households with substantial hot water use.

DOE estimated yearly energy use and cost

The Department of Energy (DOE) provides standardized estimates to help compare models. For the Accelera 220 E, the estimated yearly energy use is 1,040 kWh with a corresponding estimated yearly cost of $169. For the Accelera 300 E, the estimated yearly energy use is higher at 1,289 kWh, but the estimated cost is listed as $155.

At first glance, it can be confusing that a unit using more kWh is shown with a lower cost. This is due to how DOE’s test procedures and rate assumptions interact with each model’s performance and tested usage patterns, not a guarantee that you will always pay less to operate the larger unit. In reality, your bill will follow your local kWh rate: if you use more electricity, you will typically pay more overall.

What matters for comparison is that both units are highly efficient heat pump designs compared with a conventional electric tank, and the 300 E’s higher UEF means it is relatively more efficient at turning electricity into hot water. If your household has high hot water usage, that higher efficiency can offset part of the extra energy needed to heat a larger tank and serve more demand.

Shared Operating Characteristics: What’s the Same?

Although the 220 E and 300 E differ in size and performance ratings, they share many core design features. Understanding these similarities can simplify the decision if you already know you want a Stiebel Eltron Accelera heat pump water heater.

Operating temperature range: 42°F to 108°F

Both models are designed to operate in ambient air temperatures between 42°F and 108°F. Within that range, the heat pump can extract heat from the surrounding air to warm your water. Below this range, performance drops and the unit may need to rely more on the electric booster element, which raises operating cost.

For homeowners, this means installation location matters more than which Accelera model you choose. Placing the unit in a space that reliably stays within this temperature window—such as a conditioned basement or interior mechanical room—will support good efficiency and consistent performance for either model.

Noise and airflow: 60 dB and 324 CFM

Both units have a sound power level of 60 dB and move 324 cubic feet per minute (CFM) of air through their heat pump. A sound pressure level of about 52 dB at 3.3 feet is comparable to a quiet conversation or a typical refrigerator compressor running in the background.

Because airflow and noise output are identical, the same acoustic and ventilation considerations apply to both models:

• Avoid installing them directly next to bedrooms or quiet living areas when possible.
• Provide adequate room volume or ducting so the unit can move enough air without creating drafts or temperature swings in tight spaces.
• In small or tightly sealed homes, working with an HVAC professional to plan airflow and ventilation can help avoid unintended cooling or noise issues.

Booster heater and electrical requirements

Both the Accelera 220 E and 300 E include a 1500W electric booster element that assists the heat pump when demand is high or ambient air conditions are less favorable. This element is less efficient than the heat pump itself but ensures you still have hot water when you need it most.

Electrically, both require 220–240 V, 60 Hz power and are designed to operate on a 15-amp circuit breaker. This relatively modest electrical requirement makes them easier to integrate into existing electrical panels than some larger electric water heaters that need 30- or 40-amp circuits.

From a homeowner’s perspective, this means that the choice between the 220 E and 300 E usually does not come down to electrical capacity. If your home can support one, it can usually support the other, though it is always wise to have an electrician confirm panel capacity and code compliance.

Physical Size, Weight, and Installation Constraints

Beyond performance, the physical dimensions and weight of the water heater have a real impact on whether it will fit in your chosen location and how difficult installation will be. The Accelera 220 E and 300 E share the same diameter but differ meaningfully in height and weight.

Height, diameter, and clearance

Both models have a diameter of about 27 1/8 inches, so floor footprint is the same. The big difference is height:

• Accelera 220 E height: approximately 60 7/8 inches.
• Accelera 300 E height: approximately 75 1/4 inches.

In spaces with low ceilings, overhead obstructions, or limited access (such as under stairs or in existing utility closets), the extra 14 inches of height on the 300 E can be a dealbreaker. You will also need to account for any manufacturer-recommended top clearance for service and proper air circulation.

If your home has a cramped mechanical room or you are replacing a shorter tank that sits under ductwork or joists, measuring carefully before choosing the 300 E is essential.

Weight: dry and full

The Accelera 220 E weighs about 264.5 pounds dry and 748.5 pounds when full of water. The Accelera 300 E is heavier, at about 297.6 pounds dry and 956.6 pounds full. These weights matter for transport, installation, and in some cases structural support.

For most ground-level or basement installations on concrete slabs, the extra weight of the 300 E is not a problem. In older homes with wood-framed floors, raised platforms, or second-floor utility rooms, it is wise to confirm that the structure can safely support a nearly 1,000-pound appliance plus any occupants and adjacent equipment.

From an installation logistics standpoint, the lighter and shorter 220 E can be easier to maneuver down tight staircases, around corners, or through narrow doors. If your access path is constrained, this may tilt the decision toward the 220 E even if the 300 E’s performance is attractive.

Comfort for Different Household Sizes and Usage Patterns

Matching a water heater to a household is less about chasing maximum specs and more about aligning capacity with real-world usage patterns. Below are common scenarios and which Accelera model tends to be the better fit in each case, assuming appropriate installation conditions.

Small households (1–2 people)

In a one- or two-person home, especially in an efficient house or apartment with low-flow fixtures, the Accelera 220 E typically offers enough capacity and first-hour performance. Daily hot water use is usually modest, and shower or bath times can be easily staggered without inconvenience.

In this size range, the 300 E may be considered if the occupants take frequent long baths, have a large soaking tub, or expect occasional guests and want extra margin. However, the larger tank will also store more hot water than is regularly needed, which can increase standby losses slightly, especially if the ambient space is significantly cooler than the house.

Average families (3–4 people)

For a typical family of three or four, the choice is more nuanced. Morning routines often involve overlapping showers, and laundry and dishwashing add to total daily use. In many of these homes, the Accelera 300 E is the more forgiving choice, thanks to its higher first-hour rating and larger tank.

If your family schedules are naturally staggered—some showers at night, some in the morning, laundry during the day—the 220 E can still work, particularly in mild climates where the heat pump operates efficiently and recoveries are faster. But if your household tends to cluster hot water use into tight windows, the 300 E’s additional buffer can reduce the risk of running short and minimize reliance on the booster heater.

Larger households (5+ people) and light-commercial applications

In homes with five or more occupants, or in light-commercial settings like small offices, studios, or shared rental properties, the Accelera 300 E is generally the more appropriate starting point. Its 79.8-gallon capacity and 74.2-gallon first-hour rating provide a substantially larger supply of readily available hot water.

Even with the 300 E, it is important to recognize that a single heat pump water heater has limits. High-usage commercial or multi-family scenarios may warrant multiple units, a centralized commercial water heating system, or additional design support from a plumbing or mechanical engineer. In such cases, the 220 E is typically better suited as a secondary or point-of-use unit rather than the main hot water source.

Homes with large bathtubs or frequent soaking

Large soaking tubs, jetted tubs, or long, hot showers can quickly draw down a smaller tank. If you frequently fill a large tub or enjoy extended shower times, the 300 E’s extra 21.8 gallons of storage and higher first-hour rating are clear advantages.

The 220 E can support occasional large draws, but recovery will be slower, and you may notice more temperature drop-off for subsequent users. In such homes, spending more upfront on the 300 E can provide a better comfort experience and more consistent hot water availability.

Climate, Location, and Interaction with Your Home

Because both Accelera models are heat pump water heaters, they do more than just make hot water. They also move heat from the surrounding air into the tank, which slightly cools and dehumidifies the space where they are installed. The impact of that interaction depends on your climate and installation location, not on whether you choose the 220 E or 300 E.

Cooler climates and basements

In cooler climates, these units are often installed in basements or utility rooms. As long as ambient temperatures stay above 42°F, the heat pump can function, but performance improves at higher temperatures. If your basement sometimes gets close to the lower end of the operating range, either model may need to rely more on the booster element during the coldest months, reducing overall efficiency.

Because both units share the same operating temperature range, this is not a deciding factor between them. Instead, consider whether the larger 300 E will cool the basement slightly more during heavy use or require more airflow planning due to higher usage. In most cases, the difference will be modest compared to the impact of installation location and overall building design.

Warm climates and garages

In warm or hot climates, installing a heat pump water heater in a garage or unconditioned space can make good use of excess heat. The ambient air is often within or above the ideal temperature range, improving efficiency. Again, since both models share the same operating range, the main question is physical fit and whether the garage remains within recommended temperatures year-round.

If the garage occasionally drops below 42°F, you may see both models rely more on the booster heater during those periods, raising operating cost temporarily. Insulation and strategic placement away from exterior doors can help moderate these swings.

Interaction with space conditioning

Inside conditioned space, a heat pump water heater effectively moves heat from your home’s air into the water. In summer, this can be a modest benefit, slightly reducing the load on your air conditioner. In winter, it adds a small heating burden, as your space heating system must compensate for the heat transferred into the tank.

Because both Accelera models share similar airflow and capacity to remove heat from the surrounding air, the difference between them in terms of whole-home energy balance is relatively small. Your choice of unit should still be guided primarily by hot water needs and installation constraints, with these secondary interactions considered as fine-tuning rather than primary decision drivers.

Maintenance, Reliability, and Service Considerations

The Accelera 220 E and 300 E share the same basic technology platform, so their maintenance requirements and reliability profiles are similar. Both are designed as premium, long-life appliances compared with commodity electric tanks, but they still benefit from consistent care and proper installation.

Routine maintenance tasks

Key routine tasks are similar for both models and typically include:

• Checking and, if needed, flushing the tank to manage sediment, especially in hard water areas.
• Inspecting the condensate drain to ensure water is draining properly and not pooling beneath the unit.
• Keeping air intake and exhaust areas free of dust or obstructions to maintain proper airflow and efficiency.
• Have a qualified technician inspect plumbing, electrical connections, and safety devices periodically.

Because these maintenance tasks are effectively the same for both units, they do not provide a strong reason to prefer one over the other. Instead, plan for a regular maintenance schedule as part of protecting your investment in either model.

Service access and space

Service access is another practical concern. Regardless of model, the unit should be installed with adequate clearances for technicians to reach controls, electrical connections, and plumbing connections safely. In very tight spaces, the shorter 220 E may be somewhat easier to work around, particularly when overhead clearance is limited.

When choosing between models in a cramped mechanical room, it may be useful to consider not just whether the taller 300 E fits, but whether it still leaves enough space for comfortable service over the life of the system. Squeezing in a larger tank without enough working room can raise labor costs for future repairs or shorten the unit’s life if maintenance is deferred because access is difficult.

Upfront Cost vs Long-Term Value

While specific prices vary by retailer, installer, and region, the Accelera 300 E generally costs more to purchase and install than the 220 E. The trade-off is greater capacity and higher efficiency, which can translate into better comfort and potentially lower cost per unit of hot water delivered over time.

When the Accelera 220 E offers better value

The 220 E can provide better overall value in situations where its capacity is a good match and the added performance of the 300 E would go largely unused. Examples include:

• Compact homes, accessory dwelling units (ADUs), or apartments with 1–2 occupants and no large soaking tub.
• Energy-efficient homes with low-flow fixtures and carefully managed hot water use.
• Installations with tight height restrictions where the 300 E simply does not fit comfortably.
• Budgets where a significant price gap exists and savings could be better spent on insulation, air sealing, or other envelope upgrades that also reduce hot water demand.

When the Accelera 300 E’s advantages justify the cost

The 300 E’s larger tank and higher efficiency can justify the additional upfront investment in scenarios where comfort and performance are at a premium:

• Families of 3–5 people who routinely stack showers, laundry, and dishwashing in short time windows.
• Homes with soaking tubs or rain showers that use a high volume of hot water per session.
• Light-commercial or rental properties where higher usage is likely and running out of hot water has a larger impact on comfort or satisfaction.
• Households in regions with high electricity prices where improved efficiency can provide more noticeable operating savings over the life of the unit.

In many typical family homes, the 300 E offers a stronger balance of comfort and efficiency, while the 220 E is better positioned as a solution for smaller, lower-demand residences or constrained installation conditions.

Strengths and Limitations of Each Model

Both Accelera models share core strengths as heat pump water heaters, such as high efficiency, reduced greenhouse gas emissions when paired with clean electricity, and quieter operation than many traditional mechanical systems. But each model also has unique advantages and trade-offs that may matter in your decision.

Accelera 220 E: strengths

Key strengths of the 220 E include:

• More compact height, making it easier to fit in low-ceiling basements, closets, and under mechanical obstructions.
• Lower dry and full weight, which can help in tight or structurally limited installations.
• Lower absolute yearly energy use (1,040 kWh estimate) compared with the 300 E, assuming similar usage profiles.
• Typically lower purchase cost, making high-efficiency water heating more accessible in smaller homes or on tighter budgets.

For homeowners whose hot water demand is modest, these attributes make the 220 E a sensible, efficient choice that avoids oversizing.

Accelera 220 E: limitations

The 220 E’s main limitations are directly tied to its smaller size and slightly lower efficiency:

• Lower first-hour rating (50.3 gallons) makes it more vulnerable to running out of hot water during peak periods in larger households.
• Lower UEF (3.115) means that, for the same delivered hot water, it uses more electricity than the 300 E under test conditions.
• Less suited to homes with large bathtubs, high-flow showers, or frequent overlapping use events.

These limitations do not matter much in smaller households, but they can become noticeable as the number of occupants and intensity of hot water use increase.

Accelera 300 E: strengths

The Accelera 300 E stands out in several ways:

• Larger 79.8-gallon tank provides a substantial buffer for peak demand periods.
• Higher first-hour rating of 74.2 gallons supports multiple back‑to‑back showers and concurrent uses.
• Higher UEF of 3.609 reflects better efficiency at converting electricity into hot water under standardized conditions.
• Better alignment with the needs of typical family homes and small commercial spaces that have higher or more clustered hot water demand.

For many homeowners, these strengths make the 300 E a more future-proof choice, offering capacity and efficiency that can comfortably handle changing household needs over time.

Accelera 300 E: limitations

The 300 E does come with trade-offs:

• Taller height (about 75 1/4 inches) can limit installation options in some basements and closets.
• Higher dry and wet weight complicates transport and may require more attention to structural support in some homes.
• Typically higher upfront cost than the 220 E, which may stretch budgets in smaller or more price-sensitive projects.

In homes where space or budget constraints are tight and hot water demand is modest, these limitations may outweigh the advantages of the larger tank and higher efficiency.

How to Decide: A Simple Step-by-Step Selection Guide

To choose between the Stiebel Eltron Accelera 220 E and 300 E, it helps to follow a structured process rather than focusing on a single spec. Here is a simple way to narrow down your choice.

Step 1: Confirm installation space and access

Measure the height and width of your intended installation location and the path you will use to move the unit into place. Confirm that:

• Either the 220 E or 300 E can physically pass through doors, stairwells, and hallways.
• Ceiling height and overhead obstructions allow for the full height of the chosen model plus any recommended clearance.
• Floor structure can support the full wet weight of the unit plus a safety margin.

If only the shorter 220 E fits with reasonable clearances and safe support, your choice may be made for you at this step.

Step 2: Estimate your household’s peak hot water demand

Consider how your household actually uses hot water during its busiest hour. Ask:

• How many showers are likely to run back-to-back or at the same time?
• Do laundry and dishwashing often happen during that same time window?
• Is there a large bathtub that gets frequent use?

If your peak demand seems modest—one or two showers and occasional appliance use—the 220 E may be sufficient. If you routinely push your existing water heater’s limits, or if running out of hot water is a common complaint, the 300 E’s higher first-hour rating will likely serve you better.

Step 3: Consider budget and long-term energy costs

Compare installed cost quotes for both models, including any electrical or plumbing modifications. Then, think about energy costs over the expected life of the unit. In high-usage homes and regions with higher electricity prices, the 300 E’s greater efficiency and capacity can reduce cost per gallon of hot water delivered and improve comfort enough to justify additional upfront expense.

In smaller homes with lower usage and limited budgets, the 220 E’s lower purchase price paired with already modest energy bills can make it the more practical choice, even if its UEF is slightly lower.

Step 4: Account for future changes

Think about how your household may change over the next decade or more—the typical service life of a well-maintained heat pump water heater. Are you planning for more occupants, adding a rental unit, or expecting higher hot water usage (such as a new soaking tub or guest suite)?

If growth is likely and your installation space can accommodate it, leaning toward the 300 E can provide more flexibility. If your household is stable or even shrinking over time, the 220 E may remain well matched to your needs.

Where a Product Carousel Fits in Your Decision

On a well-designed e-commerce site, you might see both Accelera models presented in a comparison carousel with quick-view specs: tank size, UEF, first-hour rating, operating temperature range, noise level, airflow, and estimated yearly energy use and cost. Use this side-by-side snapshot to quickly confirm that both units meet your basic requirements for electrical service, operating range, and size, then apply the decision framework in this article to choose the one that aligns best with your household’s daily reality rather than simply defaulting to the largest tank or lowest estimated cost.

Unbiased Recommendations: Which Model Is Better for You?

Summarizing all of the above, neither the Accelera 220 E nor the Accelera 300 E is universally “better.” Each model is stronger in specific situations. The right choice is the one that aligns with your home’s constraints and usage patterns without overspending on capacity you don’t need or undersizing in a way that compromises comfort.

Choose the Accelera 220 E if:

• Your household has 1–3 occupants with modest, mostly staggered hot water use.
• You do not have a large soaking tub or frequent long, high-flow showers.
• Your installation space has limited height or challenging access that favors a shorter, lighter unit.
• Upfront cost is a key constraint, and you are comfortable with a slightly lower first-hour rating and UEF.
• You prioritize lower absolute electricity consumption over maximum capacity, and your usage patterns support that choice.

Choose the Accelera 300 E if:

• Your household has 3–5 or more occupants, or you often host guests who increase hot water demand.
• You want a higher first-hour rating and larger tank to handle overlapping showers, laundry, and dishwashing.
• Your home includes a large bathtub, multiple full bathrooms, or high-flow fixtures that can quickly draw down a smaller tank.
• You are willing to invest more upfront for higher efficiency and improved comfort, especially in areas with higher electricity costs.
• Your installation space can comfortably accommodate the greater height and weight of the unit with proper clearances.

In other words, the Accelera 220 E is well suited to smaller, lower-demand homes with limited space or tighter budgets, while the Accelera 300 E is generally better matched to typical family homes and higher-demand scenarios where comfort and capacity are top priorities.

Sources

• Stiebel Eltron — Accelera E Heat Pump Water Heaters technical specifications and data (rev. 2.2024) https://www.stiebel-eltron-usa.com/sites/default/files/pdf/tech-specs-accelera-e.pdf
• U.S. Department of Energy — Water heater energy factor and Uniform Energy Factor test procedures https://www.energy.gov
• U.S. Environmental Protection Agency — Heat pump water heater technology overview and efficiency guidance (ENERGY STAR Water Heaters) https://www.energystar.gov