Transformers are designed to minimize energy losses, but not all types achieve the same efficiency. With advancements in materials and design, certain transformers stand out as more efficient in specific applications. So, what makes one transformer more efficient than another, and which type leads in performance?
What Does Transformer Efficiency Mean and How Is It Measured?
Transformers are among the most energy-efficient machines in the electrical industry, with modern units achieving efficiency levels of 97–99.5%. However, losses—no matter how small—translate into wasted energy, higher operating costs, overheating, and faster insulation aging. For utilities and industries, understanding what transformer efficiency means and how it is measured is critical for lifecycle cost optimization, load planning, and sustainability goals.
Transformer efficiency refers to the ratio of output power delivered to the load versus the input power supplied to the transformer, expressed as a percentage. It is measured by comparing input and output power during test conditions, while accounting for no-load losses (core losses) and load losses (copper and stray losses). Efficiency testing typically uses methods such as direct load tests, back-to-back tests, and loss segregation techniques defined by IEC and IEEE standards.
Efficiency is not fixed—it varies with load, power factor, and operating conditions, which means real-world performance must be evaluated carefully.
Transformer efficiency is always 100% because it is a passive device.False
All transformers have internal losses due to core magnetization, winding resistance, and stray flux, so efficiency is less than 100%.
The efficiency of a transformer is defined as the ratio of output power to input power under given load conditions.True
Efficiency calculation involves comparing the useful output power with total input, including losses.
1. Definition and Formula of Transformer Efficiency
The basic formula:
\eta = \frac{P{out}}{P{in}} \times 100
Where:
- $P_{out}$ = Output Power (kW delivered to load)
- $P_{in}$ = Input Power (kW supplied to transformer)
Alternatively:
\eta = \frac{P{out}}{P{out} + P_{loss}} \times 100
Where:
- $P_{loss}$ = Total transformer losses = No-load losses + Load losses
2. Losses That Influence Efficiency
- No-load losses (Core losses): Due to hysteresis and eddy currents in the core, nearly constant regardless of load.
- Load losses (Copper losses): Due to I²R heating in windings, increasing with square of load current.
- Stray losses: Caused by leakage flux heating tank, clamps, and structural parts.
| Loss Type | Load Dependency | Typical Contribution to Total Loss |
|---|---|---|
| Core (No-load) | Independent of load | 20–40% |
| Copper (Load) | ∝ I² (varies by load) | 40–60% |
| Stray Losses | Load-dependent | 5–10% |
| Dielectric Losses | Minor | <2% |
3. Measurement Methods
- Direct Load Test: Input and output power measured directly using wattmeters at different loads (feasible for small transformers).
- Back-to-Back Test (Sumpner’s Test): Two identical transformers connected to measure full-load losses without needing full-rated power (standard for large power transformers).
- Loss Segregation: Separately measuring no-load loss (open-circuit test) and load loss (short-circuit test), then combining for efficiency calculation.
| Test Type | Best For | Key Advantage |
|---|---|---|
| Direct Load Test | Small distribution units | Simple, direct |
| Back-to-Back Test | Medium & large power transformers | Full-load test without large power source |
| Loss Segregation | All transformers | Separates no-load and load losses clearly |
4. Variation with Load
Transformer efficiency is not constant:
- At light load → Core losses dominate → Efficiency drops
- At heavy load → Copper losses dominate → Efficiency drops
- At optimal load (where copper losses ≈ core losses) → Maximum efficiency
| Load Level (% of Rated) | Efficiency Trend |
|---|---|
| 20% | Low (core loss dominates) |
| 50–70% | Peak efficiency |
| 100% | Slightly reduced (copper loss dominates) |
Case Example: Utility Distribution Transformer
- Rating: 1 MVA, 11/0.415 kV
- No-load losses: 1.5 kW
- Full-load copper losses: 9 kW
- Efficiency at 50% load = 98.3%
- Efficiency at 100% load = 98.1%
This shows efficiency remains very high, but optimal point is at partial loading.
Why Are Oil-Immersed Transformers Considered Highly Efficient?
Transformers are critical assets in power systems, and their efficiency directly impacts energy costs, reliability, and lifespan. While both dry-type and oil-immersed transformers are widely used, oil-immersed units dominate in medium- and high-voltage applications because of their superior performance under real operating conditions. Inefficiency leads to excessive losses, overheating, accelerated insulation breakdown, and higher operating costs. The solution is found in the oil-immersed design, which provides exceptional cooling, insulation, and energy transfer efficiency.
Oil-immersed transformers are considered highly efficient because the insulating and cooling oil minimizes thermal stress, reduces winding resistance, and supports compact core-winding arrangements. This design ensures lower copper and stray losses, effective dissipation of heat, extended insulation life, and high efficiency levels—often exceeding 98–99% in distribution and power applications. The oil also enables transformers to handle overloads and harmonics better than dry-type units.
This is why oil-immersed designs are the preferred choice in utilities, renewable energy projects, and heavy industries where continuous, high-efficiency operation is critical.
Oil-immersed transformers achieve higher efficiency mainly because of their oil-based cooling and insulation system.True
The oil absorbs heat from the windings and core, then circulates to radiators for dissipation, keeping losses lower and efficiency higher.
Dry-type transformers are always more efficient than oil-immersed transformers.False
While dry-type units are safer in indoor settings, oil-immersed transformers typically achieve higher efficiency due to better cooling and loss management.
1. Superior Cooling System
The efficiency advantage begins with oil circulation, which can be natural (ONAN), forced (ONAF), or directed with pumps (ODAF). Cooling oil directly contacts windings and core, removing heat far more effectively than air cooling. Lower operating temperature = lower resistance in copper windings = reduced I²R losses.
| Cooling Method | Typical Use Case | Efficiency Impact |
|---|---|---|
| ONAN (Oil Natural, Air Natural) | Distribution transformers | High efficiency under normal load |
| ONAF (Oil Natural, Air Forced) | Power transformers up to 100 MVA | Enhanced cooling, supports overload |
| ODAF/OFWF (Oil Directed/Air or Water Forced) | Extra high-voltage transformers | Very high efficiency under continuous heavy load |
2. Reduced Electrical Losses
- Core Losses: Oil cooling allows for tighter lamination stacking and advanced core materials, minimizing hysteresis and eddy currents.
- Copper Losses: Lower winding resistance at stable operating temperatures leads to reduced load losses.
- Stray Losses: Oil immersion reduces localized hot spots from leakage flux.
3. Compact Design with High Insulation Strength
Mineral oil (or alternative ester fluids) offers high dielectric strength, enabling smaller clearances between windings compared to air. This compactness reduces conductor length and resistance, further improving efficiency.
4. Longevity and Reliability
Efficiency is tied not only to energy transfer but also to durability. Oil prevents hot-spot overheating and moisture ingress, keeping insulation paper intact. Studies show oil-immersed units can operate reliably for 25–35 years with proper maintenance.
| Transformer Type | Typical Efficiency | Average Service Life |
|---|---|---|
| Dry-type | 95–97% | 15–25 years |
| Oil-immersed | 98–99.5% | 25–35 years |
5. Overload and Harmonic Handling
Oil-immersed units can sustain short-term overloads without catastrophic heating, thanks to oil’s thermal inertia. They also dissipate additional losses from harmonics better than dry-type designs, making them suitable for data centers, railways, and renewable plants.
Case Study: Utility Upgrade
A European utility replaced 50 aging dry-type units (95.5% average efficiency) with oil-immersed amorphous-core transformers (98.7% average efficiency).
- Annual energy savings: 3.2 GWh
- CO₂ reduction: 2,400 tons per year
- Payback period: <4 years
How Do Dry-Type Transformers Compare in Efficiency?
When choosing a transformer, one of the most common dilemmas is whether to select an oil-immersed transformer or a dry-type transformer. Efficiency is a key factor because it directly impacts energy costs, operating reliability, and long-term performance. The problem is that while dry-type transformers are safer and environmentally friendlier, they tend to have higher losses and reduced efficiency compared to their oil-immersed counterparts. Without understanding this trade-off, users risk paying more in electricity bills and shortening equipment lifespan. The solution lies in carefully comparing efficiency metrics under realistic operating conditions.
Dry-type transformers are generally less efficient than oil-immersed transformers, with typical efficiency ranging from 95% to 97%, compared to 98% to 99.5% for oil-immersed units. The lower efficiency is mainly due to air cooling, which limits heat dissipation and increases winding resistance, leading to higher copper and stray losses. However, dry-type transformers are often chosen for indoor, fire-sensitive, and environmentally sensitive locations where safety outweighs efficiency.
This comparison highlights that efficiency should not be the only selection criterion; application environment and safety requirements must also be considered.
Dry-type transformers can achieve the same efficiency levels as oil-immersed transformers in all operating conditions.False
Due to their cooling limitations and higher winding resistance at elevated temperatures, dry-type units generally operate at lower efficiency levels.
Dry-type transformers are less efficient but are preferred in indoor or fire-sensitive environments.True
Their solid insulation and absence of flammable liquid make them suitable for areas where safety is the primary concern.
1. Efficiency Performance of Dry-Type Transformers
Dry-type transformers rely on air circulation (natural or forced) to dissipate heat from windings and the core. This makes them inherently less efficient than oil-immersed transformers:
| Transformer Type | Typical Efficiency | Cooling Method | Application |
|---|---|---|---|
| Dry-Type (Cast Resin) | 95–97% | Air natural or forced | Indoor, fire-prone sites |
| Oil-Immersed | 98–99.5% | Oil circulation + radiators | Utilities, outdoor, heavy load |
Key factors affecting dry-type efficiency:
- Higher copper losses because resistance rises with temperature.
- Limited cooling reduces capacity to handle overloads.
- Larger conductor cross-sections are often required, increasing cost.
2. Efficiency Under Different Loading Conditions
Dry-type transformers operate best near their rated capacity but experience steeper efficiency drops at partial or heavy overloads compared to oil-immersed units.
| Load Level (% of Rated) | Dry-Type Efficiency | Oil-Immersed Efficiency |
|---|---|---|
| 25% | 94–95% | 97–98% |
| 50% | 95.5–96.5% | 98–99% |
| 100% | 95–97% | 98–99.5% |
This demonstrates that oil-immersed units maintain consistently high efficiency across load variations, while dry-type units show noticeable drops.
3. Safety and Environmental Trade-Offs
Although less efficient, dry-type transformers offer distinct advantages:
- Fire Safety: No flammable oil, reducing fire risk.
- Eco-Friendly: No risk of oil leakage or contamination.
- Installation Flexibility: Can be installed indoors (malls, hospitals, tunnels, data centers).
However, these benefits come at the cost of increased operating energy losses over the transformer’s lifetime.
4. Case Example – Commercial Building
A hospital replaced five 1,000 kVA oil-immersed units with dry-type cast resin transformers due to fire regulations:
- Efficiency loss: \~2% lower per transformer
- Extra annual energy cost: ≈ \$25,000
- Benefit: Fire safety compliance, reduced insurance premiums, improved indoor installation feasibility
5. Lifecycle Cost Comparison
| Factor | Dry-Type Transformer | Oil-Immersed Transformer |
|---|---|---|
| Initial Cost | Higher | Moderate |
| Efficiency | Lower (95–97%) | Higher (98–99.5%) |
| Maintenance | Low | Moderate (oil checks) |
| Safety | High (non-flammable) | Lower (flammable oil) |
| Lifespan | 15–25 years | 25–35 years |
What Role Do Amorphous Core Transformers Play in Reducing Losses?
The rising demand for energy efficiency and sustainability has made transformer losses a critical concern for utilities, industries, and data centers. Conventional silicon-steel core transformers, while widely used, still suffer from significant core losses (no-load losses) due to hysteresis and eddy currents in their laminated cores. The pain point is that these losses are continuous, even when the transformer is not heavily loaded, leading to wasted energy and higher lifecycle costs. The solution lies in amorphous core transformers, which use advanced core materials to drastically cut down on no-load losses and improve overall efficiency.
Amorphous core transformers reduce losses primarily by lowering no-load (core) losses by 60–70% compared to conventional silicon-steel transformers. The amorphous metal alloy, made of thin, non-crystalline ribbons, exhibits low hysteresis loss and high electrical resistance, which minimizes both hysteresis and eddy current losses. This makes them particularly effective in distribution networks where transformers operate continuously at varying load levels.
By replacing conventional cores with amorphous alloys, utilities and industries can significantly improve energy efficiency, reduce operating costs, and support decarbonization goals.
Amorphous core transformers can eliminate all transformer losses.False
They significantly reduce no-load losses but load losses from winding resistance and stray flux still exist.
Amorphous core transformers can reduce core losses by up to 70% compared to silicon steel core transformers.True
The amorphous alloy structure minimizes hysteresis and eddy currents, leading to much lower core losses.
1. How Amorphous Cores Work
Amorphous metal is produced by rapid cooling of molten alloy, creating a non-crystalline atomic structure. This leads to:
- Low hysteresis loss: Atoms have no long-range order, reducing energy lost during magnetization cycles.
- Thin ribbons (≈0.025 mm): Minimize eddy currents.
- High resistivity: Further suppresses eddy current losses.
2. Loss Comparison: Amorphous vs. Silicon Steel
| Transformer Type | No-Load Loss Reduction | Load Losses | Typical Efficiency | Application |
|---|---|---|---|---|
| Silicon Steel Core | Baseline (100%) | Normal | 98–99% | General use |
| Amorphous Core | 60–70% lower | Slightly higher (due to larger size) | 98.5–99.5% | Energy-saving distribution |
Although amorphous core transformers may have slightly higher copper losses due to larger winding requirements, the huge reduction in no-load losses outweighs this in most real-world cases.
3. Performance Across Load Profiles
| Load Level (% of Rated) | Silicon Steel Efficiency | Amorphous Core Efficiency |
|---|---|---|
| 25% | 97.2% | 98.0% |
| 50% | 98.3% | 98.8% |
| 75% | 98.8% | 99.1% |
| 100% | 99.0% | 99.2% |
This shows that amorphous core units outperform conventional transformers especially at light to medium loading, where no-load losses dominate.
4. Case Study – Utility Network
A utility replaced 10,000 silicon-steel 200 kVA distribution transformers with amorphous core units:
- Annual energy savings per unit: \~700–1000 kWh
- Total system savings: \~9 GWh/year
- CO₂ reduction: ≈ 7,000 tons annually
- Payback period: 3–5 years due to energy savings
5. Challenges and Considerations
- Cost: Amorphous core transformers are typically 20–30% more expensive initially.
- Size & Weight: Bulkier due to lower flux density handling.
- Noise: Magnetostriction can be higher, requiring acoustic management.
Despite these factors, lifecycle cost savings and regulatory push for efficiency make them increasingly attractive.
6. Role in Grid Modernization
Amorphous core transformers support:
- Smart grids with reduced base losses.
- Renewable integration, where fluctuating loads make low no-load losses crucial.
- Sustainability goals, aligning with global efficiency standards like DOE (U.S.) and EcoDesign (EU).
Are Solid-State Transformers the Future of High Efficiency?
The power industry faces increasing pressure to deliver higher efficiency, flexibility, and integration with renewable energy sources. Conventional oil-immersed and dry-type transformers, while reliable, are based on electromagnetic induction and limited in adaptability. The pain point is that traditional transformers cannot easily handle bi-directional power flows, variable frequency environments, or advanced power quality management, all of which are essential for modern grids. The solution being developed is the solid-state transformer (SST), a next-generation device that replaces bulky iron cores with power electronic converters and high-frequency transformers.
Solid-state transformers are considered the future of high efficiency because they minimize core and copper losses by operating at higher frequencies, enable real-time power conversion with active control, and provide advanced features like voltage regulation, harmonic filtering, and renewable integration. Unlike conventional transformers, SSTs combine transformation, rectification, and inversion functions in one compact system, making them more efficient under variable loads and modern grid conditions.
This positions SSTs as not just energy-saving devices but as enablers of smart grids, electric mobility, and renewable-heavy networks.
Solid-state transformers are already widely deployed in utility distribution networks.False
SSTs are still in pilot and demonstration stages due to cost, scalability, and reliability challenges, though adoption is growing.
Solid-state transformers improve efficiency by operating at higher frequencies and reducing no-load losses.True
By shifting to high-frequency operation and replacing bulky iron cores, SSTs minimize hysteresis and eddy current losses, while providing flexible electronic control.
1. What Makes Solid-State Transformers Different?
SSTs are built on power electronic conversion stages:
- AC/DC Conversion: Rectifies grid input.
- High-Frequency DC/AC or DC/DC Conversion: Uses a small, lightweight high-frequency transformer for isolation.
- Inversion & Conditioning: Provides output with regulated voltage, frequency, and quality.
This architecture enables:
- Compact size compared to conventional transformers.
- Dynamic efficiency tuned for variable loads.
- Multi-port functionality, connecting AC, DC, storage, and renewables.
2. Efficiency Comparison
| Transformer Type | No-Load Losses | Load Losses | Features | Typical Efficiency |
|---|---|---|---|---|
| Oil-Immersed Transformer | Higher (core loss) | Moderate | Basic transformation | 98–99% |
| Amorphous Core Transformer | Very Low | Slightly higher | Reduced no-load loss | 98.5–99.5% |
| Solid-State Transformer | Minimal | Converter losses (IGBTs, SiC MOSFETs) | Smart grid ready | 98–99.5% |
While SSTs show comparable peak efficiency, their real strength lies in adaptive performance under partial load, multi-functionality, and power quality benefits.
3. Performance Under Load Variation
| Load Level (% Rated) | Oil-Immersed Efficiency | Amorphous Core Efficiency | Solid-State Efficiency |
|---|---|---|---|
| 25% | 97.3% | 98.0% | 98.6% (adaptive control) |
| 50% | 98.5% | 98.9% | 99.1% |
| 100% | 99.0% | 99.2% | 99.3% |
| 120% | 98.6% | 98.8% | 98.9% (thermal managed) |
This shows SSTs can maintain higher efficiency across a broader load spectrum due to intelligent switching and real-time optimization.
4. Case Study – Electric Mobility Integration
A pilot project in Germany deployed SSTs for fast-charging EV stations:
- Enabled direct DC output, eliminating multiple conversion stages.
- Reduced footprint by 40%.
- Improved efficiency by 2–3% compared to conventional transformer + rectifier systems.
- Provided harmonic suppression and voltage regulation for grid stability.
5. Challenges Facing SST Deployment
- Cost: Semiconductor devices (e.g., SiC, GaN) are still expensive.
- Reliability: Sensitive electronics compared to rugged iron-core designs.
- Thermal Management: Requires advanced cooling solutions.
- Scalability: Larger SSTs (>10 MVA) are still in experimental stages.
6. The Future of High-Efficiency Grids
Solid-state transformers are expected to play a pivotal role in:
- Smart grid modernization with digital monitoring and control.
- Renewable integration, allowing smooth DC/AC interfacing.
- EV charging infrastructure, enabling ultra-fast, grid-friendly charging.
- DC microgrids, where bidirectional flow and voltage adaptation are essential.
Which Type of Transformer Is the Most Efficient in Practice?
When selecting a transformer, efficiency is often the most critical performance indicator because it directly influences operating costs, energy savings, and long-term reliability. The challenge is that different transformer technologies—oil-immersed, dry-type, amorphous core, and emerging solid-state transformers—each optimize efficiency under different conditions. The pain point for users is balancing initial investment versus lifetime operating savings, as the “most efficient” transformer depends heavily on the actual application and load profile. The solution lies in understanding how each transformer type performs under real-world conditions.
In practice, oil-immersed transformers with optimized designs are generally the most efficient due to superior cooling and lower load losses, reaching peak efficiencies above 99%. Amorphous core transformers are highly efficient at reducing no-load losses, especially in distribution networks with light to medium loading. Dry-type transformers, while safer in indoor applications, usually have slightly lower efficiency than oil-filled units. Solid-state transformers promise advanced adaptive efficiency and grid functionality, but they are not yet widely commercialized.
Choosing the most efficient transformer therefore requires evaluating not just peak efficiency but also lifecycle performance, load conditions, and application environment.
Oil-immersed transformers always have the lowest losses in every application.False
Oil-filled units usually achieve the highest efficiency at full load, but amorphous core transformers can outperform them in light-load conditions due to reduced no-load losses.
Amorphous core transformers reduce no-load losses by 60–70% compared to silicon-steel core designs.True
The amorphous alloy’s non-crystalline structure minimizes hysteresis and eddy current losses, making them superior in low-load operations.
1. Efficiency Comparison by Transformer Type
| Transformer Type | No-Load Losses | Load Losses | Typical Peak Efficiency | Best Use Case |
|---|---|---|---|---|
| Oil-Immersed Transformer | Low | Very Low (good cooling) | 98.8–99.5% | High-capacity substations, heavy load duty |
| Dry-Type Transformer | Moderate | Higher due to air cooling | 97.5–98.5% | Indoor, fire-safe environments |
| Amorphous Core | Very Low (up to 70% less) | Slightly higher | 98.5–99.2% | Distribution networks with variable/light loading |
| Solid-State Transformer | Minimal core loss, but converter losses | Moderate (semiconductors) | 98–99.2% | Future smart grids, EV fast charging, DC networks |
2. Efficiency Across Loading Conditions
| Load Level (% Rated) | Oil-Immersed Efficiency | Dry-Type Efficiency | Amorphous Core Efficiency | Solid-State Efficiency |
|---|---|---|---|---|
| 25% | 97.5% | 96.8% | 98.3% | 98.6% |
| 50% | 98.7% | 97.8% | 98.9% | 99.0% |
| 75% | 99.1% | 98.2% | 99.1% | 99.2% |
| 100% | 99.4% | 98.5% | 99.2% | 99.3% |
This data shows:
- Oil-immersed transformers dominate at full load.
- Amorphous core units excel at partial loads where no-load losses matter more.
- Dry-types sacrifice some efficiency for safety and installation benefits.
- Solid-state transformers offer adaptability but are still in development for widespread deployment.
3. Case Study – Utility Distribution Network
A utility compared 500 kVA units of different types in operation:
- Oil-immersed: Highest peak efficiency, but higher base no-load losses.
- Amorphous core: 15–20% lower annual energy consumption under 40% average loading.
- Dry-type: Lower efficiency, but essential in fire-prone urban installations.
- Solid-state: Pilot-tested for renewable integration, outperforming in load flexibility but cost-prohibitive.
4. Factors That Decide Real-World Efficiency
- Load Profile: Constant high load favors oil-immersed; variable/low load favors amorphous.
- Environment: Indoor/fire-risk areas require dry-type despite lower efficiency.
- Technology Maturity: Solid-state is promising but not yet economical for mass deployment.
- Regulations: EcoDesign (EU) and DOE (US) standards push for low-loss designs, favoring amorphous and high-grade silicon steel.
Conclusion
The most efficient transformer type depends on usage. Oil-immersed units often achieve the highest efficiency in large power applications, while amorphous core transformers excel at reducing no-load losses in distribution systems. Dry-type transformers trade a bit of efficiency for safety, and solid-state transformers, though still emerging, promise new levels of functionality. In short, the “most efficient transformer” is the one that best balances efficiency with the specific needs of its application.
FAQ
Q1: What is the most efficient type of transformer?
The amorphous core transformer is considered the most efficient type due to its very low core (no-load) losses. Its amorphous steel core reduces hysteresis and eddy current losses compared to conventional silicon steel transformers, achieving efficiency levels above 98–99% in many applications.
Q2: How do amorphous core transformers compare to conventional transformers?
Amorphous core transformers have 70–80% lower core losses than standard CRGO (cold-rolled grain-oriented steel) transformers. This makes them particularly efficient under light load conditions where core losses dominate. However, they are typically more expensive.
Q3: Are oil-filled transformers more efficient than dry type?
Yes, generally oil-filled transformers have better cooling capability and lower losses compared to dry type transformers. They operate more efficiently under heavy load but may not be suitable in environments where fire risk or contamination is a concern.
Q4: Do high-efficiency transformers reduce operational costs?
Yes. High-efficiency transformers reduce energy losses (both core and copper), lowering electricity costs over their service life. For utilities and industries, even a 1% efficiency improvement can result in significant long-term savings.
Q5: Which factors determine transformer efficiency in practice?
Key factors include:
Core material (amorphous vs CRGO steel)
Winding material (copper vs aluminum)
Cooling method (oil vs dry type)
Load profile (light load vs full load efficiency)
Power quality and harmonics
References
IEEE Std C57 – Transformer Efficiency Standards: https://ieeexplore.ieee.org
IEC 60076 – Power Transformer Efficiency Guidelines: https://webstore.iec.ch
NEMA – High Efficiency Transformer Standards: https://www.nema.org
Electrical4U – Amorphous Core Transformers Explained: https://www.electrical4u.com
EEP – High Efficiency Transformers and Energy Savings: https://electrical-engineering-portal.com
All About Circuits – Transformer Efficiency Basics: https://www.allaboutcircuits.com
Energy.gov – High Efficiency Distribution Transformers: https://www.energy.gov
