What Is BESS Round-Trip Efficiency? How Cooling, PCS and Auxiliary Loads Affect RTE

What Is BESS Round-Trip Efficiency?

Round-trip efficiency, commonly called RTE, is one of the most important—and sometimes misunderstood—performance metrics in a battery energy storage system (BESS).

In simple terms, RTE tells us how much of the electricity used to charge a BESS can later be recovered as usable electricity.

The basic formula is:

RTE (%) = Energy Discharged ÷ Energy Charged × 100

For example, if a BESS receives 100 MWh during charging and later delivers 90 MWh back at the same measurement point, its round-trip efficiency is:

90 ÷ 100 × 100 = 90%

The remaining 10 MWh has not disappeared. Most of it has been converted into heat through battery internal resistance, power conversion, transformers, electrical cables and auxiliary systems.

However, there is an important detail:

RTE only has meaning when the measurement boundary is clearly defined.

A battery-cell efficiency, DC-block efficiency and complete AC-to-AC system efficiency are not the same thing.

For example, the NREL 2024 Annual Technology Baseline for Commercial Battery Storage uses 85% round-trip efficiency as a representative assumption for commercial battery storage.

This illustrates why real project-level RTE is normally lower than battery-cell efficiency.

Why Is BESS RTE Lower Than Battery Efficiency?

A complete BESS contains much more than lithium-ion battery cells.

Energy typically passes through:

Grid → Transformer → PCS → DC Bus → Battery

and then during discharge:

Battery → DC Bus → PCS → Transformer → Grid

Meanwhile, many supporting systems consume electricity, including:

• Battery Management System (BMS)

• Energy Management System (EMS)

• Power Conversion System cooling

• Liquid cooling pumps

• Chillers or air conditioners

• Ventilation fans

• Fire detection systems

• Communication equipment

• Lighting

• Control power supplies

Therefore, battery efficiency alone cannot represent the efficiency of the entire system.

A simplified system-level calculation might look like this:

System RTE ≈ Battery DC RTE × PCS Charge Efficiency × PCS Discharge Efficiency × Transformer Efficiency² × Auxiliary Load Factor

Consider the following simplified example:

The approximate system efficiency becomes:

0.95 × 0.985 × 0.985 × 0.995 × 0.995 × 0.98 ≈ 89.4%

Therefore, a battery system with approximately 95% battery-level efficiency may ultimately deliver only around 89% net system RTE.

This difference is also visible in real-world testing.

A published Journal of Energy Storage study reported battery efficiency of approximately 95.71%, while complete system round-trip efficiency was around 87.95% after inverter and transformer losses were considered.

This is why buyers should be careful when a BESS supplier simply advertises “95% efficiency.”

The first question should be:

95% measured where?

How Does the PCS Affect BESS Round-Trip Efficiency?

The Power Conversion System (PCS) is one of the most important sources of conversion loss in a BESS.

During charging, the PCS converts:

AC → DC

During discharging:

DC → AC

Because electricity passes through the PCS twice during one complete charge-discharge cycle, even relatively small conversion losses accumulate.

For example, if PCS efficiency is 98.5% in each direction:

98.5% × 98.5% ≈ 97.0%

This means approximately 3% of the energy may already be lost during the two conversion stages before battery, transformer and auxiliary losses are fully considered.

However, another important factor is often overlooked:

Peak PCS Efficiency Is Not the Same as Annual PCS Efficiency

A PCS may achieve its highest efficiency near a particular operating power.

At very low loads, conversion efficiency may decline.

For example, a BESS performing frequent low-power frequency regulation may achieve a different annual efficiency from the same system operating in regular peak-shaving cycles at higher power.

This is why project engineers should examine the PCS efficiency curve, rather than relying only on the maximum efficiency printed on the datasheet.

Merus Power's discussion of round-trip efficiency as a BESS performance guarantee also highlights the importance of defining operating conditions when evaluating RTE.

For a deeper explanation of how PCS interacts with other major BESS control systems, see our guide:

Difference Between PCS, EMS and BMS in Energy Storage

How Does Cooling Affect BESS RTE?

Cooling has an interesting relationship with battery efficiency because it can both improve battery operating conditions and consume electricity.

Cooling Can Help Battery Efficiency

Lithium-ion batteries generate heat during charging and discharging.

If battery temperature becomes too high or temperature differences between cells become excessive, internal resistance and degradation may increase.

Good thermal management helps maintain a relatively stable and uniform operating temperature.

This is particularly important for modern high-energy-density systems such as a 5MWh containerized BESS, where thousands of cells operate within a relatively compact space.

But Cooling Also Consumes Energy

Cooling equipment itself requires electricity.

A liquid-cooled BESS may contain:

• Coolant pumps

• Chillers

• Heat exchangers

• Control valves

• Fans

• Cooling control systems

Air-cooled systems may rely more heavily on:

• Air-conditioning units

• Fans

• Ventilation systems

All of this electricity becomes part of the system's auxiliary consumption, reducing net AC-to-AC RTE.

Therefore, the statement:

“Liquid cooling always gives higher round-trip efficiency.”

is too simplistic.

Liquid cooling generally provides better thermal uniformity and heat-transfer capability for high-density BESS installations. However, pumps and chillers consume electricity.

Air cooling may consume relatively little power under mild conditions but can require significant HVAC energy when operating in hot climates.

For this reason, the better engineering question is not:

“Which cooling technology consumes less power?”

but:

“How much annual cooling energy will this system require under the actual site climate and duty cycle?”

You can read more about this topic in our comparison:

Air-Cooled vs Liquid-Cooled BESS: Key Differences Explained

Research published in Energy also shows that thermal management and environmental conditions can significantly influence battery energy storage performance.

Auxiliary Loads: The Hidden BESS Efficiency Loss

Auxiliary consumption is one of the most commonly underestimated parts of round-trip efficiency.

Typical auxiliary loads include:

• BMS

• EMS

• Controllers

• Communications

• Fire detection

• Fire suppression controls

• Chillers

• Pumps

• Fans

• HVAC

• PCS cooling

• Lighting

The important point is that auxiliary power does not always vary proportionally with battery output.

Some auxiliary systems continue running even when the battery is idle.

For example, imagine a BESS finishes a high-power discharge cycle.

The batteries, PCS and electrical equipment may still be warm, so the cooling system can continue operating even though the BESS is no longer exporting electricity.

Similarly, communication equipment, the BMS, EMS and control systems may remain powered 24 hours a day.

This creates an important effect:

High-Utilization vs Low-Utilization BESS

Suppose two identical BESS units each consume 100 kWh of auxiliary electricity per day.

System A cycles 5 MWh every day.

System B cycles only 1 MWh every day.

The same 100 kWh auxiliary load represents:

System A: 100 kWh ÷ 5,000 kWh = 2%

but:

System B: 100 kWh ÷ 1,000 kWh = 10%

So even though the hardware is identical, System B may show much poorer net efficiency.

This is why real BESS round-trip efficiency depends heavily on utilization and dispatch strategy.

A useful engineering overview of BESS sizing and auxiliary effects is available from BESS Courses.

Example: How Much Energy Does a 5 MWh BESS Actually Deliver?

Consider a 5 MWh BESS.

Suppose the system receives:

5.00 MWh

from the grid measured at the agreed AC connection point.

If the complete system's net AC-to-AC round-trip efficiency is:

89%

then the energy returned is:

5.00 MWh × 89% = 4.45 MWh

Total energy loss during the cycle is therefore:

5.00 − 4.45 = 0.55 MWh

That 0.55 MWh may include:

• Battery electrochemical losses

• DC cable losses

• PCS conversion losses

• Transformer losses

• Cooling consumption

• Pump and fan consumption

• BMS and EMS consumption

• Other auxiliary loads

Now imagine two suppliers both advertise:

“Efficiency: 95%”

Supplier A means:

95% battery DC efficiency

while Supplier B means:

95% net AC-to-AC efficiency including auxiliaries

These two specifications have completely different commercial meanings.

That is why buyers should never compare RTE numbers without understanding their measurement conditions.

What Should Be Included in a BESS RTE Guarantee?

When purchasing a commercial or utility-scale BESS, simply requesting:

“RTE ≥ 90%”

may not be sufficient.

A better technical specification should define the following.

1. Measurement Boundary

Is RTE measured at:

• Battery DC terminals?

• PCS AC terminals?

• MV transformer?

• Point of interconnection (POI)?

For most project owners, POI-level AC-to-AC RTE is the most commercially meaningful metric.

2. Auxiliary Consumption

Clarify whether the RTE calculation includes:

• Cooling

• Pumps

• Fans

• HVAC

• Control equipment

• BMS

• EMS

• PCS auxiliaries

3. Charge and Discharge Power

RTE should be measured under a specified operating power or C-rate.

4. SOC Window

For example:

10–90% SOC

or:

5–95% SOC

Different SOC windows can produce different efficiency values.

5. Ambient Temperature

A BESS operating at 25°C may consume much less cooling energy than the same BESS operating outdoors at 45°C.

6. Beginning of Life vs End of Life

Battery resistance generally increases as cells age.

Therefore, a Beginning-of-Life (BOL) RTE should not automatically be assumed to remain unchanged throughout a 15- or 20-year project.

7. Test Duration

A short charge-discharge test may not fully capture:

• Overnight HVAC consumption

• Standby losses

• Cooling after the cycle

• Seasonal temperature changes

For project economics, annual or duty-cycle-based energy efficiency can sometimes be more meaningful than a single laboratory test.

Why RTE Matters Financially

Round-trip efficiency directly affects how much electricity a BESS must purchase to deliver a given amount of electricity later.

Suppose an energy storage plant must deliver:

100,000 MWh per year

With 90% RTE, the required charging energy is:

100,000 ÷ 0.90 = 111,111 MWh

At 85% RTE:

100,000 ÷ 0.85 = 117,647 MWh

That represents an additional:

6,536 MWh

of charging electricity each year.

Over a 15-year project life, even a few percentage points of efficiency difference can therefore have a meaningful financial impact.

RTE should consequently be considered alongside:

• Battery degradation

• Cycle life

• Availability

• Warranty throughput

• Augmentation

• Electricity price spread

rather than being treated as an isolated specification.

Video: Understanding Important BESS Performance Metrics

For readers who want a deeper technical discussion, Energy-Storage.News and Wärtsilä have published a useful webinar covering important battery storage metrics including State of Charge (SOC), State of Health (SOH), cell imbalance and round-trip efficiency:

This is particularly useful for understanding why BESS performance should be evaluated at system level, rather than only at cell level.

Conclusion

BESS round-trip efficiency is not simply a battery specification.

It is the combined result of:

Battery electrochemistry + PCS conversion + transformers + cooling + auxiliary loads + operating strategy

A battery may achieve more than 95% DC round-trip efficiency, while the complete system delivers less than 90% AC-to-AC efficiency.

Likewise, a PCS advertising 99% peak efficiency does not mean the complete BESS has 99% efficiency.

When evaluating a battery energy storage system, the most useful question is therefore not:

“What is your battery efficiency?”

Instead, ask:

“What is the guaranteed net AC-to-AC round-trip efficiency at the agreed project measurement point, including defined auxiliary loads and under the expected operating conditions?”

This turns RTE from a marketing percentage into a measurable engineering and financial parameter—and allows BESS suppliers to be compared on a genuinely equivalent basis.

For businesses planning commercial, industrial or utility-scale energy storage projects, you can also explore ELITE ESS containerized battery energy storage solutions for configurable BESS designs including thermal management, PCS integration and containerized system engineering.