A home battery does not return every unit of electricity that goes into it.
Some energy is lost while the electricity is charged, stored, converted and discharged again.
The simplest way to understand round-trip efficiency is:
10 kWh in → 9 kWh out = 90% round-trip efficiency
That sounds straightforward, but there is one important catch:
An efficiency percentage is only useful when you know where the measurement starts and where it ends.
Two batteries can advertise very different efficiency figures while measuring different parts of the energy journey.
Round-trip efficiency tells you how much of the electricity put into storage survives the journey back out again.
What Is Battery Round-Trip Efficiency?
Round-trip efficiency, often shortened to RTE, measures how much of the energy put into a battery system is returned later as useful energy.
The basic formula is:
Round-Trip Efficiency = Energy Out ÷ Energy In × 100
For example, energy into storage of 10 kWh and energy returned later of 9 kWh gives 9 ÷ 10 × 100 = 90%.
That means approximately 1 kWh was lost during the energy journey being measured.
Round-trip efficiency measures the difference between the energy that enters the storage journey and the energy that comes back out.
What Does 90% Battery Efficiency Actually Mean?
A 90% round-trip efficiency means that around 90% of the energy put into the measured storage journey is returned.
So if 10 kWh goes in, around 9 kWh comes back out under those conditions.
The same calculation can also be turned around.
If you want 1 kWh of useful electricity later and the battery system is 90% efficient, you need approximately 1 ÷ 0.90 = 1.11 kWh of input energy.
This becomes very important when batteries are charged using cheap electricity tariffs.
A 90% efficient battery needs more than 1 kWh of input energy to deliver 1 kWh back to the home.
Where Does the Missing Energy Go?
The missing energy is not usually lost in one place.
Small losses occur throughout the storage system.
These can include:
- battery internal resistance
- AC-to-DC conversion
- DC-to-AC conversion
- DC-to-DC conversion
- inverter electronics
- wiring
- control equipment
- heat
The exact losses depend on the battery architecture and how the system is operating.
The missing energy is usually the combined result of several small losses across the complete storage system.
The Measurement Boundary Changes the Efficiency Number
This is one of the most important things to understand when comparing batteries.
An efficiency percentage needs a defined starting point and finishing point.
For example, a manufacturer might measure DC into battery → Battery → DC out.
Another might measure AC into inverter → Battery → Inverter → AC out.
Those are different energy journeys.
The second includes more conversion stages. So it would normally be expected to show more losses.
The same battery can show different efficiency figures depending on where the measurement starts and finishes.
DC Round-Trip Efficiency vs AC Round-Trip Efficiency
- DC Round-Trip Efficiency
- Normally focuses on the battery or DC side of the system and may exclude some inverter conversion losses. A simplified path could be DC Energy → Battery → DC Energy.
- AC Round-Trip Efficiency
- Can include the full conversion path, for example Grid AC → Inverter → Battery → Inverter → Home AC. That usually includes more of the equipment the homeowner actually uses, and is particularly relevant for batteries charged from the grid.
Battery specifications sometimes show both DC and AC efficiency figures. They should not be treated as the same thing.
A 96% DC efficiency figure should not be compared directly with a 90% AC round-trip figure.
Why Headline Battery Efficiency Comparisons Can Be Misleading
Example
Battery A — advertised efficiency 98%.
Battery B — advertised efficiency 90%.
At first glance, Battery A appears dramatically better. But what if Battery A is quoting battery-level DC efficiency while Battery B is quoting full AC round-trip efficiency?
The two numbers are measuring different journeys. Ranking the products by those percentages would therefore be misleading.
Before comparing efficiency figures, check:
- AC or DC?
- battery only or complete system?
- inverter included?
- typical or maximum figure?
- test conditions?
Do not rank batteries by efficiency percentage until you know the percentages measure the same journey.
How Much Energy Do Battery Losses Actually Represent?
| Round-Trip Efficiency | Approx. Energy Returned | Approx. Loss |
|---|---|---|
| 95% | 9.5 kWh | 0.5 kWh |
| 90% | 9.0 kWh | 1.0 kWh |
| 85% | 8.5 kWh | 1.5 kWh |
These examples only make sense when the efficiencies use the same measurement boundary.
That qualification is important.
A 95% DC figure cannot simply be compared with an 85% AC figure and treated as a 10-percentage-point performance advantage.
How Round-Trip Efficiency Changes Cheap-Tariff Charging
Battery efficiency has a direct effect on the real cost of tariff shifting.
Imagine a cheap electricity rate of 10p/kWh and a battery round-trip efficiency of 90%.
To receive 1 kWh later you need approximately 1.11 kWh in.
So the input energy costs 1.11 × 10p = around 11.1p per usable kWh returned from the battery.
The electricity did not really cost 10p per usable kWh after it passed through storage. It effectively cost around 11.1p, before considering any other costs.
The cheap tariff rate is not the true cost of the electricity that eventually leaves the battery.
The Tariff Spread Has to Survive the Battery Losses
This is one of the most important financial consequences of round-trip efficiency.
Imagine cheap electricity at 20p/kWh and battery efficiency of 90%.
Effective input cost per usable kWh returned: 20p ÷ 0.90 ≈ 22.2p.
Now imagine the expensive tariff you are trying to avoid costs 22p/kWh.
The battery has not created a meaningful saving on the energy price alone.
And that is before considering battery degradation, equipment cost, standby consumption and other losses.
The tariff spread has to be wide enough to survive the battery losses.
High Efficiency Does Not Create a Good Tariff Opportunity
- Strategy A
- Battery efficiency 90%. Cheap electricity 8p/kWh. Expensive electricity avoided 30p/kWh. There is a substantial price difference.
- Strategy B
- Battery efficiency 96%. Cheap electricity 20p/kWh. Expensive electricity avoided 21p/kWh. The battery may be more efficient, but there is very little price difference to exploit.
High efficiency improves an opportunity. It does not create the opportunity.
How Round-Trip Efficiency Affects Stored Solar
The same losses apply when solar energy is stored.
Imagine 1 kWh of surplus solar is placed into the battery. At 90% round-trip efficiency, around 0.9 kWh may be returned later under the measured conditions.
That means storage changes the amount of solar energy eventually available to the home.
Storing solar moves energy through another conversion journey, so not every kWh put into storage comes back out again.
Stored Solar Is Not Automatically Free Electricity
Surplus solar often feels free because it has already been generated.
But it may have another value: export income.
Suppose 1 kWh of solar could either be exported to the grid now, or stored in the battery and used later.
If stored, some energy will be lost during the round trip.
So the correct comparison is not simply free solar vs expensive electricity.
It is the value of exporting 1 kWh now versus the value of the smaller amount of energy recovered later.
Stored solar is not automatically free electricity if you could otherwise have been paid to export it.
When Can Exporting Solar Be Better Than Storing It?
Imagine an export payment of 15p/kWh, a later import cost of 30p/kWh and a battery RTE of 90%.
If 1 kWh of solar is stored, around 0.9 kWh may be returned later.
The value of avoiding later import is approximately 0.9 × 30p = 27p.
In this simplified example, storing the energy appears more valuable than exporting it for 15p.
But if the export rate were much higher, or the later import rate much lower, the answer could change.
The point is the method.
Compare the value of 1 kWh exported now with the value of the smaller amount of energy recovered later.
Efficiency Depends on Where the Electricity Came From
A home battery may store energy from solar, the grid, or both. Those sources can follow different electrical paths.
Grid charging may follow: Grid AC → Inverter → Battery DC → Inverter → Home AC
Solar charging in a DC-coupled system might be closer to: Solar DC → Battery DC → Inverter → Home AC
The conversion journey is different. That means the practical efficiency can also be different.
There is not always one universal efficiency path through a home battery system.
AC-Coupled vs DC-Coupled Battery Efficiency
DC-coupled solar batteries can often reduce the number of conversions required when storing solar energy.
For example, solar energy may remain on the DC side while charging the battery.
An AC-coupled battery may require additional conversion stages. That can increase losses.
However, the exact result depends on:
- inverter architecture
- battery voltage
- converter design
- operating conditions
- system configuration
So the simple rule DC = efficient, AC = inefficient is too crude.
DC coupling can reduce conversion losses, but actual efficiency depends on the complete system architecture.
More Efficient Does Not Automatically Mean Better Retrofit
Suppose a home already has working solar panels, a good existing solar inverter and reliable monitoring.
Replacing that inverter and redesigning the system purely to reduce storage conversion losses by a few percentage points may not make financial sense.
The extra equipment, installation, disruption and compatibility work could cost far more than the lifetime value of the additional efficiency.
The most efficient electrical route is not automatically the most economical system design.
Why Real-World Battery Efficiency Can Be Lower Than the Datasheet
A homeowner may calculate battery performance using energy-monitoring data and find that the result is lower than the advertised efficiency.
That does not automatically mean there is something wrong with the battery.
Possible reasons include:
- the datasheet uses a different measurement boundary
- standby consumption
- inverter losses
- low-power operation
- temperature
- state of charge
- charge and discharge power
- control strategy
- measurement accuracy
The home may effectively be measuring the complete operating system while the datasheet describes a smaller part of it.
Your home may be measuring a much larger energy journey than the datasheet percentage describes.
Maximum Efficiency Is Not Available at Every Power Level
Inverters and battery electronics do not necessarily operate at exactly the same efficiency across their full power range.
A system supplying 300 W may operate differently from one supplying 3 kW or 5 kW.
This is why a single headline efficiency percentage cannot perfectly describe every operating condition.
The inverter does not necessarily operate at its headline efficiency at every power level.
Charge and Discharge Power Can Affect Efficiency
Moving more power through a battery means higher electrical current.
Higher current can increase resistive losses.
That means battery efficiency can vary with charge power, discharge power and operating conditions.
This does not mean slower charging is always better.
It simply means maximum power and maximum efficiency are not necessarily the same operating point.
Battery efficiency can change with charge and discharge power, so maximum power is not always maximum efficiency.
Temperature and State of Charge Matter Too
Battery performance can also change with:
- cold temperatures
- high temperatures
- very high state of charge
- very low state of charge
The battery management system may alter charging or discharging behaviour to protect the battery.
That can affect real-world conversion performance.
Round-trip efficiency is a useful summary number, not a constant that perfectly describes every operating condition.
Standby Consumption Is Not the Same as Round-Trip Efficiency
A battery system uses some electricity simply to remain operational.
Possible standby loads include:
- inverter electronics
- battery management system
- communications
- gateway
- monitoring
- thermal management
That energy consumption is conceptually different from the losses that occur while electricity is actively being charged and discharged.
Round-trip efficiency measures energy lost while moving electricity. Standby consumption is energy used even when relatively little electricity is moving.
Why Standby Consumption Matters More on Lightly Used Batteries
Example
Home A moves 10 kWh/day through the battery.
Home B moves 1 kWh/day through the battery.
If both systems have similar fixed operating consumption, that overhead represents a much larger percentage of the electricity moved through Home B's battery.
This is one reason whole-system efficiency can look worse when a battery is only lightly used.
Fixed system losses matter more when very little energy passes through the battery.
What Is Battery Self-Discharge?
Self-discharge is different again.
It describes stored energy gradually reducing while the battery sits unused.
It should not be confused with:
- conversion losses
- inverter losses
- standby consumption
- round-trip efficiency
These can all affect energy performance, but they describe different processes.
Round-Trip Efficiency vs Usable Capacity
These two specifications are often mixed together.
Imagine a battery with a nominal capacity of 10 kWh, usable capacity of 9 kWh and round-trip efficiency of 90%.
It is not automatically correct to say 9 kWh × 90% = 8.1 kWh usable battery.
Usable capacity and round-trip efficiency describe different things.
- Usable Capacity
- How much of the battery's storage can normally be accessed.
- Round-Trip Efficiency
- How much energy survives the defined charging and discharging journey.
Usable capacity tells you how much storage you can access. Round-trip efficiency tells you how much energy survives the journey through it.
Depth of Discharge Is Different Too
Depth of discharge relates to how much of the battery’s stored capacity is allowed to be used.
Round-trip efficiency relates to energy losses while that stored energy moves through the system.
They are separate specifications.
A battery can have high usable capacity, high depth of discharge and a completely different round-trip efficiency percentage.
Round-Trip Efficiency vs Battery Degradation
- Round-Trip Efficiency
- Describes energy lost during charging and discharging.
- Battery Degradation
- Describes storage capacity lost as the battery ages.
For example, a battery that originally stored 10 kWh but can only hold 9 kWh several years later has lost capacity.
That does not mean its round-trip efficiency is 90%.
Efficiency is energy lost during use. Degradation is storage capacity lost with age.
High Efficiency Does Not Mean Long Battery Life
A battery with high round-trip efficiency is not automatically longer lasting, more durable, better warranted or more resistant to degradation.
Those characteristics depend on other factors, including:
- chemistry
- cycle life
- throughput
- operating conditions
- warranty terms
- thermal management
Efficiency tells you about energy loss, not battery lifespan.
Do Small Efficiency Differences Matter Over a Year?
They can.
Suppose a battery receives 10 kWh per equivalent cycle and operates at 90% round-trip efficiency.
The simplified loss is around 1 kWh per equivalent cycle.
Across 300 equivalent cycles, that represents roughly 300 kWh of energy difference between input and output.
A few percentage points can therefore become more noticeable when a battery moves large amounts of energy every day.
A few percentage points can look small on one cycle and meaningful across hundreds of cycles.
But Comparable Batteries May Be Much Closer Than Their Marketing Suggests
This is the important counterpoint.
Once battery systems are compared using the same AC or DC measurement boundary, similar operating conditions and the same definition of round-trip efficiency, the differences between good modern systems may be much smaller than their headline percentages initially suggest.
That means efficiency should not automatically dominate the purchasing decision.
Efficiency matters, but once you compare like with like, it may be a much smaller product differentiator than the headline numbers suggest.
Is the Most Efficient Battery Automatically the Best Battery?
No.
Round-trip efficiency is only one part of the specification.
A battery should also be assessed for:
- usable capacity
- maximum charge power
- maximum discharge power
- backup capability
- modular expansion
- controls
- compatibility
- warranty
- installation cost
- system architecture
A slightly less efficient battery may still be the better system if it fits the property better, charges inside the required tariff window, captures more useful solar, supports the required loads or integrates properly with existing equipment.
The most efficient battery is not necessarily the battery that makes the best use of your energy.
Efficient Hardware Can Still Be Used Inefficiently
Battery controls matter.
A highly efficient system can still waste energy if it:
- charges unnecessarily
- repeatedly charges and discharges
- stores energy when export is more valuable
- discharges into an EV at the wrong time
- moves electricity through unnecessary conversion stages
Efficiency is therefore about more than hardware. It is also about how that hardware is used.
Efficient hardware cannot compensate for inefficient control decisions.
Good Controls Improve the Value of Every Stored kWh
A well-controlled battery can decide when to charge, hold energy, discharge, export or preserve backup reserve.
The objective is not simply to move as much electricity through the battery as possible.
It is to move electricity when storage creates the most value.
That may mean:
- charging cheaply overnight
- holding energy before an expensive tariff period
- storing solar instead of exporting
- exporting rather than storing
- preserving energy for backup
The correct decision can change through the day.
Which Battery Efficiency Figure Are You Looking At?
| Figure | What It Describes |
|---|---|
| Battery / DC efficiency | Battery-side energy movement |
| AC round-trip efficiency | Defined AC input → storage → AC output journey |
| Inverter efficiency | Efficiency of an individual conversion stage |
| Round-trip efficiency | Energy returned from a defined charge/discharge journey |
| Standby consumption | Energy used while the system operates or waits |
| Usable capacity | Storage capacity available for use |
| Degradation | Capacity lost as the battery ages |
Worked Example: 10 kWh In, 9 kWh Out
Example
Energy entering the measured battery journey: 10 kWh. Energy returned: 9 kWh. Round-trip efficiency: 90%. Energy difference: 1 kWh.
That is straightforward. But there is still one important question: what does the 10 kWh input measurement include?
If it starts on the DC side of the battery, inverter losses may not be included. If it starts at the AC supply and finishes at the AC output, much more of the system is being measured.
The 90% figure only becomes useful once the boundary is understood.
Worked Example: Cheap Tariff Charging
Example
Cheap tariff: 10p/kWh. Battery round-trip efficiency: 90%.
To return 1 kWh later requires approximately 1.11 kWh input, costing 1.11 × 10p ≈ 11.1p.
So the effective energy cost leaving the battery is approximately 11.1p/kWh before any other costs.
If that battery energy replaces electricity costing 30p/kWh, there is still a large price difference. If it replaces electricity costing 12p/kWh, the opportunity is much smaller.
Worked Example: Store Solar or Export It?
Example
Solar export payment: 15p/kWh. Later import price: 30p/kWh. Battery RTE: 90%.
Export the solar — 1 kWh exported is worth 15p.
Store the solar — 1 kWh stored returns approximately 0.9 kWh, giving an avoided import value of 0.9 × 30p = 27p.
In this simplified example, storing appears more valuable. But if the export payment increased, the later import price fell, or battery efficiency changed, the answer could change.
That is why solar storage should be compared using export value vs recovered stored-energy value, not simply “store everything because solar is free”.
What Should You Check on a Battery Datasheet or Quote?
When comparing battery efficiency, look for:
- quoted round-trip efficiency
- whether it is AC or DC
- where the measurement starts
- where the measurement ends
- whether the inverter is included
- whether the figure is typical or maximum
- test conditions
- standby consumption
- usable capacity
- charge power
- discharge power
- system architecture
If two products quote 90% and 96%, do not assume the second is automatically more efficient in your home.
First make sure they are measuring the same thing.
An efficiency percentage is only useful when you know what was measured to produce it.
Common Battery Round-Trip Efficiency Questions
What is battery round-trip efficiency?
Battery round-trip efficiency measures how much energy is returned after electricity has been charged into storage and later discharged.
What does 90% battery efficiency mean?
In a simplified example, 10 kWh entering the measured storage journey would return around 9 kWh.
How much energy is lost through a home battery?
It depends on the battery, inverter, measurement boundary and operating conditions. A 90% round-trip journey would represent roughly 10% less energy out than energy in.
How much electricity is needed to get 1 kWh back from a 90% efficient battery?
Approximately 1.11 kWh of input energy.
Is 95% battery efficiency good?
Possibly, but the measurement boundary should be checked first. A 95% DC figure cannot be directly compared with a lower full AC round-trip figure.
What is AC round-trip efficiency?
It measures an AC-to-storage-to-AC energy journey and can include inverter conversion losses.
What is DC round-trip efficiency?
It generally measures a DC-side battery journey and may exclude some inverter losses.
Why are some batteries advertised at 98% efficiency?
The figure may refer to battery-level, DC or inverter efficiency rather than full AC round-trip efficiency. Check what the percentage actually measures.
Can manufacturer battery efficiency figures be compared directly?
Only when the figures use comparable measurement boundaries and test conditions.
Why is my real battery efficiency lower than the datasheet?
Your monitoring may include standby loads, inverter losses, low-power operation and other system losses that the published figure does not.
Does standby consumption count as round-trip efficiency?
Not necessarily. Standby consumption is a separate operating loss and depends on how the efficiency measurement is defined.
Does fast charging reduce battery efficiency?
Efficiency can vary with charge power, but slower charging is not automatically more efficient in every system.
Does temperature affect battery efficiency?
It can. Battery and inverter performance can change with temperature and operating conditions.
Is battery efficiency the same as usable capacity?
No. Usable capacity measures available storage. Efficiency measures energy lost during the storage journey.
Is round-trip efficiency the same as battery degradation?
No. Efficiency describes energy losses during use. Degradation describes loss of storage capacity with age.
Does higher efficiency mean longer battery life?
Not necessarily. Battery life depends on other factors such as chemistry, cycles, throughput and operating conditions.
Is DC coupling more efficient?
It can reduce conversion stages for some solar-storage energy flows, but overall system efficiency depends on the complete architecture.
Is storing solar always better than exporting it?
No. The answer depends on export value, later import cost, battery efficiency and the wider tariff strategy.
How does battery efficiency affect smart tariffs?
Battery losses increase the effective cost of electricity stored during the cheap tariff period, so the price spread needs to be large enough to justify shifting that energy.
Follow the Whole Energy Journey
A battery efficiency percentage should never be viewed on its own.
You need to understand:
- how much energy entered
- where the measurement began
- which conversion stages were included
- how much energy returned
- what the electricity cost going in
- what that energy was worth coming back out
- how the battery actually operates in the home
Round-trip efficiency tells you how much of the electricity put into storage survives the journey back out again.
An efficiency percentage is only useful when you know where the measurement starts and where it ends.
The tariff spread has to be wide enough to survive the battery losses.
High efficiency improves an opportunity. It does not create the opportunity.