When electricity is stored in a solar battery, not every kilowatt hour that enters the system comes back out as usable electricity. Some energy is lost during charging, storage, power conversion, and discharge.
Battery round trip efficiency (RTE) measures how effectively an energy storage system returns the electricity used to charge it. It is an important specification when comparing solar batteries because higher efficiency generally means more stored solar energy is available for later use.
The National Laboratory of the Rockies’ Annual Technology Baseline defines round trip efficiency as the ratio of useful energy output to useful energy input. Its 2024 utility scale battery model assumes an RTE of 85%, although actual performance depends on battery technology, system design, and operating conditions.
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What Is Battery Round Trip Efficiency?
Round Trip efficiency compares the energy recovered from a battery with the energy originally supplied to it.
The basic RTE formula is:
Round Trip efficiency (%) = Energy output ÷ Energy input × 100
Consider a battery that receives 10 kWh while charging and later supplies 9 kWh during discharge:
9 kWh ÷ 10 kWh × 100 = 90%
The battery’s round trip efficiency is therefore 90%.
The missing 1 kWh has been consumed by losses occurring during the storage cycle rather than disappearing at one specific point.
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Why Battery Round Trip Efficiency Matters
Higher RTE means a greater percentage of the electricity used to charge a battery is available later.
| RTE | Energy input | Usable energy returned |
|---|---|---|
| 80% | 10 kWh | 8 kWh |
| 85% | 10 kWh | 8.5 kWh |
| 90% | 10 kWh | 9 kWh |
| 95% | 10 kWh | 9.5 kWh |
These differences can accumulate across hundreds or thousands of battery cycles.
RTE can therefore influence solar self consumption, operating costs, energy arbitrage, backup planning, and overall storage economics.
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Where Does Battery Energy Get Lost?
Energy losses occur at several points during a complete battery cycle.
Battery cell losses
Charging and discharging battery cells involves internal electrical and electrochemical losses. Some energy is converted into heat rather than being recovered as electricity.
These losses are why even a battery measured only from its DC input to DC output cannot achieve perfect efficiency.
Inverter losses
Battery systems also require power electronics.
Batteries store electricity as DC, while homes, businesses, and the electric grid predominantly operate using AC. An inverter therefore converts electricity between DC and AC.
These conversions introduce additional losses.
A DOE energy storage assessment illustrates the distinction clearly. For the lithium ion systems it modeled, DC to DC RTE was approximately 89.55%, while AC to AC RTE at the inverter level was 86%. When transformer losses were also included, modeled RTE fell to 82.59%.
This demonstrates why buyers need to know where efficiency is being measured.
DC DC vs. AC AC Round Trip Efficiency
Not every RTE specification measures the same thing.
DC DC RTE measures efficiency through the battery on the DC side.
AC AC RTE includes the conversion required to charge the battery from AC electricity and return usable AC electricity.
For many real world applications, AC AC efficiency provides a more complete picture because it accounts for power conversion losses.
NREL research has specifically warned that DC DC RTE figures require additional consideration of power conversion losses when evaluating storage for grid applications.
Buyers should therefore avoid directly comparing a 95% DC DC rating with a 90% AC AC rating as though the second battery were automatically less efficient.
The measurement boundaries are different.
What Are Battery Idle Losses?
A battery can also consume electricity when it is not actively charging or discharging.
These are commonly called idle, standby, auxiliary, or parasitic losses.
Energy may be required to operate:
- Battery management systems
- Monitoring equipment
- Communications
- Control electronics
- Heating and cooling equipment
- Other supporting systems
NREL notes that reported RTE figures may not always include these parasitic loads and that heating and cooling requirements can vary substantially depending on climate, usage, and storage cycle duration.
DOE/Sandia research similarly notes that real world AC level RTE can be lower than manufacturer DC level figures once resistance, onboard controls, HVAC, and other auxiliary consumption are considered.
Idle losses become particularly relevant when a battery spends long periods waiting between charge and discharge events.
System Architecture Can Affect Efficiency
The way solar and storage equipment is connected also influences efficiency.
In an AC coupled system, solar electricity may need to undergo additional AC/DC conversions before being stored and later used.
A DC coupled system can avoid some of those conversions when solar energy charges the battery directly on the DC side.
The 2024 Annual Technology Baseline models this difference using an 85% round trip efficiency for grid charging and 87% when a DC coupled battery charges from the connected PV system, with the difference attributed to avoided AC/DC conversions.
Those values are modeling assumptions rather than universal performance ratings, but they demonstrate how system architecture can influence overall efficiency.
How to Compare Battery Round Trip Efficiency
Round Trip efficiency should be evaluated alongside other battery specifications.
Buyers should determine whether the published RTE is measured DC DC or AC AC, whether inverter and auxiliary losses are included, and under what operating conditions the test was performed.
Other important specifications include:
- Usable battery capacity
- Power rating
- Cycle life
- Depth of discharge
- Battery chemistry
- Throughput warranty
- Capacity retention warranty
- Operating temperature range
A battery with the highest advertised RTE is not automatically the best option if the specifications were measured under different conditions.
Conclusion
Battery round trip efficiency measures how much electricity can be recovered from a battery compared with the amount required to charge it.
The formula is:
RTE = Energy output ÷ Energy input × 100
If 10 kWh enters a battery and 9 kWh comes back out, the RTE is 90%.
But real world battery efficiency involves more than the battery cells themselves. Inverter conversion, auxiliary equipment, thermal management, and idle consumption can all reduce the amount of electricity ultimately available to the user.
That makes the measurement boundary especially important.
When comparing solar batteries, buyers should determine whether RTE is measured on a DC DC or AC AC basis, what losses are included, and how closely the published test conditions resemble the battery’s intended operating profile.
Frequently Asked Questions
How do you calculate battery round trip efficiency?
Divide recovered energy by charging input energy and multiply by 100. A battery receiving 10 kWh and returning 9 kWh has 90% round trip efficiency at the stated measurement boundary.
Can I compare a DC rating directly with an AC rating?
No. The boundaries differ. AC measurements can include inverter conversion losses that a DC measurement excludes. Compare figures measured at the same boundaries and under similar operating conditions.
Does a high efficiency rating include standby losses?
Not necessarily. Ask whether controls, communications, heating, cooling and other auxiliary consumption are included, along with the duration of the test.
Does efficiency describe battery capacity or cycle life?
No. Efficiency measures the share of charging energy recovered. Usable capacity measures energy available for storage, while cycle life describes durability under defined cycling conditions. Evaluate all three when selecting equipment.
Sources
National Laboratory of the Rockies: 2024 Annual Technology Baseline: Utility Scale Battery Storage
U.S. Department of Energy: Energy Storage Grand Challenge Cost and Performance Assessment
NREL: Electricity Storage Technical Report
U.S. Department of Energy / Sandia: Energy Storage Financing Study