Round-Trip Efficiency: Why 10 kWh Gives 8.5 kWh

Last updated: November 2026.

You sell a 10 kWh battery, and the client expects 10 kWh back. At an 85% round-trip efficiency, the figure NREL assumes in its 2024 Annual Technology Baseline for utility-scale batteries, 10 kWh in returns 8.5 kWh out. That gap is not a defect. It is round-trip efficiency, the most misunderstood number in storage design.

Designers who assume 100% efficiency in self-consumption calculations overstate what a battery can shift. Solar PV design software should build these losses into the financial model, not only the hardware specification sheet. When it does not, the gap between proposal and reality turns into a warranty complaint.

In this guide:

• What round-trip efficiency means and why it is never 100%
• The exact RTE formula with a worked example using a 10 kWh system
• The difference between DC and AC round-trip efficiency, and which one matters for your project
• Where energy is lost: inverters, BMS, heat, and wiring
• Chemistry comparison: LFP, NMC, lead-acid, flow, and NaS
• Published datasheet and planning figures, with sources
• How RTE affects payback, self-consumption, and client ROI
• Practical steps installers can take to minimize losses

What Is Round-Trip Efficiency?

Round-trip efficiency is the percentage of electrical energy recovered after a complete charge-store-discharge cycle. If you feed 10 kWh into a battery and retrieve 9 kWh, the RTE is 90%. It is a system-level energy metric, not a cell-level chemistry score, and it is never 100% because physics demands losses at every conversion step.

The term applies to the full pathway: AC power from the grid or solar inverter passes through a battery inverter to reach DC storage, then reverses direction to supply AC loads. Each conversion, electrical to chemical, chemical back to electrical, generates heat, resistance, and parasitic overhead. A lithium cell can test at a high DC efficiency in a laboratory, but by the time two inverter conversions, BMS balancing, and wiring resistance are included, the homeowner sees a lower figure on an AC-coupled system.

RTE is often confused with two related metrics. Coulombic efficiency measures charge carriers: amp-hours out divided by amp-hours in. A battery can show 99.9% coulombic efficiency while RTE sits at 90% because voltage drops during discharge. Voltage efficiency captures the ratio of average discharge voltage to average charge voltage. Neither metric alone describes the energy balance the client pays for. Only RTE, energy out divided by energy in, matches the kilowatt-hours on the utility bill and the monitoring dashboard.

Think of a water tank. You pour 10 liters in through a narrow funnel (inverter losses), the tank walls absorb some moisture (chemistry inefficiency), the float valve draws a trickle continuously (BMS power), and when you drain it through a second funnel (second inverter), you collect 8.5 liters. The tank holds 10 liters. The system delivers 8.5. That is round-trip efficiency.

Designers who confuse these terms quote DC cell figures to clients who pay for AC meter readings. The result is misaligned expectations. Solar software should distinguish between DC battery RTE and AC system RTE automatically so proposals reflect what the homeowner actually experiences.

The RTE Formula: How to Calculate Battery Efficiency

The formula is simple. The application is not.

RTE = (Energy_discharged / Energy_charged) × 100%

Both measurements must be taken at the same electrical point, typically the AC meter, to produce a system-level figure. Measuring at the battery terminals gives DC RTE. Measuring at the household distribution panel gives AC RTE. Mixing the two produces meaningless numbers and unreliable financial projections.

Here is a worked example for a 10 kWh AC-coupled lithium iron phosphate (LFP) battery in a residential retrofit. The efficiencies are assumptions chosen to mirror a typical datasheet, not measurements.

Step 1: Energy charged. The homeowner draws 10.00 kWh from the grid during off-peak hours, measured at the AC meter.

Step 2: Inverter charge conversion. The battery inverter converts AC to DC at an assumed 97% efficiency. 10.00 kWh × 0.97 = 9.70 kWh reaches the battery DC bus.

Step 3: Battery DC round trip. The battery’s DC round-trip efficiency is assumed at 96%, covering cell resistance and BMS losses across a full charge and discharge. 9.70 kWh × 0.96 = 9.31 kWh is available at the DC bus on the way out.

Step 4: Discharge inversion. The battery inverter converts DC back to AC at an assumed 97% efficiency. 9.31 kWh × 0.97 = 9.03 kWh reaches the household loads.

Step 5: System RTE. (9.03 kWh / 10.00 kWh) × 100% = about 90% AC-AC.

Step 6: Depth of discharge is a separate limit. RTE is an efficiency, while depth of discharge (DoD) limits how much of the nameplate capacity you may use. A 10 kWh battery with an 85% DoD limit cycles 8.5 kWh, and at 90% RTE it delivers about 7.65 kWh to the loads.

The 10 kWh that went in becomes about 9 kWh at the load, so the 1 kWh gap is lost to conversion, cell resistance and BMS overhead. Add thermal management, standby draw and wiring, and the delivered figure falls further, as the loss budget below shows.

Now consider a DC-coupled system on a new build. The solar array charges the battery through a DC-DC stage, skipping the AC-to-DC charge conversion. In the same illustration, losing the 97% charge-side step lifts the chain from about 90.3% to about 93.1% (0.96 × 0.97). Real products differ, so use the manufacturer’s published figure.

Publish the round-trip figure, and its AC or DC boundary, in every proposal. A solar design suite that applies a flat 95% assumption to all batteries regardless of architecture produces payback estimates that can fail in the field.

AC vs DC Round-Trip Efficiency: What Solar Designers Actually Need to Know

Manufacturers quote DC round-trip efficiency because the number is higher. Installers need AC round-trip efficiency because the number is real. Understanding the gap between the two separates accurate proposals from disappointed clients.

DC RTE measures losses inside the battery pack only. It includes cell chemistry limitations, internal DC resistance, BMS balancing and cell-to-cell variation. It does not include any conversion outside the battery enclosure.

AC RTE measures energy from the point where AC power enters the system to the point where AC power exits. For an AC-coupled battery, that means two inverter conversions, one to charge and one to discharge, plus wiring resistance, breaker losses and any thermal management power drawn during the cycle. For more on this topic, see AC Coupled vs DC Coupled Battery Solar.

The difference matters because datasheets do not always say which figure they show. Enphase’s IQ Battery 5P datasheet lists both: 96% DC and 90% AC. A client who reads a DC figure and expects it at the meter will feel misled when monitoring shows the AC number.

Use this decision framework on every project:

• Retrofit / AC-coupled: Use AC RTE. The battery connects to the existing AC bus through a separate inverter. Two conversions are unavoidable.
• New build / DC-coupled: Use DC RTE plus one inverter loss for discharge. The charge controller handles DC-DC conversion from the array. Only the discharge path needs inversion.
• Hybrid inverter: Check whether the manufacturer quotes RTE at the battery terminals or the inverter AC terminals. Some hybrid inverters include battery conversion losses in their system RTE; others do not.

A further caution: some datasheets omit wiring and protection losses. A long cable run, a fused disconnect and a distribution breaker add small resistive losses, so keep a margin in the loss budget for detailed designs.

Where the Energy Goes: Inverter Losses, BMS Overhead, and Thermal Load

Energy does not disappear. It converts to heat, parasitic power and resistive dissipation. NREL defines round-trip efficiency as the system efficiency through a charge and discharge cycle, including losses from cooling systems and battery control equipment (NREL/TP-6A20-73222). The loss stack below shows why 10 kWh can become 8.5 kWh or less.

1. Inverter losses Modern battery inverters have high peak efficiency, but efficiency usually drops at low load. Because a round-trip cycle involves two conversions in an AC-coupled system, even a small per-pass loss adds up.

2. BMS parasitic loads The Battery Management System monitors cell voltages, balances charge across cells and manages safety contactors. That needs continuous power, which counts against throughput on a daily-cycled battery.

3. Thermal management Batteries perform best in a moderate temperature range. In extreme climates, active heating or cooling adds loss per cycle, and cold garages or poorly ventilated rooms can trigger heaters or fans that draw energy which never reaches the load.

4. Wiring and protection DC cabling between inverter and battery, AC cabling to the distribution panel, fuses, breakers and contactors all add resistive loss. Well-designed systems keep this small, while long runs, undersized conductors or corroded connections raise it.

This is an illustrative loss budget built on assumed values, not measured data. Poor installations or extreme climates can push total loss higher, which pulls delivered energy lower than the 8.74 kWh shown. Treat the table as a template for your own assumptions.

Round-Trip Efficiency Comparison by Battery Chemistry

DC round-trip efficiency varies across storage chemistries, and chemistry also affects cycle life, safety, temperature tolerance and cost. The ranges below are commonly cited general guidance, not product specifications. Check the datasheet for the exact model.

LFP is often cited as leading on DC RTE. Its flat discharge curve keeps the battery near a steady voltage through most of the cycle, which limits resistive loss. NMC offers higher energy density, meaning more kilowatt-hours per liter, and typically trades some efficiency for it.

Lead-acid remains common in legacy off-grid systems, but high internal resistance and reduced capacity at high discharge rates limit its efficiency. For new solar-plus-storage designs, lead-acid rarely competes on lifetime cost. See our guide on Battery Solar System Design UK for more.

Vanadium flow batteries trade round-trip efficiency for longevity. Their efficiency is lower than mainstream lithium-ion, but long cycle life makes them attractive for long-duration utility discharge where lifetime throughput matters more than per-cycle efficiency.

Aging can degrade RTE over time. As internal resistance rises, resistive losses per cycle rise with it. A battery can retain most of its nameplate capacity while its round-trip efficiency slips, so model capacity fade and efficiency fade separately.

Published RTE Figures: What a Datasheet and a Planning Study Show

Manufacturer datasheets provide a baseline, but the conditions matter. The table uses the Enphase IQ Battery 5P datasheet (document DSH-00010-11.0, dated 6 May 2025) and the NREL 2024 Annual Technology Baseline, checked November 2026. Other manufacturers publish their own figures on their own sites, and some do not state the AC or DC boundary, so read each datasheet directly.

Manufacturers state conditions for a reason. Enphase quotes its AC figure at 50% power rating, so a battery run harder or in extreme temperatures can return less. Treat a quoted figure as a starting point and keep a margin in the model.

Products that quote only a DC figure require the designer to model inverter losses separately. Do not mix DC and AC figures in the same financial model: assuming the 96% DC value at the meter overstates performance by several percentage points against the 90% AC value for the same battery.

For a direct comparison, see Arka 360 vs SurgePV.

How RTE Affects Solar Self-Consumption and Financial Returns

Round-trip efficiency is not an abstract specification. It directly determines how much money a battery saves, how fast it pays back, and whether a self-consumption target is achievable.

This illustration assumes a 10 kWh battery cycled once daily, 365 days per year, at a utility rate of $0.15 per kWh. The inputs are assumptions, not market data.

At a hypothetical 100% RTE, the battery shifts 10 kWh × 365 = 3,650 kWh annually. At $0.15 per kWh, that equals $547.50 per year in avoided grid imports.

At an assumed 90% AC RTE and 85% DoD, usable energy per cycle is 7.65 kWh. Annual shifted energy becomes 7.65 kWh × 365 = 2,792 kWh, and savings drop to $418.84 per year.

The gap is $128.66 per year, or about $1,930 over a 15-year life. If the designer promised an 8-year payback using 100% efficiency and full capacity, the same project pays back in roughly 10.5 years.

The error also reaches self-consumption design. Imagine a 5 kW array generating 7,000 kWh per year, with a 60% self-consumption target (4,200 kWh kept on site). The battery is sized to absorb excess daytime production and discharge it in the evening.

If the designer assumes 95% RTE but the system delivers 88%, the battery returns less energy per cycle than the model expects. The shortfall is met by grid imports, the client misses the self-consumption target, and the installer faces a callback.

Warranty language adds another layer. A capacity warranty covers remaining capacity, and a datasheet may not warrant round-trip efficiency. Enphase’s datasheet, for example, lists a capacity warranty of more than 60% for up to 15 years or 6,000 cycles, whichever occurs first, and lists RTE as a specification. Solar proposal software should model capacity fade and RTE fade separately so clients see the full depreciation curve.

Sources for the figures above: the Enphase IQ Battery 5P datasheet, the NREL 2024 ATB utility-scale battery page and the NREL cost projections report for utility-scale battery storage (June 2019), all checked November 2026.

Improving Round-Trip Efficiency: What Installers Can Control

Not all losses are fixed. Design choices and maintenance can recover some of them.

1. Prefer DC-coupled or hybrid architectures Eliminating one AC-DC conversion step removes one loss in the chain. On new builds, specify DC-coupled battery storage with a hybrid inverter. On retrofits where AC-coupling is unavoidable, compare battery inverter efficiency across candidates. See Adding Battery Storage Services for detailed guidance.

2. Right-size the inverter An oversized inverter operating at a small fraction of rated load wastes conversion efficiency. Match the inverter continuous rating to the battery’s maximum charge and discharge rate, unless future expansion is planned.

3. Manage temperature Install batteries in conditioned, shaded or ventilated spaces. Unheated garages in cold climates can trigger heating draws, and direct sun in hot climates can force cooling cycles. Cooler, steadier cell temperatures also slow aging. Shadow analysis software identifies shading on the array before installation.

4. Limit charge and discharge rates Lower C-rates reduce resistive losses. Charging a battery at 0.2C instead of 0.5C cuts I²R heat generation, so where time-of-use windows allow, program slower charging schedules.

5. Keep firmware updated BMS firmware updates can improve cell balancing and state-of-charge estimation. Apply manufacturer updates, and note any change in standby draw.

6. Use accurate modeling software Build real RTE figures into every proposal. Show year-1 RTE and an aged-battery case side by side, so clients see realistic expectations. Solar design software that automates these projections prevents manual spreadsheet errors and keeps every proposal consistent.

Conclusion

A 10 kWh label does not mean 10 kWh delivered. Round-trip efficiency, depth-of-discharge limits, inverter losses, BMS overhead and wiring resistance all reduce usable energy. In the worked example, a 10 kWh battery at 90% RTE and 85% DoD delivers about 7.65 kWh to the load. That is physics, not a defect.

Chemistry matters, but system architecture and the stated boundary decide what the homeowner sees. DC-coupled designs skip one inverter conversion, and AC-coupled retrofits face two. Model the figure that matches your architecture.

The financial impact is measurable. In the illustration above, ignoring RTE and DoD overstates savings by about $1,900 over 15 years. A designer who models it accurately builds trust and sizes storage that performs as promised.

Use PV design software that applies architecture-specific RTE assumptions and year-by-year capacity fade, then book a SurgePV demo to see the generation and financial workflow, or compare plans on the pricing page. Your clients will see realistic numbers, and your callbacks should drop.

Frequently Asked Questions

What is round-trip efficiency in a solar battery?

Round-trip efficiency is the percentage of energy you get back after storing and discharging electricity. If you put 10 kWh into a battery and get 9 kWh out, the RTE is 90%. It accounts for inverter, chemistry and BMS losses in a complete cycle.

How do you calculate round-trip efficiency?

Divide the energy discharged by the energy charged: RTE = (Energy_discharged / Energy_charged) × 100%. Measure both values at the same point, typically the AC meter, to get a system-level figure.

What is a good round-trip efficiency for a solar battery?

Published figures cluster around 85% to 96% depending on the boundary. NREL’s 2024 ATB assumes 85% for utility-scale batteries, and Enphase publishes 90% AC and 96% DC for its IQ Battery 5P. Always check which boundary a datasheet uses.

What is the difference between AC and DC round-trip efficiency?

DC RTE measures losses inside the battery only. AC RTE includes inverter and wiring losses for the complete system. AC RTE is what the homeowner’s meter measures and what matters for financial modeling.

Which battery chemistry has the highest round-trip efficiency?

Lithium iron phosphate (LFP) is commonly cited as leading on DC RTE, with NMC close behind. Lead-acid and flow batteries are commonly cited as lower. Check the datasheet of the exact model rather than relying on a chemistry average.

What causes energy loss in a battery round-trip cycle?

Energy is lost to inverter conversion heat, BMS balancing and monitoring power, thermal management, resistive losses in wiring and protection devices, and internal cell resistance. NREL defines round-trip efficiency as including cooling systems and battery control equipment losses.

Does round-trip efficiency drop over time?

Yes, it can. Aging increases internal resistance, which raises resistive losses per cycle, so a battery can keep much of its capacity while its efficiency slips. Model capacity fade and efficiency fade separately.