Pin lưu trữ LiFePO₄LiFePO₄ storage · BàiLesson 4/10
Round-Trip Efficiency (RTE) of Energy Storage Systems
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Solar Knowledge HubHiệu suất vòng tròn RTERound-trip efficiency (RTE)
Điện vào pin rồi ra không bao giờ nguyên vẹn. RTE pin DC–DC và RTE cả hệ AC–AC là hai con số khác nhau: nhầm chúng dẫn tới tính toán sai.Energy into a battery never comes back intact. Battery DC–DC RTE and whole-system AC–AC RTE are different numbers: confusing them causes calculation errors.
Tổng năng lượng hao sau một vòng qua kho pin. Cứ 10 kWh đưa vào → lấy lại được khoảng 8.5 – 9.2 kWh.Total loss per round trip through the storage system. Every 10 kWh in → about 8.5 – 9.2 kWh back out.
- Khi sạc: nhiệt trong cell + tiêu hao BMS + dây dẫn → mất ~1–3%.During charge: cell heat + BMS draw + cabling → ~1–3% lost.
- Khi xả: tương tự, lại mất ~1–3% ở cell và van bán dẫn.During discharge: similar, another ~1–3% in cells and switches.
- Tự tiêu hao: LFP ~1–3%/tháng khi để yên (không phải mỗi vòng sạc xả ngắn).Self-discharge: LFP ~1–3% per month at rest (not per short cycle).
- Biến tần: mỗi lần chuyển đổi DC↔AC thêm ~2–4% hao.Inverter: each DC↔AC conversion adds ~2–4% loss.
Hệ đấu một chiều nối tấm pin trực tiếp vào thanh DC rồi vào pin: khi xả chỉ chuyển đổi một lần nên RTE cả vòng cao hơn. Hệ đấu xoay chiều đổi DC→AC để sạc rồi AC→DC trở lại, thêm tổn hao ~2–6 điểm phần trăm. Khi ưu tiên tự dùng từ pin, cấu hình DC-coupled nhỉnh hơn.DC-coupled systems connect panels directly to the DC bus and battery: only one inversion on discharge, so round-trip efficiency is higher. AC-coupled systems convert DC→AC to charge then AC→DC back, adding ~2–6 percentage points of loss. For self-consumption priority, DC-coupled is more efficient.
Khách thắc mắc "sạc 10 số mà tối dùng được hơn 9 số"? Giải thích: mọi kho lưu trữ đều hao một phần khi nạp và xả, giống rót nước qua phễu. LFP hao ít nhất và phần hao đã tính vào bài toán tiết kiệm. Tránh hứa RTE 95% cho cả hệ: đó là con số riêng pin trong phòng thí nghiệm; thực tế qua biến tần là 85–92%.Customer asks why charging 10 kWh only gets 9+ kWh back? Explain: every storage system loses some energy in and out, like pouring water through a funnel. LFP loses the least, and the loss is already factored into the savings estimate. Never promise 95% system RTE: that's battery-only in a lab; real-world through the inverter is 85–92%.
Energy that enters a battery and then leaves is never fully recovered: a portion is always lost as heat. That loss is measured by round-trip efficiency, abbreviated RTE. Understanding RTE correctly matters because it bridges the gap between a datasheet specification and the real electricity-bill arithmetic, and it is also the point where numbers measured at different boundaries are most easily confused.
For: technicians calculating yield and designing systems, sales staff explaining why energy in does not equal energy out, and newcomers who need to distinguish battery efficiency from whole-system efficiency.
Quick summary
RTE is the ratio of energy discharged to energy charged over one complete charge-discharge cycle. There are two levels of RTE: do not confuse them.
The battery’s own RTE is measured directly at the battery terminals, from DC in to DC out, and reaches roughly 93 to 96% for LFP cells. The whole-system RTE also includes the inverter’s conversion losses in both directions, so it falls to roughly 85 to 92%.
Losses originate from heat inside the cells, the BMS circuitry, wiring resistance, and the inverter’s conversion process. High temperatures and high charge/discharge currents both push efficiency down, so the best figures are only achieved when the battery is cool and operating at a light load.
What RTE is and where it is measured
RTE stands for round-trip efficiency: the efficiency of one complete charge-then-discharge cycle. The core formula is straightforward: RTE equals energy discharged divided by energy charged, multiplied by 100%. Charge 10 kWh in and discharge 9.5 kWh out and the RTE is 95%.
What determines the meaning of the number is the measurement boundary. Measured directly at the battery terminals (DC energy in to DC energy out), the result is the battery’s own RTE. Measured from the AC side of the building, through the inverter to charge the battery, then back through the inverter to discharge as AC, the result is the whole-system RTE. The two figures differ because the system adds two inverter conversion steps, each consuming a few percent.
Compared with older battery technologies, LFP stands out precisely on this metric. Lead-acid batteries achieve only about 70 to 85% RTE, gel and AGM types reach about 80 to 90%, while standard LFP reaches 93 to 96% and premium LFP reaches 95 to 97%.
| Battery type | Battery-level RTE |
|---|---|
| Lead-acid | 70 – 85% |
| Gel | 80 – 90% |
| AGM | 80 – 90% |
| LiFePO₄ standard | 93 – 96% |
| LiFePO₄ premium | 95 – 97% |
Where the losses occur
The energy that is lost does not disappear: it is converted mainly into heat, spread across three stages. During charging, heat generated inside the cells plus consumption by the BMS and wiring accounts for roughly 1 to 3%. While the battery sits idle in storage, self-discharge takes a small additional fraction. During discharging, roughly another 1 to 3% is lost in the cells, semiconductor switches, and wiring. Added together, these produce the battery-level RTE loss.
One clarification on self-discharge is needed to avoid a unit confusion. LFP batteries self-discharge at roughly 1 to 3% per month when left idle. The much smaller figure of 0.5 to 1% is only correct when calculated across the short charge-discharge window of a few hours to a few days that a solar system operates on, not per month. In daily operation, a battery charged during the day and discharged in the evening makes self-discharge negligible.
From battery efficiency to whole-system efficiency
This is where calculation errors most often occur. Battery efficiency and inverter efficiency are measured at different boundaries, so they cannot simply be multiplied together and assigned to the whole system.
Inverter efficiency is measured across only the DC-to-AC conversion process, typically reaching 96 to 98%. Whole-system efficiency must follow the complete round trip: AC power through the inverter into the battery, then from the battery back through the inverter to AC output. Because there are two conversion steps on top of the battery’s own losses, real-world whole-system efficiency lands at roughly 85 to 92%, not the higher figure that is often assumed.
A full-cycle example makes this concrete. Start with 10 kWh of AC power; after passing through the inverter and into the battery, roughly 9.5 kWh remains; after discharging back through the inverter to AC, roughly 9.2 to 9.3 kWh is available. The round-trip system efficiency is approximately 92%, meaning about 0.7 to 0.8 kWh is lost for every 10 kWh cycled through the battery bank.
A DC-coupled system connects the solar array directly to the DC bus and into the battery. On discharge there is only one conversion step, so the whole-system round-trip efficiency is higher. An AC-coupled system must convert from the array to AC, then back to DC to charge the battery, adding extra losses that reduce round-trip efficiency by roughly 2 to 6 percentage points. When advising on a system that prioritises self-consumption from storage, DC-coupled configurations generally have the efficiency advantage.
A quoted RTE of 95% is typically measured in a laboratory at 25 °C and at a low charge/discharge current (low C-rate). In the field, elevated temperatures and higher C-rates both increase internal resistance and therefore increase losses, pulling RTE down. Always use realistic field ranges when calculating yield. Do not use the peak nameplate figure.
Quick estimation rules
To estimate storage losses without detailed calculation, use three round numbers. The LFP battery itself contributes roughly 4 to 7% loss over the charge-discharge cycle. Each inverter conversion adds roughly 2 to 4%. Combined, the total loss after energy passes through the battery bank and returns is roughly 8 to 15% depending on conditions.
In practical terms: for every 10 kWh put into the battery bank, roughly 8.5 to 9.2 kWh is available for use. This is the figure to plug into the economic model in document 03-06, rather than assuming the battery returns energy without any loss.
Sales takeaway
When a customer asks why they charged 10 units but can only use just over 9 in the evening, the answer is that every storage system loses a small portion during charging and discharging. LFP has the lowest losses of any common battery chemistry, and those losses are already factored into the savings calculation, so there are no hidden costs.
Avoid promising whole-system efficiency of 95%. That figure belongs to the battery alone under laboratory conditions. A real system through the inverter reaching roughly 85 to 92% is accurate and honest, and customers will trust you more when the number on their monitoring app matches what they were told.