For developers, engineers, and financial modelers
BESS Round Trip Efficiency (RTE) Calculator
Equipment manufacturers quote DC block round-trip efficiency at 94–95%. That figure excludes the PCS, transformers, cables, and auxiliary power between the DC block and the Point of Interconnection. This calculator shows what actually reaches the grid.
Equipment efficiency
Set each value with the slider, a preset, or your own number. Single-path efficiencies are squared automatically — energy passes through on both the charge and the discharge.
Auxiliary power
HVAC, cooling, BMS, plant controllers, SCADA, lighting. Not an equipment efficiency — modeled as a percentage of the energy discharged.
Arbitrage impact
How plant-level RTE affects the effective cost of energy sold back to the grid.
How to use this calculator
Each row is a stage between the battery cells and the Point of Interconnection. Set each efficiency with the slider, one of the preset values, or by typing your own number. The gauge and the figures beside it update live, showing the single-path efficiency, the auxiliary power losses, and how much of every 100 MWh charged is returned to the grid.
Only the DC block is specified as a round-trip figure. Every other stage — the PCS, the two transformers, and the cabling — is specified as a single-path efficiency, which the calculator squares so both the charge and the discharge pass are counted. Use the toggle to include or exclude auxiliary power from the plant-level result.
New to the concept? What Is Round Trip Efficiency (RTE) in BESS? explains what RTE is, where the losses occur, and why auxiliary power changes the equation.
How the calculation works
Energy passes through every component twice per cycle — once charging, once discharging — so each single-path efficiency η contributes η² to the round trip. The equipment round-trip efficiency is the product of every stage's round-trip contribution, and the single-path figure shown is its one-way equivalent (the square root of that product).
Auxiliary power is applied on top, as a percentage of the energy discharged: Plant RTE = Equipment RTE × (1 − aux). This is a deliberate simplification. Auxiliary load actually runs across the whole cycle, including idle time, so a plant that sits idle or cycles slowly carries a larger real auxiliary hit than a single fixed percentage captures — modeling that precisely needs the duty cycle and idle time, which belong in a dispatch or revenue model rather than a round-trip figure.
Who this calculator is for
- Financial modelers building revenue cases that need realistic RTE assumptions instead of datasheet figures.
- Project developers evaluating how equipment selection and plant layout affect whole-plant efficiency.
- Technical advisors checking whether the RTE assumptions in a financial model are reasonable.
- Engineers quantifying where losses occur and how design changes move the POI-level result.
Frequently asked questions
Why are single-path efficiencies squared in this calculator?
Energy passes through every component twice per cycle: once on the way in while charging, and once on the way out while discharging. A component with a single-path efficiency of η therefore contributes η² to the round trip, so the calculator squares each single-path value automatically. The DC block figure is already a round-trip number — it covers both the charge into and the discharge out of the cells — so it is used directly and not squared.
What is the difference between the single-path and round-trip figures shown?
The single-path efficiency is the one-way efficiency through all the equipment, from the Point of Interconnection to the DC block. The round-trip figure counts both passes, so it is roughly the single-path value squared. The Plant RTE on the gauge is that round-trip figure after auxiliary power is taken out — the number that actually determines how much energy reaches the grid.
Is auxiliary power taken as a percentage of charged or discharged energy?
This calculator treats auxiliary power as a percentage of the energy discharged and reduces the plant RTE accordingly: Plant RTE = Equipment RTE × (1 − aux). It is a deliberate simplification — auxiliary load actually runs across the whole cycle, including idle time, so a plant that sits idle or cycles slowly carries a larger real auxiliary hit than a single fixed percentage captures. Use the toggle to see the result with and without it.
Why is the plant RTE lower than the DC block datasheet number?
The datasheet figure covers only the DC block. The plant-level result also includes the PCS, both transformers, the cabling, and the auxiliary load between the DC block and the Point of Interconnection — each adding losses on both passes. The plant RTE is what reaches the grid, and it is always lower than the datasheet number.
How does round-trip efficiency affect arbitrage economics?
Every point of RTE lost raises the effective cost of each MWh sold, because more energy has to be bought than is delivered. The arbitrage section shows this directly: the effective cost per MWh sold is the buy price divided by the plant-level RTE, so a lower RTE compresses the margin between the buy and sell prices.