Calculating Round Trip Efficiency (RTE) for Co-located Storage
Round trip efficiency (RTE) is defined as the ratio of total AC energy discharged from a battery energy storage system (BESS) to the total AC energy injected during charging, inclusive of all conversion and balance-of-system losses.
Most EPCs rely solely on battery datasheets, leading to optimistic projections. A manufacturer might claim 95% DC-side efficiency, but project viability depends on AC-to-AC meter readings. When determining how to calculate auxiliary load losses in co-located solar projects, you must account for transformer losses, HVAC draw, and controllers. If these are ignored, your pro-forma becomes a work of fiction, failing to address why solar plant performance ratio is lower with co-located batteries.
The RTE Formula
To determine actual system performance, use this formula:
RTE (%) = (Total AC Discharge Energy / Total AC Charge Energy) × 100
You must capture these values at the Point of Interconnection (POI). To verify your assumptions and run sensitivities on calculating round trip efficiency for dc coupled solar storage systems, test the calculations using the SolarMetrix performance simulator at solarmetrix.app/tool.
Numerical Example
- Total Charged: 1,000 kWh at the AC meter.
- Total Discharged: 850 kWh at the AC meter.
- Calculation: (850 / 1,000) × 100 = 85% RTE.
If your model promised 92%, your debt service coverage ratio (DSCR) is likely already tanking.
Rule of Thumb
Rule of thumb: Always subtract an additional 3–5% from the manufacturer's stated DC efficiency to account for "parasitic" auxiliary loads and site-specific balance-of-system (BOS) losses.
5 Common Causes of RTE Degradation
If your RTE is lower than your energy model, analyze these factors:
- Transformer Inefficiency: Low-load operation causes disproportionate core losses.
- HVAC Draw: Excessive cooling in poorly insulated enclosures increases parasitic load.
- BMS Cycling: High-frequency, shallow-depth discharge cycles increase thermal waste.
- Inverter Conversion: Operating inverters outside their optimal efficiency curve during low-solar charging hours.
- DC-Coupling Impedance: Excessive cable lengths between the solar array and the BESS cause voltage drops.
If you suspect deep-seated performance issues, evaluate how to optimize battery dispatch for co-located solar pv assets to ensure you aren't masking underlying system degradation.
FAQs
What is a healthy round-trip efficiency for a commercial BESS? A healthy, modern lithium-ion co-located system typically yields between 85% and 89% AC-to-AC efficiency. Anything below 80% usually indicates excessive auxiliary consumption, transformer mismanagement, or significant environmental thermal losses that require immediate site inspection.
How does battery depth of discharge (DoD) affect RTE calculations? Operating at extreme DoDs (below 10% or above 90%) triggers non-linear resistance. This increases heat generation and internal energy dissipation during charging. To maintain high RTE, keep your operational setpoints within the "sweet spot" defined by the manufacturer’s charge/discharge efficiency curve, typically 20% to 80% state of charge.
Does ambient temperature significantly impact my RTE model? Yes, ambient temperature dictates the duty cycle of your battery HVAC system. Every degree above the optimal operating temperature (usually 20–25°C) increases cooling load. In hot climates, ignoring the thermal management overhead will result in a 2–4% drop in net RTE compared to standard laboratory conditions.