About the Auxiliary Load
Auxiliary load calculation estimates the parasitic power consumption of a solar plant or BESS — the energy used by the plant's own systems (inverters in standby, monitoring systems, HVAC, lighting, pumps, security) that reduces the net export to the grid. Auxiliary load directly impacts the performance ratio and revenue of the plant.
How to calculate it
- 1List all auxiliary consumers: inverter standby power, transformer no-load losses, SCADA/monitoring systems, HVAC (inverter room, BESS container), lighting, security cameras, pumps (cleaning water, cooling).
- 2Inverter standby: typically 0.1–0.3% of rated power when operating; 20–50 W/inverter in night standby.
- 3Transformer no-load losses: 0.1–0.3% of rated kVA continuously.
- 4HVAC: 2–5 kW per inverter room (depending on room size and ambient); 1–3 kW per BESS container.
- 5SCADA/monitoring: 50–200 W total.
- 6Total annual auxiliary energy: sum all loads × hours of operation.
- 7Auxiliary load ratio = annual auxiliary energy ÷ annual gross generation × 100%.
Worked example
A 1 MWp plant: inverter (0.2% × 1,000 kW × 8 hours/day = 1,600 Wh/day), transformer no-load (1.5 kW × 24 h = 36 kWh/day), HVAC inverter room (3 kW × 16 h = 48 kWh/day), SCADA (100 W × 24 h = 2.4 kWh/day). Total daily auxiliary: ~88 kWh. Annual gross generation: 1,500 MWh. Auxiliary ratio = 88 × 365 ÷ 1,500,000 × 100 = 2.1% — within the typical 1–3% range for well-designed plants.
What you can calculate
- Parasitic kW
- HVAC load
- BMS power
- Net efficiency
Frequently asked questions
Does auxiliary load affect the solar plant's PR (Performance Ratio)?
Yes — Performance Ratio is calculated on net energy export, which is gross generation minus auxiliary load. A plant with a 2% auxiliary load has its PR reduced by approximately 2 percentage points. For bankable PR calculations, lenders require that auxiliary load is measured and reported separately from generation losses. High auxiliary loads (> 3%) typically indicate oversized HVAC, inefficient transformer selection, or energy waste in monitoring systems.
How is auxiliary power supplied in a solar plant?
Auxiliary power is typically taken from the plant's own AC output (self-supply from the LT bus) via a dedicated auxiliary transformer or directly from the ACDB. At night, when the solar inverters are off, auxiliary power comes from the grid through the plant's transformer. Some off-grid plants use a small dedicated auxiliary solar + battery system for night-time monitoring and security loads.
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