Performance Ratio: Where a PV System’s ~15% of Lost Energy Actually Goes

Key Takeaways

1. Performance Ratio links solar resource with delivered AC energy. A PR of 0.82 means the meter recorded 82% of the reference energy.

2. PVGIS’s 14% input covers one layer of the energy reduction. Temperature and incidence-angle effects are calculated separately.

3. Loss percentages compound through the energy path. Adding them directly overstates the combined reduction.

4. Loss patterns require different responses. Light-induced degradation occurs near the start of module operation, while soiling returns with site conditions and availability loss follows outages.

5. The Project Builder starts with a 14% system-loss allowance. This figure covers soiling, wiring, inverter conversion, mismatch, nearby shading and availability. PVGIS calculates temperature separately.

What Performance Ratio Measures

Performance Ratio uses the two energy figures from the introduction. The 950 MWh is reference energy, calculated from plane-of-array irradiation and installed DC capacity at a reference irradiance of 1 kW/m². The AC meter supplies the delivered energy of 779 MWh.

For the example:

779 MWh ÷ 950 MWh = 0.82

A PR of 0.82 means the array delivered 82% of its reference energy.

Module efficiency measures the share of incident sunlight converted to electricity under stated test conditions. Module area and efficiency determine rated power, expressed in Wp or kWp. Capacity factor compares annual production with continuous operation at rated capacity for 8,760 hours, so local irradiation influences the result. PR normalises production for available irradiation and focuses the assessment on system performance.

The PVGIS loss input covers selected system losses. PVGIS applies this percentage after modelling temperature and irradiance effects, placing the 14% default within the full PR reduction. IEC 61724-1 sets out the monitoring terminology and assessment methods.

The Loss Stack, Step by Step

Performance Ratio states how much reference energy reached the AC meter. The table traces each loss from the module surface to grid delivery and identifies its place in the Project Builder calculation. The 14% system-loss allowance covers most operating losses. PVGIS calculates temperature and irradiance effects separately. The Builder applies annual module degradation over the project life.

These ranges help estimate individual losses. Their combined effect requires multiplication because each percentage applies to the energy remaining after the preceding loss.

Sunlight first reaches the module glass, where dust reduces the irradiation passing to the cells. Local dust conditions, rainfall, module tilt and cleaning frequency therefore set the soiling allowance. Once the cells receive this energy, their operating temperature affects DC output. Restricted airflow behind the array raises cell temperature and increases the reduction.

Light-induced degradation occurs near the start of module operation, following first exposure to sunlight. Long-term degradation develops over years. Separating these timescales prevents the initial reduction from reappearing as an annual rate.

The remaining DC electricity passes through connected modules and cables. Differences in current and voltage create mismatch. Cable resistance dissipates part of the electrical energy as heat:

After the DC cables, the inverter converts the remaining energy to AC. Its efficiency varies with loading and input voltage, linking the conversion loss to equipment selection and operating conditions. Resistance in AC cables and connections dissipates another share on the way to the meter.

Availability loss falls outside this electrical path. A fault, maintenance period or grid outage removes production for part of the year. PVsyst separates optical, array and system losses in its energy balance, linking the final PR to individual causes.

How Loss Percentages Combine

Loss percentages apply in sequence. NREL’s PVWatts assigns default values of 2% for soiling, 3% for shading, 2% for mismatch, 2% for wiring, 0.5% for connections, 1.5% for light-induced degradation, 1% for nameplate rating and 3% for availability.

Their straight sum is 15%. Compounding produces 14.08% because each percentage acts on the remaining energy:

Applied to the PVWatts values:

1 − (0.98 × 0.97 × 0.98 × 0.98 × 0.995 × 0.985 × 0.99 × 0.97) = 14.08%

PVWatts calculates module temperature and inverter conversion elsewhere in its model. The Project Builder draws a different boundary: its 14% system-loss input includes inverter conversion but excludes temperature. The shared percentage does not mean both models contain identical loss categories.

Which Losses Can Be Reduced?

The Project Builder starts with a 14% system-loss allowance. Project documents support any reduction from this reference, while difficult site conditions may justify a higher figure.

Design choices influence wiring, mismatch and inverter losses. A larger conductor area cuts resistance, though extra copper raises capital cost. The investment becomes economical once recovered energy offsets the added expense.

Maintenance determines much of the soiling and availability allowance. Cleaning removes deposits but consumes labour and water. Monitoring shortens fault detection time, while spare parts reduce repair delays.

Site weather sets the operating conditions behind temperature loss. Ventilated mounting limits heat buildup, and a favourable temperature coefficient reduces the power penalty at high cell temperatures. Procurement choices also influence initial light-induced degradation and ageing over the project life.

Cable schedules and inverter datasheets support the system-loss estimate. Cleaning logs and outage records document operating losses. Module qualification data and warranty terms support the annual degradation rate.

Setting Honest Loss Inputs in the Project Builder

The Project Builder now separates system losses from annual module degradation. Its System losses slider starts at 14%, the PVGIS reference value, and accepts values from 9% to 20%.

This percentage covers soiling, DC and AC wiring, inverter conversion, module mismatch, nearby shading and availability. The selected value is sent to PVGIS through its loss parameter. Temperature remains outside this input because PVGIS calculates it from module technology, irradiance and mounting conditions.

The PV module losses slider sets the annual degradation rate. Its starting value is 0.45% per year, with an available range of 0.2% to 0.8%. The Builder applies this reduction year by year during the financial projection. It does not form part of the system-loss percentage sent to PVGIS.

Light-induced degradation also remains outside both calculations. The current Builder notes an initial loss of about 1% but omits it from the model.

Begin with the 14% system-loss reference where project information remains limited. A different value needs support from the site layout, cable calculations, inverter data, cleaning plan and expected availability. Set annual degradation from the module datasheet or warranty terms. Record both assumptions in the project comment.

Run the yield calculation after entering the system-loss value. Then compare the financial projection under the selected degradation rate. This separates the first-year production estimate from the gradual decline over the project life.

Test Your Project Losses

Place your project point on the SolarBrief Project Builder map. Review the 14% system-loss allowance and adjust it where project evidence supports another value. Set the annual module-degradation rate from the equipment data, then calculate the PVGIS yield. Record both assumptions with the result for later project review.

FAQ

# Does a 14% PVGIS loss input mean the PR is 86%?

The 14% input covers system losses such as soiling, wiring, inverter conversion, mismatch, nearby shading and availability. PVGIS calculates temperature and irradiance effects separately. The final PR therefore reflects a broader energy reduction than the 14% input alone.

# Does temperature loss belong in the 14% system-loss input?

PVGIS calculates temperature from the module technology, irradiance and mounting conditions. Including another temperature allowance in the system-loss percentage counts the effect twice and understates projected yield.

# How does module degradation enter the calculation?

The Builder applies the selected annual degradation rate over the project life. Its starting value is 0.45% per year. This rate affects the financial projection and remains separate from the system-loss percentage sent to PVGIS.

Glossary: Key Terms

Sources: PVGIS 5 User Manual, PVGIS Frequently Asked Questions, PVGIS Data Sources and Calculation Methods, NREL PVWatts Version 5 Manual, IEC 61724-1, PVsyst Performance Ratio, PVsyst Array and System Losses, PVsyst Ohmic Losses