Solar performance ratio in Perth: what it is and whether your system is healthy

Your solar system has a rated capacity (say, 6.6kW). In theory, on a Perth day with 5 peak sun hours, it "should" generate 33kWh (6.6 × 5). In practice, it never generates exactly this. The performance ratio (PR) is a measure of how efficiently the system converts available solar resource into delivered electricity, accounting for real-world losses.

Understanding PR lets you compare your system's performance against expectations and spot when something is wrong.

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What performance ratio means

Formula: PR = Actual generation ÷ (System capacity × Peak sun hours × Days)

Example:

• System: 6.6kW
• Perth annual peak sun hours: 5.0 × 365 = 1,825 PSH
• Theoretical maximum annual generation: 6.6 × 1,825 = 12,045kWh
• Actual annual generation (measured): 9,700kWh
• PR = 9,700 ÷ 12,045 = 0.806 (80.6%)

What constitutes good PR in Perth:

Perth's climate context: In hot climates, PR is naturally lower than in cooler climates: temperature losses are higher. A Perth summer PR of 0.75 may be perfectly normal on days when panel temperatures reach 60–70°C, because heat reduces output efficiency (approximately 0.4–0.45% per °C above 25°C for PERC panels). Annual PR averaging 0.78–0.83 is typical for a well-maintained Perth system.

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The main sources of performance ratio losses

Temperature losses (typically 5–10%): Perth's heat is the largest single PR reducer. Panels operate at 25°C in standard test conditions (STC); on a 35°C day with full sun, panel temperature may reach 60–65°C, triggering a 14–18% derating in output. This is not a fault; it's physics. LFP batteries handle heat better, but panels lose efficiency regardless of battery chemistry.

Wiring and inverter losses (typically 3–5%): DC cabling from panels to inverter, inverter conversion efficiency (typically 97–98%), and AC cabling to the switchboard all introduce losses. Well-designed systems minimise these.

Shade losses (variable, 0–30%+): Any partial shade during peak hours significantly reduces string inverter output. A tree branch shading one panel on a 12-panel string can reduce the entire string's output substantially. Microinverters and optimisers minimise this.

Soiling losses (typically 1–7%): Accumulated dust, bird droppings, and pollen reduce PR. In Perth's extended dry summer, soiling losses are higher than in wetter cities.

Panel degradation (typically 0.3–0.5%/year): Panels produce slightly less each year. A system installed 10 years ago may have lost 3–5% capacity from normal degradation. This shows up as slightly lower PR over time.

System clipping losses: When a 6.6kW panel array is connected to a 5kW inverter, any generation above 5kW is "clipped": the inverter limits output to its rated capacity. This is by design (the standard Australian oversizing practice) and accounts for typical 2–5% clipping loss on the sunniest summer days.

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How to check your system's PR

Step 1: get your annual generation figure From your solar monitoring app or Synergy DEBS export data, find your total generation for the past 12 months.

Step 2: calculate theoretical maximum System capacity (kW) × 1,825 (Perth annual peak sun hours at 5.0 PSH × 365 days)

Step 3: calculate your PR PR = Actual ÷ Theoretical

Example calculation:

• 6.6kW system installed February 2021 (operating approximately 4 years)
• Annual generation 2025: 9,200kWh
• Theoretical: 6.6 × 1,825 = 12,045kWh
• PR = 9,200 ÷ 12,045 = 0.764 (76.4%)
• Assessment: acceptable but below typical good performance, investigate shade changes (tree growth?), soiling, or potential inverter efficiency drop

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Year-on-year trend monitoring

Single-year PR can vary due to weather. More useful is the trend:

• Calculate PR for each full calendar year of system operation
• A declining PR trend over 2–3 years (after accounting for normal degradation ~0.3%/yr) indicates a developing issue
• A sudden drop in PR from one year to the next (>5–8% drop) warrants investigation

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Seasonal PR variation

Perth systems naturally show lower PR in summer than winter (despite higher total generation):

• Summer: high temperatures depress efficiency; more clipping losses near midday
• Winter: cooler temperatures → higher efficiency; less clipping

A summer PR of 0.73–0.78 paired with a winter PR of 0.80–0.85 is typical for Perth. What matters is whether your seasonal PR is consistent with prior years.

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When to investigate PR issues

Investigate if:

• Annual PR < 0.70 (once the system is more than 2 years old)
• PR drops > 8% from the prior year without an obvious cause (tree grew, soiling season)
• Monitoring app shows an output drop on clear days compared to the same days last year
• Any string in a multi-string system shows significantly lower output than other strings (inverters typically show per-string data)

Investigation steps:

1. Check for new shading sources (trees, new structures)
2. Check panel soiling visually
3. Check inverter fault codes in monitoring app
4. Review inverter efficiency in monitoring data (should be ≥97%)
5. If no obvious cause: engage a licensed solar electrician for an assessment

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If you have access to your annual generation data from your monitoring app, calculate your PR and compare it to the ranges above. A PR below 0.72 for a Perth system more than 3 years old is worth investigating with a professional inspection.