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Solar Energy Yield and Performance Ratio Calculator
Any yield estimate is irradiation multiplied by capacity multiplied by everything that goes wrong on the way. The number that separates a credible estimate from a sales figure is the loss chain: which losses were counted, and at what value.
This calculator makes each loss explicit so you can see what the performance ratio is built from rather than accepting a single rounded percentage.
Build the loss chain
From PVGIS at your tilt and azimuth.
Performance ratio
81.5%
Typical
Specific yield
1,427 kWh/kWp
Annual production
8,561 kWh
Average day
23.5 kWh
Typical peak month
1,106 kWh
Around June or July in the northern hemisphere
Typical low month
321 kWh
Around December
How the calculation works
Figure — The sizing chain
Start from plane-of-array irradiation in kWh/m² per year. Because module capacity is defined at 1,000 W/m², that irradiation figure is numerically the number of full-sun equivalent hours, so kWp × irradiation gives the ideal DC energy.
Everything after that is subtraction. Each loss is applied multiplicatively, not by addition, because they act in series on the same energy flow — seven losses of 3% each give 19.1%, not 21%.
Performance ratio is the product of all the loss factors. It is the single figure used to compare systems across different climates, and modern well-designed European systems land between 0.78 and 0.86.
The formula
PR = Π (1 − Lᵢ) Y = H_POA × PR E = Y × P_kWp H_POA plane-of-array irradiation, kWh/m²·yr Lᵢ individual loss fractions applied in series Y specific yield, kWh/kWp · E annual energy, kWh
Assumptions behind the numbers
- Irradiation is entered for the actual tilt and azimuth, not the horizontal plane.
- Losses are applied in series and treated as annual averages, which hides the fact that thermal loss is concentrated in summer and soiling in dry seasons.
- The monthly figures use a generic mid-latitude seasonal shape and are indicative only; use an hourly model for anything financed.
- Degradation is not included. Year one is the reference; subtract roughly 0.5% per year thereafter.
- Inverter loss uses the European weighted efficiency, which already accounts for part-load operation.
How to read the result
A performance ratio below 0.75 on a new system is a design problem, not bad luck. The usual causes are shading that was accepted rather than modelled, a badly ventilated array, or an inverter operating far outside its efficient range.
Above 0.85 is achievable with an open-rack array, module-level electronics and clean conditions, but treat any quoted figure above 0.88 with suspicion — some loss has been left out.
For monitoring, the useful number is not annual production but the ratio of measured to expected on a clear day. That comparison detects a fault in a week instead of a year.
Worked example
A 6 kWp array on a south-facing 30° roof in southern Europe receives 1,750 kWh/m² in the plane of the array. Losses are estimated at 7% thermal, 2.5% soiling, 2% shading, 2.5% mismatch and light-induced degradation, 2% wiring, 3% inverter and 1% downtime.
The performance ratio is 0.93 × 0.975 × 0.98 × 0.975 × 0.98 × 0.97 × 0.99 = 0.815. Specific yield is 1,750 × 0.815 = 1,426 kWh/kWp, giving 8,556 kWh a year, or about 23 kWh a day averaged over the year.
In practice that arrives as roughly 40 kWh on a clear July day and 11 kWh on a clear December day, which is why sizing to an annual average without checking the winter month misleads anyone planning to run a heat pump.
Mistakes that skew the answer
- Adding losses instead of multiplying them, which overstates the total and produces conservative-looking numbers that are wrong in the other direction on large chains.
- Using horizontal irradiation with a tilted array.
- Counting inverter efficiency twice — once in the loss chain and again in an already AC-referenced yield figure.
- Omitting shading because it is 'small', when a single chimney can cost 5% of annual energy.
- Quoting year-one yield for a 25-year financial model without applying degradation.
Questions readers ask
- What is a good performance ratio?
- For a residential rooftop system in Europe, 0.78 to 0.85 is the normal band. Ground-mounted utility plants with tracking and professional cleaning reach 0.83 to 0.88. Anything under 0.72 warrants a site investigation.
- How accurate is this kind of estimate?
- With good irradiation data and honest loss assumptions, an annual estimate is typically within 5 to 8% of the measured first-year figure. Inter-annual weather variability alone is ±5%, so higher precision is not meaningful without multi-year data.
- Why is my system below the estimate in its first year?
- Check three things in order: shading that was not modelled, an inverter clipping or tripping on grid voltage, and string-level faults such as a disconnected string that halves output without triggering an alarm. Weather explains only a few percent.
- Should I use DC or AC capacity?
- Use DC capacity in kWp with this method, because specific yield is conventionally expressed per kWp of modules. If a supplier quotes yield per kW of inverter, the numbers are not comparable when the DC/AC ratio is above 1.0.
- How does degradation change the total?
- At 0.5% a year, cumulative production over 25 years is about 93.5% of what a non-degrading system would produce. Financial models should apply degradation year by year rather than using a flat average, because early years are discounted less.