Solar PV Plant Performance – Capacity Utilisation Factor(CUF) Vs Performance Ratio(PR)

In the Indian solar context, CUF is perhaps the most overused word when it comes to measuring a solar power plant performance. The interesting thing is, not many people outside India understand the CUF because most of them use a much better metric – Performance Ratio(PR). When we posted the CUF of solar plants in India available in the MNRE website, many readers outside India emailed us asking how CUF is calculated and how different it is from the globally accepted standard of Performance Ratio(PR) (Some discussions on the topic in Linkedin are available here and here).
CHROSIS, a Germany based consulting firm, has published this excellent whitepaper on the subject that gives some good insights on the subject.
Update: A concise version of the whitepaper in the form of a power point presentation can be viewed below.

Some of the highlights of the whitepaper are given below.
Performance Ratio (PR) of a plant for a period of time =Energy measured(kWh)/(Irradiance(kWh/m2) on the panel x Active area of PV module(m2) x PV module efficiency)
Capacity Utilisation Factor(CUF) =Energy measured (kWh) / (365*24*installed capacity of the plant).
So on one side, PR is a measure for the performance of a PV system taking into account environmental factors (temperature, irradiation, etc.) and on the other side is CUF that completely ignores all these factors and also the de-rating or degradation of the panels.
Some more factors that can also be important when comparing PR vs. CUF:
– PR will take into account the availability of the grid, CUF will not.
– PR will take into account the minimum level of irradiation needed to generate electrical energy, CUF will not.
– PR will take into account irradiation levels at a given period of time, CUF will not
PR can be used as a tool to compare different solar PV systems with each other – even if they are located at different locations since all environmental factors will be taken into account. Therefore only the design and the ability of the system to convert solar energy into electrical energy will be compared with each other.
Therefore we are not convinced that the CUF is a good tool to provide insights into a solar PV system.
The whitepaper gives a comprehensive overview of the factors that affect PR and how to monitor and control it. The whitepaper can be downloaded here.
(This blog is supported by Aspiration Cleantech Ventures)
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46 thoughts on “Solar PV Plant Performance – Capacity Utilisation Factor(CUF) Vs Performance Ratio(PR)”

  1. Personally I feel CUF, does not have any meaning in SPV plants. Plant utilisation factor ( read CUF in this discussion ) is important in those powerplants which operate 24/7/365.
    Ashok Toshniwal

    1. With advancements and improvements in storage technologies, a solar power plant can be made to run 24/7/365

  2. CUF has been defined by CERC wrt the required RE plant availability.Whereas PR establishes the performance level of Solar PV power plants.The following should be clearly borne in mind:
    1) CUF is linked to Generation
    2) Performance ratio establishes the level of performance of a solar PV Power Plant.
    As a Thermal Power Plant Engineer with over 3 decades of experience and a Certified Energy Manager, I wish to inform that CUF and PR are two different parameters and RE Professional should not confuse these two parameters, each of which has it’s own reckoning.

  3. Good article.Your analysis on Solar PV plant performance in India is simply superb.
    Dr.A.Jagadeesh Nellore(AP),India

  4. Hi madhavan,
    I have a solar radiation values in W/m^2. To find the performance ratio of the solar plant, how to convert W/m^2 to Kwh/m^2?, what should be the value of no. of hours?

  5. Dear Madhavan sir
    What is key factor as Active area of PV module/m2 for calculation of PR.Plz reply becoz i working in 2MW & 50MW solar power plant in project so its important factor for me
    Thanking you

  6. Dear Mr. R.Pon Perumal
    It is recommended to use the average GHI either on per day basis or yearly basis to calculate the PR of the solar plant because the irradiance keeps on changing throughout the day. So simple multiplication with sunshine hours will not help to calculate the exact PR.

  7. Dear Mr. Amol
    Active area of modules to calculate the PR is simply the multiplication of no. of modules and area of one modules installed in a solar plant. To be more precis manual calculations use cell area.

  8. Dear Mr. Hari,
    The formula of PR can be more simplified as per NREL/PVSYST as under
    PR = Energy Output (KWh)/Installed capacity (KWp)
    Irradiance on in plane plane (KWh/m2) / Irradiance at STC (1KW/m2)

  9. Dear Mr Madhavan
    When it is necessary to link the generation from a solar plant for which parameter would be more appropriate , CUF or PR.
    We face this problem when negotiating EPC contracts with the EPC agencies.

    1. Hi Mr. Sudhir,
      From our opinion it is better to give a performance guarantee based on PR than on CUF.
      Here is why:
      – PR is independent from climatic events like heavy rainfalls etc.
      – PR is independent from grid-availability
      The worst case could be that you guarantee a certain CUF and the following happens:
      – There are climatic events that prevent to plant to go to full power (heavy rain, dust / sandstorm, etc.) for one month of the year. As a result the CUF achieved will be very low.
      – There is a grid failure so the plant can not feed-in any energy into the grid. This will affect the CUF again in a negative way.
      Bottom line is: You will be held responsible for things you can’t influence (weather, grid, etc.).
      Therefore it is better to go with PR
      If the weather is bad, the expected PR will be less compared to sunny weather – this is a fair comparision.
      If the grid fails, your expected PR will be set to zero – so no negative impact on that.
      It is hard to communicate these things to clients. Most of them expect that everything will work out just fine, but from our experience it is better to tell the facts and educate them about possible solutions like PR instead of CUF.
      Sebastian Duerr/Madhavan

  10. Dear Mr.Nampoothiri,
    Being a power generation professional, presently involved in the designing, EPC, O&M of grid connected PV solar plants of MW capacity, I have prepared a practical guide for the total works of pv solar plants. Please let me know how can I share it with the engineers who are working in this field,

  11. Dear Sir,
    I am working in solar power plant.As you have given PR formula I couldn’t find correct value of my plant PR.Here I have given all values of plant… Please say how to find PR value.
    Generation :12,754kwh
    Irradiation :866w/m2
    Module area :1.4m * 1.0m
    Total modules :45360
    Module power :125 w
    Voc :59.7v
    I sc :3.37A
    PR :0.76
    This is correct or not?
    PR ={(12754)/[(1.4*1.0)*(.866)*(0.892)]}
    I used following formula to find efficiency.
    Efficiency =P max /(module area)
    Please say PR value by using above value.
    Thanking you.

  12. Dear one please advise how best way S P V module can be used in school& Industry& small shop having domestic articles along with minimal finance limits as I have read solar theory up to Intermediate with physics on reading your views on P R & C U F effectively explaining, its best use in public can be communicated to people.
    With regards;
    From D K Bajpai(Mb:09793039400);
    Re tiered communication engeneer

  13. What is the difference between annual estimated energy production calculated by accounting for losses as mentioned in the loss diagram and performance ratio? Can this be used as a tool for evaluating / deciding offers from different bidders.
    santanu kr saikia

  14. Hello Sir,
    In CUF formula, at denominator What value we have to consider, Is it DC Capacity or AC Capacity? For Example DC Capacity: 40MWp AC Capacity: 30MW with over sizing features.
    Please Reply.

  15. PR is undoubtedly the right index for PV power plants. However, PR also varies with environmental factors as a second order effect. For example, due to higher ambient temp in summer, thermal loss is increased and PR is reduced in summer. PR normalized to ambient temp will be a better index to compare performance of PV plants at different locations and different ambient conditions. Likewise PR can be normalized to other weather factors. Further, DC PR and AC PR can be evaluated to determine underperformance of PV array and Inverter resp. We will be happy to undertake plant performance evaluation and improvement exercise on case to case basis.

  16. I agree with comments made by Vaman. So the PR calculation steps followed in comments by Sugu needs refinement.
    A PR of 78% is mentioned as minimum in many solar power policy documents. Is this minimum for the entire 20 year period which I think should be the right approach.
    As regards negotiating for EPC contracts, a first year guaranteed generation number with a preset degradation on yearly basis. Globally First year guaranteed generation is more in vogue.

  17. After going through all discussions, I should say that to know the performance of the plant for EPIC point of view PR is better tool, but to ascertain the economic viability CUF is logical and appropriate tool.

  18. Prashant pokarna

    When we talk about installed capacity is it the DC installed capacity or the inverter rating on the AC side ?

  19. Dear Sir,
    It is nice to see the difference and calculations of PR and CUF.
    I appreciate for this this will give you awareness towards
    Solar power plants.
    Thank you.
    P.uma maheshwar rao

  20. Dear All,
    Ofcourse PR is the correct factor to establish the performance of PV solar plant, but this factor alone cannot certify that plant is doing well. The reason is the Weather Station which gives you the ir-radiation value. If WS is not installed properly, it is in shading area, it is covered with dirt, its tilt angle is not as per the tilt of plant modules then you will get absurd results.For certified performance of any PV plant you have to calculate both factors-PR and CUF. PR should touch 80% and CUF should be around 20% only then you can say that the plant is working OK.

  21. If i will connect 1000kw dc capacity to a 1 mw inverter then can i get 1000kw ac at inverter output?

    1. Not exact , it will based on STC of module, but even if it meets conditions, there is some minute loss in conversion process . The inverter efficiency around 99%

  22. In my opinion, both are very pertinent parameters for assessing & evaluating the solar power plant functional capability.
    CUF: Capacity Utilisation Factor is a measure of pre-estimating the plants’ generation capacity/availability against the installed capacity, anticipating and/or assuming certain critical parameter, including the available time of the day. Whereas, the PR (Performance Ratio) is again a calculated value ratio based on the similar assumptions or anticipation of uncontrollable parameters in our hand.
    PR (Performance Ratio may be more meaningful, if calculated, at least after the first year of actual performance study of the plant and extrapolate the values based on the observed external parameters effect during the year of study.
    In my opinion, I guess, the EPC contracts based on genuinely assumed CUF, based on historical facts,and current technology breakthrough in solar panel performance should not have much difference against the future PR (Performance ratio).
    One point I agree with Mr.Madhavan is the EPC contractor and developer should understand fully the plus and minuses of solar power plant performances against many uncontrollable natural factors and the present cell technology on hand and should get fully educated before making the contract. Meaning both should get to a same page of understanding.

  23. panga narayana rao

    Dear All,
    The formula given above is the basis formula to obtain the Performance ratio of solar power plants. According to my view always PR has to be temperature corrected as module power starts degrading after STC conditions.
    Also PR is simply nothing but replica of losses occurs in solar PV plant. Generally PR varies from 75% to 85%. remaining 15% to 25% losses shall be in account of temperature, weather conditions, losses of the power system, cable losses,near shading losses, IAM, Mismatch, Soiling and LID effect.

  24. When is ready for PR test? Immediately after commissioning the solar power plant or after one year operation of power plant? Thanks.

  25. Hiii sir,
    Could please provide a document about solar performance monitoring and solar analytics, key parameters which will give about the total system performance.

  26. Joining this such a late point (around 7 years from the start of this chain), I am feeling a little apprehensive about not being taken seriously. The problem first started somewhere around 3 years back, when I first started venturing into this segment of power generation starting my own business with an aim to work in small domestic segments. In those days, when I started designing systems for very small residential units etc., we used to assess the power consumption in an interactive manner with the house owners, asking for the installed loads approximate running hours of those devices separately for day and night. Thereafter, when we tried to calculate the solar panel capacity, our first supplier informed us that we may assume that a 100 Wp panel is active for 5 hr average (a rule of thumb) in a day generating around 500 WHr of energy considering various factors. During the next 2 years, the thumb changed somewhat and now I hear people say that it is better to take 4 hours in a conservative estimate (but with a possibility of the customer being driven away due to my conservative mindset, because ultimately he pays for the panels). Some other people informs that be optimistic and go for 5 hours or even for 5.5. This essentially means a business risk, which one should take when he is skilled in customer management. Honestly, these day when some customer approaches us, we take a figure of 4, 4.5 & 5 in different cases, after seeing the customers, their financial condition, budget etc.
    For last one year or so, I have seen that we have to look at bigger systems (mainly rooftop grid-connected system) and not being able to grab any order, due to various reasons (which is not relevant to the present discussion),I tried to embrace RESCO model. Here, I tried to work on the tariff calculation, the CUF and PR issues came running in as I was trying to adopt some models of CERC or some other ERCs, though their model calculation was generally for higher size utility plants. Their, I saw that they are using either CUF or PR parameters to decide on the tariff. Firstly, most literature as well as the early comments on this very discussion declares PR to be a much better method than CUF, with quite a few (very) valid reasons. My first doubt came when, to the best of my knowledge, I found the PR surely is a post-installation Performance Parameter. And CUF is a predictive parameter using yearly average solar resource (which PR also do). But, where do I get my PR value for a proposed system when I have to explain a customer and investor / financier about the sanctity of th tariff in case of a RESCO model or the ROE / ROI for a Capex model Customer.
    So, I decided to go back to the rule of thumb. Only thing that I still have to look at the Customer’s thumbs, which are not identical for each of them.

  27. P.C.Sarma- India

    I am empanelled Inspector for Grid Connected Solar plants of Solar Energy Corporation of India (SECI).
    SECI requires that PR is determined on real time basis at the time of Inspection of the plant.
    PR=(Measured Output in kW)/(Installed capacity in kW×Measured radiation intensity in kW/m2) × 100
    It appears that the requirement specified by SECI is rational as errors in measurement of size of panels, loss of generation due to breakdown of components etc. in the formula specified in this document is avoided.
    Request offer views on the following –
    Is the measurement by handheld solar power meter is reliable and generally accepted for small plants?
    As the sensor is placed on the solar panel , it appears that the actual radiation received by the panel at a particular tilt gets measured and temperature correction also not necessary

  28. Sir, while calculating CUF of a plant, we take energy measured and plant rated capacity. Is it taken before the inverter convirsion i. e DC or AC?

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