7 Reasons Why You Should Oversize Your PV Array

From on December 15, 2015 in Category Technology with 27 Comments
PV_Oversizing

Oversizing a PV array, also referred to as undersizing a PV inverter, involves installing a PV array with a rated DC power (measured @ Standard Test Conditions) which is larger than an inverter’s rated AC output power (i.e. DC @ STC > AC). It can be a valuable tool for system designers seeking to deliver a maximum amount of energy at a lowest possible specific cost. Reasons for oversizing PV arrays and important factors to consider are summarised below.

 

1. Make better use of the inverter’s AC output

PV modules have ratings which define how they will operate. Their power, current and voltage ratings are all defined at Standard Test Conditions (STC). STC are defined as operating at:

  • 25º Celsius
  • Air Mass 1.5
  • Insolation 1000W/m2

 

However it is obvious that a PV module would very rarely be subjected to these conditions under real world operating scenarios. Operating conditions can vary throughout the day and temperature can greatly impact the output power of a PV array. As the temperature of a PV array increases, its voltage and power will decrease. Typically at solar noon (maximum solar irradiation), a PV array will have its STC output power de-rated by between 20-25%, due to the array operating above25ºC. That would mean that at solar noon on a clear sunny day a 100kWp PV array would probably be generating approximately 77kW. That’s 23% of the array’s rated power not being delivered!

If a PV array will never deliver its rated power, sizing an inverter to match that array’s typical peak power can make better use of the inverter’s AC output capacity.

 

2. Lower the specific cost of energy delivered

By oversizing a PV array, a lower cost of delivered energy can be realised (lower $ or €/kWh). Oversizing a PV array will increase the cost of PV modules and array racking for a system. However, since this can be achieved without necessarily increasing either the quantity of rating of other balance of system components, the increased energy production is achieved with a lower $ or €/kW installed cost. This in turn yields in a lower specific cost of energy delivered by the system. An example comparison made using Sunny Design shows that by oversizing a PV array with a 5kW inverter, the annual energy yield of a system can be increased by over 28% for only a ~10% increase in the total cost of installation.

 

 

3. Reduce inverter costs

By oversizing a PV array, the DC energy output of that array can better match the rated AC power of an inverter. This means that an inverter with a lower AC rating (hence lower cost) can be used. Consequently, this can decrease the relative cost of inverters compared to the total system cost.

 

4. Achieve favourable energy output when installing inverters in limited space

Inverters sometimes need to be installed in specific locations, either due to constraints from the owner or local electrical regulations. This may mean it would not be possible to install as many inverters at a site as would be desired for a perfectly sized system. However by oversizing PV arrays, it may be possible to achieve almost the same annual energy output with fewer installed inverters. For example, using Sunny Design, a 100kWp PV array with three STP25000TL-30 inverters (i.e. 75kW of inverters) would only produce ~2% less annual energy compared to the same PV array with  four STP25000TL-30 inverters (i.e. 100kW of inverters). This means that there is only a ~2% lower energy output for 25% fewer inverters.

 

5. Maximise the value of daytime energy to the system owner

For a business which operates during normal business hours, the value of daytime energy from their PV system might be different depending on individual circumstances. The PV output may be used to avoid peak-capacity grid charges or to offset constant loads which may be operating on the site. In such cases, oversizing a PV array could provide a business with greater certainty in their energy costs, especially given the low price of PV modules in today’s market. By oversizing a PV array, the inverter can reach its rated AC capacity earlier in the day, and continue operating at that point until late in the afternoon as shown in the following graph.

 

9kW_inverter_3

 

6. Better match the inverter to the PV array, in the event an inverter needs to be replaced

Sometimes if an inverter which is no longer within its warranty fails, it is not always possible to replace it with the same model inverter. In such cases an inverter of a different AC output power may need to be purchased and installed. By installing an inverter with a lower AC output power, the existing PV array could be better matched to the inverter’s capacity and the replacement cost to the system owner minimised.

 

7. Make the most of East-West PV arrays

Often, PV arrays are installed to maximise energy output and so are tilted towards the equator (south facing in the northern hemisphere, north facing in the southern hemisphere). Sometimes however the array plane available for installing PV modules with these ideal orientation conditions may not be as great as other less ideal array planes. In cases where the area available for east and west facing orientations is greater, a PV array might be split into some east facing strings and some west facing strings. Since an east and west PV array will peak in output power at different times of the day, it is possible to greatly oversize a PV array (e.g. install a DC input power equal to the inverter AC output power for EACH of the east and west PV arrays). Using an inverter’s sizing capability in such a way can deliver greater overall energy output, and a more levelled AC output each day.

 

FACTORS TO CONSIDER WHEN OVERSIZING

There are two important factors which must be considered when designing for and installing oversized PV arrays:

1. Inverter input conditions

The most important input characteristic which should NEVER be exceeded for any SMA inverter is the input voltage limit. Inverters and their constituent components are designed and rated for certain input voltage levels. If an input voltage were to exceed this rating, it will almost certainly result in the inverter’s immediate failure. When oversizing a PV array, it is important to never exceed an inverter’s maximum input voltage. Consideration should also be given to the maximum power point tracker’s operating voltage range, to make sure that the PV array will not go outside that range. When a PV array voltage is outside an MPPT voltage range, the inverter is not able to maximise the performance of the system.
To most easily design an oversized PV array, Sunny Design is the simplest tool since it can provide warnings if a design will exceed an inverter’s critical input parameters.

 

2. Inverter Operating Efficiency and Heat Generation

Broadly speaking, the efficiency loss from an inverter is realised as heat generation. An inverter has a different operating efficiency at different output powers. The example curve below shows that for an STP25000TL-30, at certain input voltage levels, operation is between 0.5-1.0% less efficient at full rated output power compared to 60% or 80% rated output power. This could result in more than double the heat generation at 100% AC output power compared to 60% or 80% AC output power. And when oversizing a PV array an inverter will be more often operate at or close to its rated AC output power, heat generation from the inverter may create an issue for the installation location especially if inverters are installed in a plant room or similar where air flow and heat dissipation might be limited.
Output power_4

 

Summary

There can be many different reasons to install an oversized PV array. Given PV array’s rarely operate at their rated peak power, oversizing a PV array can make better use of an inverter’s rated AC output and deliver a lower cost/watt system resulting in a lower specific cost of energy delivered ($ or €/kWh). When oversizing PV arrays it is important that an inverter’s critical input limits are never exceeded. Sunny Design is the perfect tool to use when attempting to design a PV system with an oversized PV array. It is important to always comply with local electrical regulations and to use appropriately qualified system designers and installers.

 

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27 Comments

  • avatar

    Shane Kelly

    April 13, 2017 at 13:08

    Hi,

    I wonder if you could offer some advice on the subject of oversized inverters, I appreciate that such a system would give a higher yield, but can you offer any advice on how to calculate what the yield would be? Taking into account days where the performance is limited by the inverter, and of course, also days where the performance is improved by the greater size of the PV system.

    Many thanks,

    Shane

    Reply »
    • avatar

      Annika Linke

      April 18, 2017 at 08:54

      Hi Shane,
      thank you for your question which is not easy to answer. That’s why I suggest that you better contact your installer. You will get the best consulting from your installer concerning your individuel context.
      Thank you and kind regards,
      Annika

      Reply »
  • avatar

    Mark

    March 10, 2017 at 09:26

    Hi I am Mark from Sri Lanka,

    Interesting article; however there is a theory here in Sri Lanka, that due to the higher temperatures experienced, the inverters would be susceptible to damage when exceeding the recommended Max input DC power by installing a oversized array. Is this true and if not what would be an ideal % increment for the country?

    Reply »
  • avatar

    Sam

    March 2, 2017 at 20:00

    I would say in most cases oversizing is not the best answer financially. If the cost of the higher rated inverter is only $100-$200, you will lose out on $1000-$2000 easily by not capturing the extra kw’s produced. I personally have done both routes with oversizing and undersizing, and I recommend matching up a 7kw array with a 7 kw inverter. I have done a 9.2 with a 7.7kw inverter as well as a 8.4 kw array with a 7.7 inverter and they produce about the same, only a couple more kw extra a day, and some kw are being wasted every day which adds up over time to more than the $100-$200 dollars for a larger inverter.

    Reply »
  • avatar

    Sergio

    January 14, 2017 at 07:41

    Hello I´m Sergio from México,

    I’m designing a system of 192PV of 320W in a industrial roof oriented south with a 10° tilt . I’m thinking of using the sunny tripower 60kW, but after reading this article not sure if it’s the best choice… what would you say about it?

    Reply »
  • avatar

    William

    November 20, 2016 at 08:58

    I m an installer. Your article changed my mind. I used to design a PV array 1:1 to the inverter. My main concern is not only the system cost but also the reliability of the inverter. I was always request myself that I should build a PV system that runs at least 25 yrs with minimum, or even zero, maintenance cost. Would the operation you suggested in this article shorten the lifetime of the inverter?
    Not only the cost of the system, Safety is the highest concern. What kind of protection does SMA inverters provided if over current occurs? (of course, i used to build a system within MPPT voltage). One of the example I got recently is a CIGS system. I built a system with 24.3kw PV array with SMA 25000TL. There is still an extra of 1.6kw PV paneles available, the customer brought more panels for educational analysis, for me to connect to the inverter. I tried not to connect those extra panels into the system since I worry CIGS may perform higher output the beginning months as they serve. Thanks your article again.

    Reply »
  • avatar

    Mitul Patel

    October 27, 2016 at 07:57

    I live in India and have a SunnyBoy 3000TL inverter connected to 4Kwp of PV panels. This setup is working since 1 year. Earlier i had 3Kwp of panels connected. My question is about oversizing the PV panels to 4.5KWp. I have fed in all the panel parameters in the Sunny Design software and it gives me a green signal to go about it. All the DC panel max. voltage and current parameters are in range as mentioned in the specs for the 3000TL. Should i go on to increase the PV to 4.5Kwp so that i get a better output from the inverter in the mornings and evenings?

    I am hesitant as the SMA India people say that you should not oversize the PV beyond 30% more than the inverter max. power. Looking at data for the past one year the inverter has never reached a peak power of 3000watts for more than a few minutes. And as i understand even if the peak power output of the panels exceed 3000watts the inverter will derate and use only 3000watts. So my second question is if this setup will in any case spoil the inverter?

    Reply »
  • avatar

    Pola

    October 20, 2016 at 10:43

    I guess over sizing is suitable for European ambient conditions
    Is it suitable for Africa which has high irradiation??

    Reply »
  • avatar

    ratnesh thepade

    October 7, 2016 at 13:30

    I am designing 50kw system using 2 25000 STP inverter. Do I need combiner box for it? I have 11 Strings of 16 module (280W each). Please let me know?

    Reply »
    • avatar

      Lucas Unbehaun

      October 13, 2016 at 07:40

      Hi Ratnesh!

      If you have an inverter equipped with two independently working tracking system (inputs A+B) that have 3 String Inputs each, and there`s 11 Strings of 16 panels, you`ll certainly need to install a sort of string combiner . When connecting a high number of strings in parallel, please think of the maximum input current of 33A per tracker!
      For further assistance, please contact our service line: http://www.sma.de/service-support/uebersicht.html

      Regards, Lucas

      Reply »
  • avatar

    Tessa

    October 7, 2016 at 00:39

    Hi, we have an 8.67kw North and West panel array with a 7kw inverter. Will the inverter cope with additional panels if we ever want to put more on? Or do we need a larger one? And is it correct that the inverter will only deliver 7175w back to the grid even if generating more electricity in optimum conditions? Thanks.

    Reply »
    • avatar

      Lucas Unbehaun

      October 13, 2016 at 07:42

      Hi Tessa,

      In case you have an North-West arrangement there, it`s quite legitim to orverdimensioning your pv-array of course. Anyway, it makes less sense to increase the module power furthermore, since the maximum ac-output power of your inverter is exactly 7000VA (active power, apparent power with cos-Phi 1). Even if you`re putting more panels, your inverter will be limiting it`s output power eventually. Those mentioned 7175W just refers to it`s maximum DC-Power! When designing a PV-System, there`s a recommended “Nominal Power Ratio (DC-Input / AC-Output) which is supposed to be in between 75% to 120% in order to run your system most efficient. In order to check it on your own, please enter Sunny Design Web.

      Regards, Lucas

      Reply »
  • avatar

    Vishnu

    September 23, 2016 at 15:20

    I am an installer. A 5kW plant with a 6000TL SMA inverter yields more kWh/kw than a 4kW plant with a 4000TL SMA or a 3600TL SMA. Why? So how could oversizing the PV array could give the best yield. Please let me know what could be the reason. All three plants are in India and are close by.
    Thank You

    Reply »
    • avatar

      Lucas Unbehaun

      September 26, 2016 at 09:37

      Hello Vishnu,

      to compare the efficiency from solar system a few more parameters are needed. You can easily check that yourself with our powerful software Sunny Design, available for free at http://www.sma.de. On the other hand feel free to contact our local services and sales team from SMA India.

      Regards, Lucas

      Reply »
      • avatar

        Vishnu

        September 30, 2016 at 10:15

        Thank You Lucas. We use the Sunny Design Software for the designing. We are happy with the performance of all plants. We find the SMA service support highly reliable and appreciable. thanks to Vishal Kedar, SMA India.

  • avatar

    Piotr

    April 30, 2016 at 07:27

    I wonder what is the limit in oversizing PV array. Is 2 times OK? i.e. 5kW inverter, 10kW solar panels? Of course provided max voltages and currents are within inverter range.

    Reply »
    • avatar

      Scott Partlin

      May 4, 2016 at 07:11

      Piotr,

      Thanks for your interest. The maximum amount which could be oversized is not a fixed number and is dependant upon the input parameters of each specific inverter. This sort of information can be found both in Datasheets and Technical Information sections in SMA inverter Operating Manuals.

      Reply »
  • avatar

    Solaris

    February 10, 2016 at 07:55

    Great article, we find that at times we may also under-size our customers PV array’s if necessary for further expansion in the future if they are seeking to add onto their system at a later time. I guess its all on preference. However, SMA PV Inverters are always a premium pick for long-lasting reliability and quality. Everytime we design a system with the SMA inverter, our installers are always pleased with the final result in energy output. Pair these with some SolarWorld panels and an Ironridge racking kit – and you are set!

    Reply »
  • avatar

    Girmay Worres

    January 27, 2016 at 04:59

    Thanks Scott. Great info.

    Reply »
  • avatar

    paul

    December 18, 2015 at 11:47

    Can you tell me if i can still get the gov sub if off line
    We have a block without power in balingup Thanks Paul

    Reply »
    • avatar

      Sarah Römsch

      December 21, 2015 at 08:35

      Hi Paul,
      thanks for your question. My colleague Scott from the Australian has sent you an e-mail with further details and questions.
      Best regards from Germany,

      Sarah

      Reply »
  • avatar

    Chris Taeni

    December 17, 2015 at 11:20

    Fantastic article. Great stuff. In Australia it also means we take advantage of STC incentives – lowering the total cost of the system since incentives are based on module power.

    Reply »
    • avatar

      Ron Flynn

      January 25, 2016 at 09:38

      Chris Taeni – The STC’s cannot be created at all if the PV Array is higher than 1.33 times the nominal output of the inverter.. IE: SB-5000TL-21 , nominal AC output: 4.6kw x 1.33 = 6.118kw of panels. Not sure why it’s not calculated on the max AC output of 5.3 (perhaps SMA Australia can address this with the CER).
      You can do it, no problems, but you are unable to apply for the STC’s for the system. Have confirmed this with theCER.

      Reply »
  • avatar

    kal patel

    December 17, 2015 at 03:56

    3600 smart energy
    how long of wait before it will be a viable in USA

    Reply »
    • avatar

      Julia Stunz

      December 17, 2015 at 10:19

      Hi Kal,
      At the moment there are no plans for the USA.

      Kind regards,
      Julia

      Reply »
  • avatar

    James

    December 16, 2015 at 12:36

    Like this
    Want it on my roof
    Great idea
    Put more cheap panels
    Get more energy from sun
    What about battery storage?

    Reply »

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