7 Reasons Why You Should Oversize Your PV Array

From on 04/13/2016 in Category Technology with 23 Comments
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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 over-sizing PV arrays and important factors to consider are summarized 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 is defined as operating at:

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

 

However, it’s obvious that a PV module would 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 percent due to the array operating above 25ºC. That would mean that at solar noon on a clear sunny day a 100kW PV array would probably be generating approximately 77kW. That’s 23 percent 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 realized (lower $/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 $/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 percent for only a roughly 10 percent 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 (and thus lower cost) can be used. Consequently, this can decrease the relative cost of inverters compared to the total system cost.

4. Achieve favorable 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 100kW PV array with three Sunny Tripower 25000TL inverters (i.e. 75kW of inverters) would produce roughly 2 percent less annual energy compared to the same PV array with  four Sunny Tripower 25000TL inverters (i.e. 100kW of inverters). This means that there is only a near 2 percent lower energy output for 25 percent fewer inverters.

5. Maximize 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 fails once it has exceeded its warranty period and 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 minimized.

7. Make the most of East-West PV arrays

Often, PV arrays are installed to maximize energy output and as 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 level 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 maximize 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 realized as heat generation. An inverter has a different operating efficiency at different output powers. The example curve below shows that for an Sunny Tripowrer 25000TL, at certain input voltage levels, operation is between 0.5-1.0 percent less efficient at full rated output power compared to 60 or 80 percent rated output power. This could result in more than double the heat generation at 100 percent AC output power compared to 60 or 80 percent 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 ($/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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The Author

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My tasks:
I lead the Application Engineering department at SMA Australia, which provides pre-sales technical support for Residential, Commercial and Utility scale systems. I also work closely with Product Management to help refine SMA's solutions to be better suited to market requirements.
My Topics:
Market issues, SMA product suitability, new business opportunities, Asia Pacific Region (APAC)
Hobbies:
Reading to my children, volunteering at my local football club, skiing, wine tasting (only good wine though)
What even my colleagues don’t know about me:
Have travelled to Nepal to deliver and install a rural hospital solar lighting aid project. Received my Gold Duke of Edinburgh’s award personally from Prince Phillip in 2004. Once spent a night lost in the wilderness on a trek because our group was reading the map upside down.

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23 Comments

  • avatar

    Ian

    04/24/2016 at 6:27 am

    I am a firm believer that the inverter should Not be the limit of the system.
    The graph does not show the energy wasted as the production would have peaked at say 13KW or 14KW.

    All of the kWh from where the kW line meets 9kW at 08:00 to where is leaves 9kW at 15:30 was lost to time.

    You could have just as easily installed 15kW of inverter and gained the following.
    I used your graph and added a typical solar inclination curve to the top with the following assumed peak power production.

    9kW cap, 92kWh, Base line from your chart
    12kW peak, 107kWh, 16% energy increase
    13kW peak, 112kWh, 22% energy increase
    14kW peak, 116kWh, 26% energy increase

    Wasting 16% to 26% system output is hard to justify. Buy the bigger inverter and maximize your production.

    If you only want 92kWh daily it would be cheaper to buy a smaller array and a larger inverter.
    Inverters are cheap compared to Space and installed panels.

    My Opinion…

    Ian

    Reply »
    • avatar

      Phil Olson

      05/13/2016 at 10:22 am

      So if you want to oversize your array and can’t exceed the voltage limit, can you exceed the current limit? It would seem by the examples that that is what’s going on.

      I have two 9000TL’s with about 9600 in panels, and would consider adding a fourth row to make that 12800 in panels but that would exceed the max current limit.

      Reply »
      • avatar

        Justin Dyke

        05/17/2016 at 12:14 pm

        Hi Phil –

        Great question. I have forwarded it to a subject-matter expert who will be in contact shortly.

        Best,
        Justin

  • avatar

    B

    05/25/2016 at 11:56 am

    I side question, which you may have answered before:
    After reading the above article it follows to ask a few battery questions. I have a SB3000TL-US-22 and a SI 6048. I have narrowed the batteries down to the NiFe batteries. Are these batteries compatible with the mentioned SI & SB? Are there and special things I would need to do? i.e. modify settings on SI, add additional equipment?

    Reply »
    • avatar

      Justin Dyke

      05/25/2016 at 6:15 pm

      Hello –

      Those are great questions. I have shared them with our applications engineering department and a technical expert will contact you soon.

      Best,
      Justin

      Reply »
  • avatar

    Zenon coronel

    12/03/2016 at 11:59 am

    I have 3640 watts in solar panel
    What inversor i Neem
    Thank

    Reply »
  • avatar

    Potiem Host

    07/04/2017 at 7:10 am

    Hey Scott, It was a complete are satisfactory answer to the question that I was searching for. But, there is one new question that is troubling me now is that what will be the status of peak of peak power. If we have an inverter SMA STP 25000TL and have safely oversized the PV array, lets assume to 30 kWpeak. What will be status of peak output power of the inverter. Will the portal show a value of 25 kW more often or will it never hit the 25kW peak mark?

    Reply »
    • avatar

      Carlo

      07/11/2017 at 8:48 am

      Hello Potiem, we’ve shared your question with our service engineers and they have provided the following:
      The arrays potential output is going to depend on the current solar irradiance among other factors (module temperature, module mismatch, shading, soiling, module tilt / orientation, etc.). Assuming the inverter is able to draw 25.5 kWdc from the array (≈98% inverter efficiency), the inverter will be able to produce 25 kW. For the same daytime conditions a system with a higher DC to AC ratio will reach the maximum output of the inverter quicker and stay at that output level longer.

      Reply »
  • avatar

    Eitan Segall

    01/27/2018 at 3:22 am

    I have 15.6KW Installed syatem with SMA 15KW, while the usuall DC high power produce is about 13-13.5KV (pick of ~15.2KW three times a year ). In spite of the efficiancy of the panels (65 units of 240W) increase every year the AC output reduce every year ( I see according the electricity company ).
    I live in Israel where the radiation and the tempeture during 6 months are relatively high so the whole system efficiancy reduce.
    what is your sugestion to me to increase the AC output?

    Reply »
    • avatar

      Mike

      01/29/2018 at 10:44 am

      Hi Eitan! It is not uncommon for the performance of PV modules to drop slowly over time. This is why there is a generous power warranty on these devices, typically ensuring they stay above 80% nameplate power even 20 or 25 years after manufacturing. You are right that hot temps will reduce the available output well below the STC rating (240W), but hitting 15.2 kW out of 15.6kW STC rating is quite good. Also, soiling of the modules over time will reduce their output, so cleaning may help recover some output. Best Regards.

      Reply »
  • avatar

    Rudi

    02/01/2018 at 11:48 pm

    If I want to oversize the PV array on a 12KW SMA, what would be a same limit to oversize by? 14KW?

    Reply »
    • avatar

      Mike

      02/02/2018 at 9:37 am

      Hi Rudi! For UL listed SMA string inverters, SMA is fine with you going to a 1.5 DC:AC ratio as long as the inverter DC voltage and short circuit current limits are not ever exceeded. As an example, the STP 12000TL-US has a 1000VDC input limit and 53A Isc limit for each of the two input channels. Please note the inverter will not pull more than 33A from either channel in operation however. Your module choice, install location and orientation of the array will otherwise determine what is a “sane” limit. Best Regards.

      Reply »
  • avatar

    ANWAR

    03/14/2018 at 10:36 am

    Hai i am having a project were i need to install a 60kW SMA and the panel capacity is 78kW. Is that ok to oversize 60kW SMA with 78kW panel.

    Thank you
    Anwar

    Reply »
    • avatar

      Mike

      03/14/2018 at 2:06 pm

      Hello Anwar! As long as the inverter is never exposed to more than 1000VDC or subject more than 150A Isc from the array (under worst case local conditions), using a PV array with STC nameplate of 78kW with the STP 60 is acceptable. Be aware the inverter will not draw more than 110ADC from the array in operation. Regards.

      Reply »
      • avatar

        Anwar

        03/20/2018 at 4:18 am

        Thank you Mike

      • avatar

        Mike

        03/20/2018 at 9:32 am

        You’re welcome! 🙂

  • avatar

    Pravin

    11/12/2018 at 5:07 am

    IF I have PV array of 15 KWP and inverter of of 12.5 KW , will it produce the same power as a inverter of 15 KW with same PV array

    Reply »
    • avatar

      Mike

      11/15/2018 at 11:30 am

      Often a PV array is not capable of operating at its STC rating, for example, if the array (surface) temperature is much warmer than 77 deg F or if the incident light levels are well below 1,000 W/m^2. Only if the environmental conditions of the 15kW STC rated array would allow it to generate more than 12.5kW would you see a difference. In such a situation, the 12.5kW inverter will operate the array slightly off the maximum power point to generate 12.5kW of AC output power. This is commonly called “clipping” and is perfectly fine as long as the array never violates the electrical limits of the inverter. Sunny Design will allow you to simulate your system (actual geographic location, module type and mounting details) with both inverters and provide an estimate of any clipping. This is shown as the “Energy Usability Factor” on the results screen – only if it is below 100% does the program estimate that clipping will occur.

      Reply »
  • avatar

    Abdullah

    11/24/2018 at 8:42 am

    hello,
    I have a project in which I have to run a three phase motor of 15 HP. Could you please guide me how to size the system with STP 25000TL-30. How much do I need to oversize the system if I had to run a similar load of 15KW or 20KW with STP 25000TL-30? Also, Could you tell me how to register on the sunny portal for WEB Connect Installed and I couldn’t find any details for how to set the country data set for Pakistan. Lastly, can you also guide me regarding the use of a cluster controller if 3 STP 25000TL-30 are to be used on a project and can you change the number of strings joined in parallel to each input while keeping the same number of modules in each string?

    Reply »
    • avatar

      Mike

      11/26/2018 at 10:33 am

      Abdullah, I am not sure your goal with the pump, but be aware the STP is a grid-tied inverter. With a utility grid present it will output current in phase with the utility provided voltage and frequency waveform. How much current depends on the installation details, specifically location, size of array and orientation of the array. You can use Sunny Design Web to get an estimate of production with this inverter and the details of your array.
      https://www.sunnydesignweb.com

      For Webconnect registration, you can reference this TechTip video: https://youtu.be/8DShVnMqHUo

      (For setting Pakistan grid parameters, they are going to have to contact Service in DE likely.)

      If using a cluster controller, you will daisy chain the ethernet cable to each inverter and finally to the Cluster Controller. You would register only the Cluster Controller with Sunny Portal in this instance.

      Sunny Design Web will tell you if the current limitation of the STP inputs are violated or not by adding another string, and if you are over the number of inputs for that channel of the STP, you would need to use a combiner box.

      Reply »
      • avatar

        Abdullah

        12/19/2018 at 8:40 pm

        Hello,
        I have a project to commission STP25000TL-30, I wanted to confirm if I could connect 90 panels to input no. A and 40 panels to input B? While keeping the same number of panels in series in each string for both inputs i.e. (10 panels in series). The voltage and amperes for both inputs are within limits as confirmed by SMA design web. I would appreciate if you could help me with this issue at your earliest as I don’t want to damage the inverter if there is a difference in the amperes going to both the inputs.
        Thank you

      • avatar

        Mike

        12/21/2018 at 11:55 am

        Hi Abdullah! You should contact the local Applications Engineering team for this kind of support.

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