Question about the importance of start-up voltage in an inverter

Hello, all.
I hope you'll forgive this one-off question; I don't really have any knowledge or experience to offer, so I hope you'll help out a consumer in need of some advice.
I live in Cyprus where there's plenty of sun, and wish to choose the best PV system for my home. The Cypriot government has limited all net-metering tariffs to a 3KWp system, and I want to get the best out of the system I eventually purchase. Since my peak production is around 3KW, all I can really do is purchase a system that will make as close to that as possible in the broad range of weather, temperature, and illumination conditions that occur here.
I've found a system that uses Silevo panels that have the lowest temperature coefficient (around -0.26% VOC, from memory), the highest efficiency I've seen available for sale here in CY (near 18%), and a high open-circuit voltage of 64.5V at 800w/m2, and 65.3V at 200w/m2. The I-V curve seems to be about as 'hard kneed' as I'm likely to find in such a small country with such a small market, and they look like a good option, if a little expensive. The system would use an array of 10 of their 300W panels.
However, another PV provider told me that it's important for the inverter to have a low turn-on (or start-up) voltage. The idea, as explained to me, is that the lower start-up voltage will maintain production in low-light and cloudy conditions, and cause production to begin sooner after the sun begins to rise. The company that provides the Silevo panels uses a Danfoss transformer-based inverter, with a start-up voltage of 230V, while the other company provides a Chinese (Goodwe) transformerless inverter that starts at 120V.
I'm sorry to ask for a favour that I'll never return, but I just can't find the answer anywhere: Is the start-up voltage issue a big deal, or will a combined array of 10 of the Silevo panels almost always deliver greater than 230V, even in very low-light conditions?
Thanks in advance!

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Question about the importance of start-up voltage in an inverter

    Welcome to the forum.

    Low turn-on Voltage is not important. Panels are a current source; if they don't have enough sun on them to produce current the Voltage is irrelevant. They will hit Voc with minimal illumination. It will not help to have low turn-on Voltage in low light or cloudy conditions. No sun = no power.

    You can have more than 3kW of PV on a 3kW inverter because on average they won't put out their nameplate rating, and the inverter will not produce more than it is rated for.

    I would avoid the low-Voltage Chinese inverter. And also look at using twelve 300 Watt panels instead of ten. It is important that the array Voc does not exceed the maximum input Voltage of the inverter (usually 600 Volts). In this case with those high Voltage panels that probably means running two parallel strings for the array: 6 * 65.3 Volts = 391.8 Volt string. Such should have no trouble providing the necessary start-up Voltage and full power abilities for that inverter. Check the panels' Vmp rating too; it is probably around 52.
  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    Re: Question about the importance of start-up voltage in an inverter

    And don't worry about panel efficiency--Unless you have only a small area (not array) to install the panels and need high efficiency to get the total amount of power you need.

    More or less, look for the least costly panels ($$$/Watt) and I would tend towards larger panels (>175 Watt) so you have fewer/larger panels to mount and connect. The smaller panels usually only make sense if you have to fit them in a limited space with obstructions.

    Make sure panels are installed with no shading by chimneys or vent pipes/overhead wires. Any shading will dramatically reduce the output of your system.

    And, I would not pay too much attention to thermal deratings... Usually the high quality panels are mono or poly crystalline cells, and there is not much difference in their performance (temperature deratings). It is all based on the property of silicon.

    And you know that with Grid Tied systems, they do not provide backup power if the utility power fails (you will be in the dark). There is (at least) one company (SMA) that makes GT inverters that can provide some backup AC power in the middle of a sunny day--If you really need a little backup power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • omannayomannay Registered Users Posts: 3
    Re: Question about the importance of start-up voltage in an inverter

    Thanks to the both of you; that clears up a lot.
    The only thing that remains is to make sure I'm absolutely clear on the temperature coefficients. The VOC value for the Silevo panels is -0.262 %/degC, versus around -0.37 for the cheapest option.
    Both panels are rated to 46 degC outside temperature, and 85 degC panel temperature. We have sustained summer temperatures of over 40 degC here in Cyprus, and I wouldn't be surprised if the panels get pretty close to 85 degrees when the sun's really shining. I know that Arizona isn't known for its blizzards and snowdrifts either, so perhaps you can give me the benefit of your experience.
    The cost of the systems range from €4800 for the cheapest (Jordanian panels, Chinese inverter) to €6100 for the most expensive (Silevo panels, Danfoss inverter). Given the lifespan, the need for quality components, and the government limit on system power (3kWp), I'm leaning toward the more expensive system (hoping it will produce at a far more optima level when it's a bit too hot, or a bit too dim), but I'd really appreciate it if you could let me know whether I'm paying a lot of extra money for something that won't produce much more electricity.
    Thanks.
  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    Re: Question about the importance of start-up voltage in an inverter

    Right now--Panels that have 20+ year warranties and companies that may not be around in 5 years to service any warranty.

    My own panels failed after ~6 years (grid tied system), the vendor did replace them--And the vendor shortly thereafter shut down their solar panel business (I assume that they are no longer providing warranty, but I have not checked). My first inverter was replaced due to mfg. problems that may cause early life failure (inverter was fine for ~5 years). The replacement failed (new and improved) failed in ~1 year. The third inverter is still working fine (new/old in box).

    Inverters--In general, electronics are good for about 10+ years. If they fail in ~5 years, you may get them repaired. After 10 years if/when there is a failure, you will probably have to replace them.

    So--Start looking at becoming your own power/utility company. And make sure you have savings to repair/replace hardware as needed.

    What you need to do is find the "right" combination of panels (Vmp/Imp--working voltage and current), and the right configuration (x panels in series by y parallel strings) that will meet the controller's input voltage and current requirements.

    For example, my panels are 175 Watt and Vmp~40 volts. I have two strings of 10 panels each for ~3,500 Watt array and a 3kWatt GT inverter. With a Vmp-array ~ 400 volts.

    If you want to get as much "legal" power from your system as possible--Depending on how your regulations are written--Usually you would have a 3kWatt GT inverter (which meets the 3,000 Watt peak rating), and the maximum array you can fit on your roof. You may "waste" a little power in the middle of the day (clipping, the solar array can output more power than the inverter can use), but you will get more energy total in the day as the array+inverter output more power in the mornings/afternoons.

    The typical over paneling (maximum cost effective array) is around:

    3,000 Watts * 1/0.77 panel+controller deratings = 3,896 Watt array

    With this size of an array, the GT inverter will spend some time (safely/reliably) "clipping" some power on cool clear days around noon time. But not lose that much power overall.

    Note the GT inverter does get warm--Keeping the inverter in shade with good ventilation will help keep it running a long time. Temperature is the enemy of electronics. For every ~10C over 25C, the life of the electronics is cut by 1/2. Put the GT inverter in a small room with poor ventilation or in direct sun, the life of the unit will be less. Also, thermal cycling (cold/hot/cold) cycling is hard on electronics too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Question about the importance of start-up voltage in an inverter

    On the bright side, the worst that can happen with a too-hot array is the inverter shuts down from low Voltage.

    Up here we have the opposite problem: the super cold Winter temps can send the Voc soaring, and too high an input Voltage can damage equipment before it turns off.
  • omannayomannay Registered Users Posts: 3
    Re: Question about the importance of start-up voltage in an inverter

    Thanks for the input. I'll ask my providers about adding more panels to the array. It's been great to finally get some answers.
  • BB.BB. Super Moderators, Administrators Posts: 32,204 admin
    Re: Question about the importance of start-up voltage in an inverter

    Please feel free to return with any other questions...

    And let us know how it all works out for you.

    Good Luck,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: Question about the importance of start-up voltage in an inverter

    1. Is your limit 3kW peak, its not watt hour per day/month limit?
    2. Is the NEM/FIT attractive, ie better than net meter?

    If the answer to both is yes, then you could consider either a tracker, or a "virtual tracker" with fixed arrays pointing in different directions. In addtion to overpaneling the array as the guys mentioned above, these tecniques will yeild the flatest top for the longest period of the day. Also good for big aircon demand.

    If the FIT isnt terribly flash, or is uncertain to last, ie not duration guaranteed, then you really ought to double check your cost beneift model on the whole proposition.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Question about the importance of start-up voltage in an inverter
    On the bright side, the worst that can happen with a too-hot array is the inverter shuts down from low Voltage.

    Up here we have the opposite problem: the super cold Winter temps can send the Voc soaring, and too high an input Voltage can damage equipment before it turns off.
    What often happens is that if there is a power outage in the middle of the day the system won't turn back on when the power returns because the string voltage has fallen below the turn on voltage from the inverter. The inverter turn on voltage is typically significantly higher than the lowest operating voltage, so the inverter usually won't shut down on its own.

    I know of a system that had to be inspected by the AHJ early in the day because if the inspector came later in the day it wouldn't turn on. As long as it was already running when the panels got hot, it ran fine. If the power failed during the day, though, it would be the next morning before the system would come back on line.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Question about the importance of start-up voltage in an inverter
    ggunn wrote: »
    I know of a system that had to be inspected by the AHJ early in the day because if the inspector came later in the day it wouldn't turn on. As long as it was already running when the panels got hot, it ran fine. If the power failed during the day, though, it would be the next morning before the system would come back on line.

    Sounds like judicious use of the garden hose (mist, not spray) would eliminate the scheduling issues.
    SMA SB 3000, old BP panels.
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