Is voltage coming from a string constant?

I am trying to make sense of the issue of voltage windows on inverters and the varying sun energy available to an array. The only way that it all falls together is if I can assume that voltage is constant based on the size of the string and the voltage rating of the panels in the string, and is not affected by the amount of sunlight available. Is this true? Thanks.


  • BB.BB. Super Moderators, Administrators Posts: 31,061 admin
    Re: Is voltage coming from a string constant?


    Not quite sure what your question is... But, in summary, the voltage of the solar panels is, more or less, "optimum" around Vmp (Voltage maximum power) once a minimum amount of light is present on the panel (sort of full shade on a bright day)...

    Once the minimum amount of sunlight is present, the output current is proportional to the amount of sun hitting the panel (square to the sun)... Anything that reduces the intensity of the sunlight, then the current is reduced (turning away from sun, dirt on panels, clouds, etc.).

    Both Vmp and Imp are somewhat affected by the temperature of the panels... As the panels get warmer, Vmp drops (cooler panels have a higher Vmp voltage). Imp is not affected as much by temperature.

    There is another voltage, Voc (Voltage open circuit)... Which is higher than Vmp... As a very rough approximation, Vmp Hot to Voc cold (min to max voltage) will vary by a factor of 1:2 (a hot panel with Vmp=15 volts on a 100 degree day with no wind, may have a Voc=30 volts on a sub freezing day with full sun that just came out from behind a cloud).

    And with no sun on the panels, the generated voltage goes down to zero volts, and the panel resistance goes fairly high (but still need a blocking diode if directly connected to a battery--such as a solar panel trickle charger--to prevent nighttime discharging).

    When panels are partially shaded, their voltage will drop (each solar cell generates about 1/2 volt). Some panels have blocking and bypass diodes to help reduce some shading effects... But, in most cases, even very partial shading a string will reduce its output wattage by 50% to near 100%.

    Regarding the effect on inverters... There are two major classes of inverters...

    One class is the standard "off grid" inverter... Put 12 volts DC (or whatever its input rating is) in, get 120 VAC at 60 Hz out (or, again, whatever the local requirements are).

    For a 12 volt inverter, the battery is really the voltage regulator... The battery is fully charged near 13 volts, and full discharged near 10.5 volts... However, with solar (or any type of charger), the battery will be charged around 13.6 to 14.2 volts (variable with temperature and what type of charging cycle), and may go as high as 15.5 volts when equalizing (and/or "fast" charging)... So, the inverter would need to support a range of input voltage of 10.5 to 15.5 volts for a typical deep cycle battery... For cars, the voltage range is probably closer to 12 to 14.2 volts (because of alternator, and you cannot deep cycle a car battery without permanently damaging it).

    The other major class is a Grid Tied (or Utility Interactive) Inverter without any battery on the DC input...

    This type of inverter may accept between 195 to 600 VDC from the solar panels... The inverter typically is a MPPT type (Maximum Power Point Tracking). It monitors the input voltage and current and tries to maximize the Power=Voltage*Current equation...

    And, as such will take the maximum power from the input solar panels (or wind turbine, etc.) and transfer it to the output 240 VAC (the GT Inverter's AC output is not regulated--and needs the utility to regulate voltage and frequency)... In this case, the electrical utility, for all apparent behaviour, looks like a 240 VAC battery... The inverter will output as much current as it can get from the solar panels (assuming there is enough sun)...

    If there is not enough sun, the input voltage will "collapse" and the inverter will go into standby for 5 minutes and try to draw power again... The inverter also monitors the AC Mains for 240 VAC and 60 Hz--if they fall out of spec., the inverter will shutdown, wait 5 minutes, and test the line again before starting up AC power transfer.

    Of course there are "hybrid" type inverters that take DC battery input and can output either Utility Interactive or Off-Grid AC power (Xantrex and Outback both make these types of inverters--have advantages of "turning your meter backwards" when the sun is up (and your batteries are charged) and being able to go "off-grid" with an AC transfer switch and run your home when utility power fails.

    Lastly, regarding "voltage windows" on a regular off-grid inverter... Typically, they will have a minimum cutoff voltage (like 10.5 volts for a 12 volt battery system)... This cutoff voltage is intended to protect the inverter from overheating (the lower the input voltage, the more current it draws). The cutoff voltage is not intended to protect the battery bank from damage... Because, by this point the battery is already functionally dead and probably badly damaged.

    You will also run into other issues with voltages and batteries... You will have voltage drop in your wiring and fuses/breakers... Batteries also have voltage drop in them too--as the current drawn goes up, their voltage falls. And, you have surge currents (such as for starting motors)... So the inverter, for a few hundred milliseconds (or up to 10 minutes--depending on the inverer), will draw substantially more current than its standard operating current--so the low voltage disconnect may have some current bias and/or time filtering to prevent the inverter from turning off every time you start a well pump motor.

    I will stop here--because I am not sure where you want to go with your questions. Please feel free to ask any more questions, or for clarifications of my ramblings.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Is voltage coming from a string constant?

    Thank you for taking the time to provide this very useful information. I will try to clarify the question, though I think you have answered it and beyond.

    I was looking to install two parallel strings on different slopes and orientations. I was wondering if the sun hits one string fully, meeting the inverter's lower voltage window requirement, but hits the second string minimally, will the minimal energy produced on the second string be used by the inverter or will it be tossed out because it is independent from the first string?

    Thanks again, Bill
  • BB.BB. Super Moderators, Administrators Posts: 31,061 admin
    Re: Is voltage coming from a string constant?

    From what I have read, and understand (Fromus or somebody did a white paper on this subject)... If you have, for example, two strings... One facing east and the second west... If you put all of the series panels in String East on the east face--and all of the series panels on string West on the west face--then tie both strings together in parallel into one inverter... You will be within a couple percent of total output vs a system that had one dedicated GT inverter for the East string, and a second dedicated GT inverter for the west string...

    Basically, the panel voltages are similar between the two strings (the one with more sun will be hotter and slightly lower voltage, but supplying more current).

    Your problem would be if you put some panels from the east and some from the west on the same series string--will reduce your output by a lot.

    So--go with what is cheaper and easier to wire (one large inverter, or two smaller inverters). Both will work fine for you. My guess is that inverters that are ~3kW and larger tend to have a better price (dollars per watt) than two smaller, for example 1.5 kW inverters.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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