Inverter to Panel Help

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  • ZnodeZnode Registered Users Posts: 18
    Re: Inverter to Panel Help
    Looking for about 60 Volts @ 100 Amps you get 6 kW of panel.

    This was the only part I was confused on, where did the 60 volts come from?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter to Panel Help
    Znode wrote: »
    This was the only part I was confused on, where did the 60 volts come from?

    To charge a battery you need more Voltage than it's got.
    A "12 Volt" deep cycle battery typically charges at 14.2-14.8 Volts. A "48 Volt" battery system is basically 12 * 4, or in this case 14.2 * 4 = 56.8 up to 14.8 * 4 = 59.2. When you consider the need to do an equalization cycle now and then you get 15 Volts * 4 = 60 at a minimum, up to 62. It all depends on which batteries you get and what the manufacturer recommends plus your particular system's efficiency (wire sizes, lengths, operating temperature, et cetera).

    That estimation I gave is all "ball park" numbers to give you an idea, not a specific design. I don't think we emphasize that enough. When you get some preliminary figures like that you can then start adjusting. For instance, increasing the battery bank size to the nearest commercially available units. Then re-adjusting the charge parameters and array size according to the needs of the selected batteries. Then re-adjusting the array size to the nearest available units. Then possibly adjusting again to reduce the efficiency losses to acceptable limits. You can't really do it in one calculation unless you just severely over-size everything to begin with, or settle for having the power give out shortly after the sun goes down. Neither is very fun. :roll:
  • BB.BB. Super Moderators, Administrators Posts: 29,811 admin
    Re: Inverter to Panel Help

    A battery bank (48 volt) will run from ~42 volts dead to ~60 volts high (equalization).

    A MPPT charge controller is a "constant power device... For example a 1,200 watts (nominal) array would output:
    • Power = Volts * Amps
    • Amps = Power / Volts
    • Amps = 1,200 watts / 42 volts = 28.6 amps into a "dead battery"
    • Amps = 1,200 watts / 60 volts = 20.0 amps into a "well charged battery"
    So, since we are looking at getting XX% of battery AH capacity into the battery, we choose the "worst case" amperage conditions for a MPPT controller--Charging a battery near Vbatt-max charging voltage.

    To charge a 48 volt battery bank at rated current and nearly fully charged, vs rated current into a well discharged battery.

    So lets say we want 60 amps into a well charged 48 volt battery bank... (I use ~58-59 volts), assume 0.77 charge+panel losses:
    • 60 amps * 59 volts * 1/0.77 system charging losses = 4,597 watt array
    If I was to use the "nominal" 48 volt battery rating (which is fairly discharged battery voltage when under charged):
    • 60 amps * 48 volts * 1/0.77 system charging losses = 3,740 watt array
    • 4,597 watts / 3,740 watts = 1.23x larger array...
    To be honest, if your "real numbers" are within 10-20% of what is calculated for your solar power system (with all of the variables)--you are doing well...

    I like to use the "worst case" values when making these estimates... It will make the array / system a little bit larger (~23% in this case)--but it does a better job of accounting for losses and how the various devices work.

    Could you build a minimum 5% rate of charge system with the smaller numbers--That would be really a ~4% rate of charge system (about 20% smaller)... Not the end of the world--but every place if we "error" to the smaller numbers--we may end up with a system that will not meet your performance expectations.

    Anyway--too many words explaining why this I (and others) do it "this way".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rplarryrplarry Solar Expert Posts: 203 ✭✭✭✭✭
    Re: Inverter to Panel Help
    BB. wrote: »
    Actually, that is correct based on 25 lights x 7 watts x 10 hours per day or 1.75 kWH per day...

    -Bill

    Darn, I forgot the 10 hours part.
    Larry
  • ZnodeZnode Registered Users Posts: 18
    Re: Inverter to Panel Help
    BB. wrote: »
    A battery bank (48 volt) will run from ~42 volts dead to ~60 volts high (equalization).

    A MPPT charge controller is a "constant power device... For example a 1,200 watts (nominal) array would output:
    • Power = Volts * Amps
    • Amps = Power / Volts
    • Amps = 1,200 watts / 42 volts = 28.6 amps into a "dead battery"
    • Amps = 1,200 watts / 60 volts = 20.0 amps into a "well charged battery"
    So, since we are looking at getting XX% of battery AH capacity into the battery, we choose the "worst case" amperage conditions for a MPPT controller--Charging a battery near Vbatt-max charging voltage.

    To charge a 48 volt battery bank at rated current and nearly fully charged, vs rated current into a well discharged battery.

    So lets say we want 60 amps into a well charged 48 volt battery bank... (I use ~58-59 volts), assume 0.77 charge+panel losses:
    • 60 amps * 59 volts * 1/0.77 system charging losses = 4,597 watt array
    If I was to use the "nominal" 48 volt battery rating (which is fairly discharged battery voltage when under charged):
    • 60 amps * 48 volts * 1/0.77 system charging losses = 3,740 watt array
    • 4,597 watts / 3,740 watts = 1.23x larger array...
    To be honest, if your "real numbers" are within 10-20% of what is calculated for your solar power system (with all of the variables)--you are doing well...

    I like to use the "worst case" values when making these estimates... It will make the array / system a little bit larger (~23% in this case)--but it does a better job of accounting for losses and how the various devices work.

    Could you build a minimum 5% rate of charge system with the smaller numbers--That would be really a ~4% rate of charge system (about 20% smaller)... Not the end of the world--but every place if we "error" to the smaller numbers--we may end up with a system that will not meet your performance expectations.

    Anyway--too many words explaining why this I (and others) do it "this way".

    -Bill

    Great info thanks for the help I'm learning quickly. So say I were to go with the proposed 7400 W array with the 2 volt cells to make a 48 v system. And to do so I used 32 240 W 36 V panels... I'd have more than enough voltage to do so if I wired in series with 4 8 panel arrays.. but how do I go about wiring these to a controller(s) that could handle the amperage? I guess I'm not quite comprehending this part and haven't been able to find a thread or article to answer this question
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter to Panel Help
    Znode wrote: »
    Great info thanks for the help I'm learning quickly. So say I were to go with the proposed 7400 W array with the 2 volt cells to make a 48 v system. And to do so I used 32 240 W 36 V panels... I'd have more than enough voltage to do so if I wired in series with 4 8 panel arrays.. but how do I go about wiring these to a controller(s) that could handle the amperage? I guess I'm not quite comprehending this part and haven't been able to find a thread or article to answer this question

    There you're looking for array combiners: http://www.solar-electric.com/sopawiinco.html
    So you can connect four panels in series by just plugging the MC4 connectors (+) to (-). This makes a string. Bring the positive and negative leads of the whole string to the combiner where circuit protection is introduced (as needed) and the (+) and (-) of each string ties to the corresponding input to the charge controller.

    Some of the things you have to watch out for when combining panels:
    Wire size is capable of handling the current.
    Wire size does not present too much Voltage drop (especially problematic on long runs).
    More than two parallel connections of panels or strings must have circuit protection per each.
    Voc on array does not exceed maximum input Voltage on charge controller (particularly in cold climates).
  • BB.BB. Super Moderators, Administrators Posts: 29,811 admin
    Re: Inverter to Panel Help

    And to add what Marc says--Configuring an Array for a MPPT charge controller is a bit on the complex side the first time you do it.

    And it is not something that I would worry too much at the beginning of a project--more towards the end. Prices and availability change on panels on a weekly/monthly basis. And until you are ready to actually order the parts--Don't get too wrapped up in the calculations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ZnodeZnode Registered Users Posts: 18
    Re: Inverter to Panel Help

    Alright that makes sense. I don't want to make you guys speculate then off fairly vague info, but so I can perhaps understand in general how it works is the attachment I have generally what you were talking about? So in effect that configuration would be able to go through a controller such as http://www.solar-electric.com/oufl80sochco.html ?
  • BB.BB. Super Moderators, Administrators Posts: 29,811 admin
    Re: Inverter to Panel Help

    Each panel has a set of parameters:
    • Voc=Voltage open circuit
    • Vmp=Voltage maximum power
    • Isc=Current short circuit
    • Imp=Current maximum power
    • Series Protection Fuse (maximum current/fuse rating per panel/series string)
    • Temperature compensation (as panels get cold, Voc rises. As panels get hot, Vmp falls)
    For the Controllers:
    • Vmax = Maximum Rated input voltage
    • Voperational = Min/max range of operating voltage
    • Ioutput-rated = Maximum continuous rated output current
    • Iinput-max = Maximum Array Isc before you need an input fuse/breaker
    So--You basically have to configure your solar array for:
    • Vmp... Typically 17.5/35/70 volts < Vmp-array < ~100 volts Note:
      • low voltage is for 12/24/48 volt banks (for a 150 Vmax controller in cold region)
    • Vmax must be less than Voc-Array on the coldest morning/ambient temperature you would expect with the sun above the horizon. Typically for a Vmax=150 vdc in cold climate, roughly Vmp-array~100 VDC maximum will work for most location.
    Note, the array can be >> than the MPPT Charge Controller's "rated output power"--MPPT controller will regulate their output current to the maximum rated current (safely).

    For example, to figure out the maximum (cost effective) array wattage for a typical MPPT system:
    • Parray ~ Controller Current * Vbatt-charging * 1/0.77 controller+panel deratings
    So, for a 12, 24, and 48 volt battery bank:
    • P-12volts @ 60 amp = 14.5 volts charging * 60 amps * 1/0.77 = ~1,130 watt maximum cost effective array
    • P-24volts @ 60 amp = 29 volts charging * 60 amps * 1/0.77 = ~2,260 watt maximum cost effective array
    • P-48volts @ 60 amp = 58 volts charging * 60 amps * 1/0.77 = ~4,520 watt maximum cost effective array
    You can go larger, but the MPPT charge controller will spend more time "current limiting" -- so the extra panel power is largely wasted.

    Note, it is not unusual for the input voltage range from Vmp-hot to Voc-cold to be almost a 2:1 range (~Vmp=70 volts; Vmp-hot to be ~60 volts or less; Voc-cold ~120 volts or more). All depends on panels and climate.

    You can go real small on a large MPPT controller--but they do have their own losses--below 400 watts, the controller operating power requirements, on a large controller, become significant and waste a fair amount of power (not very efficient with small arrays on large controllers).

    For many MPPT controllers--their "optimum" efficiency is Vmp-array about 2x the Vbatt voltage.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter to Panel Help
    Znode wrote: »
    Alright that makes sense. I don't want to make you guys speculate then off fairly vague info, but so I can perhaps understand in general how it works is the attachment I have generally what you were talking about? So in effect that configuration would be able to go through a controller such as http://www.solar-electric.com/oufl80sochco.html ?

    In my estimation that is a bad drawing. It does not show two wires from each panel, only one. The two become very important when you're talking about how to hook up an array. That pic seems to show strings being combined before the combiner too. Let's try a "word picture":

    (-)PANEL(+)---(-)PANEL(+)---(-)PANEL(+)---(-)PANEL(+)

    That makes a string (all panels connected via their MC4 plugs). Wires from the (-) and (+) ends of each string meet up in the combiner box:

    STRING(+)---FUSE---(+)CHARGE CONTROLLER
    STRING(-)
    (-)CHARGE CONTROLLER

    The negatives are all common and connect to a (-) bus bar. The positive leads of each string connect to a fuse or breaker (size for the panels' Isc) and then connect to a (+) bus bar. The (-) and (+) of the bus bars are wired to the charge controller.

    (I know we need a picture library of basic schematics for solar wiring but I'm not going to mention it 'cause I'll get volunteered to create it. :p)
  • ZnodeZnode Registered Users Posts: 18
    Re: Inverter to Panel Help

    (I know we need a picture library of basic schematics for solar wiring but I'm not going to mention it 'cause I'll get volunteered to create it. :p)

    This would be IMMENSELY helpful, its too bad you didn't mention it though.. 8)
  • ZnodeZnode Registered Users Posts: 18
    Re: Inverter to Panel Help

    The two controllers would be required because about the biggest one you can get is the Outback FM80, which handles 80 Amps max. A 1000 Amp hour battery bank would need a potential 100 Amp charging rate, which can't be accommodated on a single controller.

    Perhaps you could paint a "word picture" on how you would wire 2 controllers to a battery bank
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter to Panel Help
    Znode wrote: »
    Perhaps you could paint a "word picture" on how you would wire 2 controllers to a battery bank

    That one is easy: positives to positive, negatives to negative. A standard parallel connection. Each controller having its own fuse on the output.
    Batteries don't care how many charge sources they are connected to, and for the most part charge controllers don't care if they're sharing duty with other charge sources.

    More expensive systems like Outback and Xantrex have communications connections between units that allow them to share data and "agree" on things like battery Voltage and temperature.
  • BB.BB. Super Moderators, Administrators Posts: 29,811 admin
    Re: Inverter to Panel Help

    Outback has a nice documentation area with lots of drawings of various configurations (using their controllers, of course).

    http://www.outbackpower.com/resources/documents/wiring_diagrams/

    The basics are very similar for other controller brands too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ZnodeZnode Registered Users Posts: 18
    Re: Inverter to Panel Help
    BB. wrote: »
    Each panel has a set of parameters:
    • Voc=Voltage open circuit
    • Vmp=Voltage maximum power
    • Isc=Current short circuit
    • Imp=Current maximum power
    • Series Protection Fuse (maximum current/fuse rating per panel/series string)
    • Temperature compensation (as panels get cold, Voc rises. As panels get hot, Vmp falls)
    For the Controllers:
    • Vmax = Maximum Rated input voltage
    • Voperational = Min/max range of operating voltage
    • Ioutput-rated = Maximum continuous rated output current
    • Iinput-max = Maximum Array Isc before you need an input fuse/breaker
    So--You basically have to configure your solar array for:
    • Vmp... Typically 17.5/35/70 volts < Vmp-array < ~100 volts Note:
      • low voltage is for 12/24/48 volt banks (for a 150 Vmax controller in cold region)
    • Vmax must be less than Voc-Array on the coldest morning/ambient temperature you would expect with the sun above the horizon. Typically for a Vmax=150 vdc in cold climate, roughly Vmp-array~100 VDC maximum will work for most location.
    Note, the array can be >> than the MPPT Charge Controller's "rated output power"--MPPT controller will regulate their output current to the maximum rated current (safely).

    For example, to figure out the maximum (cost effective) array wattage for a typical MPPT system:
    • Parray ~ Controller Current * Vbatt-charging * 1/0.77 controller+panel deratings
    So, for a 12, 24, and 48 volt battery bank:
    • P-12volts @ 60 amp = 14.5 volts charging * 60 amps * 1/0.77 = ~1,130 watt maximum cost effective array
    • P-24volts @ 60 amp = 29 volts charging * 60 amps * 1/0.77 = ~2,260 watt maximum cost effective array
    • P-48volts @ 60 amp = 58 volts charging * 60 amps * 1/0.77 = ~4,520 watt maximum cost effective array
    You can go larger, but the MPPT charge controller will spend more time "current limiting" -- so the extra panel power is largely wasted.

    Note, it is not unusual for the input voltage range from Vmp-hot to Voc-cold to be almost a 2:1 range (~Vmp=70 volts; Vmp-hot to be ~60 volts or less; Voc-cold ~120 volts or more). All depends on panels and climate.

    You can go real small on a large MPPT controller--but they do have their own losses--below 400 watts, the controller operating power requirements, on a large controller, become significant and waste a fair amount of power (not very efficient with small arrays on large controllers).

    For many MPPT controllers--their "optimum" efficiency is Vmp-array about 2x the Vbatt voltage.

    -Bill


    Alright so basically to get up to the wattage I'm looking at I would have to up the voltage of my battery bank... or is there something about not going past 48 volts?

    Otherwise the calculations would be:
    • 60v= 72.5v * 60 amps * 1/.77 = ~5649 Watt array
  • BB.BB. Super Moderators, Administrators Posts: 29,811 admin
    Re: Inverter to Panel Help

    Historically, Off Grid Solar power has been limited to 48 volt battery banks.

    The history, I would guess, is that 60 volts is, typically, the maximum "safe to touch" voltage in many regulatory/safety circles (it can be as low as 30 VDC, and 8 amps maximum, etc..).

    There are inverters that use >> 60 volts for their battery banks (computer rooms and larger). But-so far, nothing really for off-the-shelf off grid battery banks for home use.

    But, to be fair, high voltage / high current DC power from Battery Banks is scary stuff. In many ways, AC voltage is safer than DC voltage/power and easier to design equipment/switches/safety devices for.

    DC power tends to sustain arcs very well when compared to an equivalent AC circuit.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 29,811 admin
    Re: Inverter to Panel Help

    By the way, around 60 VDC is the maximum recommend charging voltage for typical lead acid 48 volt battery bank--so upping the charging voltage for a 48 volt bank is not really an option either.

    -Bill "depending on what question you were asking" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ZnodeZnode Registered Users Posts: 18
    Re: Inverter to Panel Help

    I see, so am I basically SOL if I want larger than a 4520 watt array? Or is there some setup way around that?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Inverter to Panel Help
    Znode wrote: »
    I see, so am I basically SOL if I want larger than a 4520 watt array? Or is there some setup way around that?

    Nope. No problem.
    You can put together a huge battery bank if you like. 1000 Amp hours @ 48 Volts can be done.
    Need lots of power to recharge it? Multiple arrays feeding multiple charge controllers all connected to the same bank.

    So 100 Amps charging @ 60 Volts is 6000 Watts/ 77% = 7792 Watt array.
    A typical charge controller handles 60 Amps, so use two such controllers and put half the array on each one.

    This is where using an Outback or Xantrex system helps, because you can have the controllers and inverter(s) communicate with each other so everything agrees on the Voltages and such.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Inverter to Panel Help

    in general you can parallel more by going with another system or systems paralleling the first one with another cc being fed by its own dedicated pvs. the only common point between the systems would be the battery bank and odds are you'd expand on that to accommodate the higher current being fed to it.

    also note that the classic controller may give you more overall power handling potential and even options for higher battery bank voltages if you find an inverter to match that voltage.
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