Question on battery size needed for proper PV grid tied operation

Rich V
Rich V Registered Users Posts: 7
New guy here with a question on battery size needed for proper PV grid tied operation.

My location:
Arizona with no obstructions to the sun and panels flat mounted on flat roof

My main system components will be the following:
30 Solar World SW-275 panels – 8250 watts total face plate,
Two Radian GS8048 inverters in parallel
Three FM-80 controllers
Two GSLC 175-PV-120/240 distribution centers
Mate 3 controller
AC disconnect & AC manual transfer switch (100A 240V)

My system use will be grid tied with infrequent need for battery backup as the power here is pretty reliable. Hoping to break even or be a little ahead on generating as much electric as I use on a yearly average.

Now my question, how much battery capacity will I need to keep the grid tied system pushing full power (~8kw) into the grid and not strain the battery bank? I want to size the batteries for the grid tied power requirements not off grid use since my off grid needs are not that large/frequent and I have a Honda EU6500 generator I can hook in if needed.

Since my off grid use will be infrequent and short duration I’m looking at the OutBack EnergyCell 220GH batteries as these have a long float lifespan.

What is the minimum battery bank size needed for my specific situation?

Thanks for your help.

Rich

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Question on battery size needed for proper PV grid tied operation

    Welcome to the forum Rich.

    Have you already bought the Radians and controllers or just contemplating it?
    The reason I ask is because where back-up is not the major concern using battery-based inverters for grid-tie is not very practical.
    In order to get best function from a 8kW 48 Volt grid-tie hybrid inverter you need 800 Amp hours of battery (minimizes the ripple). That would be for each inverter. Now, do you want to invest in 1600 Amp hours of 48 Volt battery and have it sit around doing nothing most of the time?

    You could have a Radian & batteries for backing up that part of your household needs that are critical, and then have the rest of the PV capacity as straight grid-tie. It's cheaper and more efficient. You may even find you only need a 4kW Radian.
  • Rich V
    Rich V Registered Users Posts: 7
    Re: Question on battery size needed for proper PV grid tied operation

    I have contracted for the system but I could change the setup if needed since nothing has been ordered yet.

    I was hoping the lower internal resistance/higher current capacity of ADM's would allow for a lower AH on the system. What would happen if I used two sets of EnergyCell 220GH batteries (~440AH @ 48V) with my proposed system?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Question on battery size needed for proper PV grid tied operation
    Rich V wrote: »
    I have contracted for the system but I could change the setup if needed since nothing has been ordered yet.

    I was hoping the lower internal resistance/higher current capacity of ADM's would allow for a lower AH on the system. What would happen if I used two sets of EnergyCell 220GH batteries (~440AH @ 48V) with my proposed system?

    With two 8kW inverters? Too much ripple current; problems maintaining sell.

    Really I think you should evaluate how much battery back-up you need for basic equipment and adjust the design accordingly. You could save yourself a lot of money and have a more efficient system.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Question on battery size needed for proper PV grid tied operation

    Battery backup grid tie will cost you efficiency, and unless you have Lots of outages, your batteries will die of old age before you get much use out of them.

    If you are planning for SHTF, you have to plan on defending your gear - at night with the lights on, you will attract the zombies.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • RONALD YAHAYA
    RONALD YAHAYA Registered Users Posts: 1
    Re: Question on battery size needed for proper PV grid tied operation

    know your total load and the hours without interruption.
  • Rich V
    Rich V Registered Users Posts: 7
    Re: Question on battery size needed for proper PV grid tied operation

    I found this on OutBack Power's website:

    How much battery capacity should I have for my grid-interactive inverter system?
    http://outbackpower.com/outback-support/frequently-asked-questions

    "This is a common question with folks looking to install a grid-interactive inverter, but want a small and low cost battery bank. There is no one right answer, but there are some good guidelines to follow.

    DO NOT: use starting or automotive batteries. These will quickly fail and will bring down system operating efficiency through their internal charging losses.

    DO: use a deep cycle battery. Deep cycle batteries have thicker cell plates and are designed for longer life.

    DO: use a minimum of 100 amp-hour batteries per 4kw of grid-interactive inverter. For example a GFVX3648 should have no less than four, 100aH deep cycle batteries. A GS8048 should have no less than four, 200aH deep cycle batteries.

    Because the inverter actually sells from the DC side of the system, it is important to have a large enough battery capacity for the hysteresis to be effective. Hysteresis is used to create a "lag" or delay in the battery voltage changing state during the charge and discharging events. Without any hysteresis the batteries could be lowered to the lower sell voltage in seconds at which point the inverter would stop selling. Then the battery voltage would quickly spike up to a higher voltage at which point the inverter would again sell the battery voltage down. This rapid micro-cycling of the battery bank would work to shorten the life of the batteries and thereby reducing the system selling efficiency due to the increased self-discharge and heat losses within the battery bank."

    So OutBack recommends 100 amp-hour of batteries per 4kw of grid-interactive inverter. In my case with two GS8048 inverters I would need 400 amp-hour of batteries per OutBack's spec.

    I cross posted my question on the OutBack forum and sent them an e-mail to clarify the battery requirements for my system.
    Thanks All for your input.
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: Question on battery size needed for proper PV grid tied operation

    As others have alluded to, why not just go with a direct grid-tie system, no batteries and no battery inverter? It will cost much less and it will deliver more PV power to the grid. The only downside is that when the grid goes down, you can't use your PV system as a backup(*).

    * Some SMA inverters have a feature where they can delivery 1.5kW to a special outlet during grid failure: http://www.sma-america.com/en_US/products/grid-tied-inverters/sunny-boy/sunny-boy-with-sps-3000tl-us-3800tl-us-4000tl-us-5000tl-us-6000tl-us.html
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Question on battery size needed for proper PV grid tied operation

    Well I hate to disagree with Outback, but I'm going to.

    When you switch over to battery power you want your inverter to be able to supply its full capacity in an outage. 4kW on 48 Volts is roughly 93 Amps (including conversion factor and tare loss). 200 Amp hour battery? Kiss it goodbye. That would be near 50% capacity in one go, meaning significant Voltage drop on loading and short operation time.

    I think they are relying too much on people under-sizing loads here. Not a safe bet.

    Although it's interesting they feel their inverters do not need as much 'filtering' as other brands do.
  • Rich V
    Rich V Registered Users Posts: 7
    Re: Question on battery size needed for proper PV grid tied operation
    Well I hate to disagree with Outback, but I'm going to.

    When you switch over to battery power you want your inverter to be able to supply its full capacity in an outage. 4kW on 48 Volts is roughly 93 Amps (including conversion factor and tare loss). 200 Amp hour battery? Kiss it goodbye. That would be near 50% capacity in one go, meaning significant Voltage drop on loading and short operation time.

    I think they are relying too much on people under-sizing loads here. Not a safe bet.

    Although it's interesting they feel their inverters do not need as much 'filtering' as other brands do.

    A couple of thoughts/questions from a guy who knows just enough to get in trouble.

    When grid tied and back feeding the mains isn’t the amount of power back feed determined by how much power is generated by the PV panels? If so then in my case this is in the range of 8KW. We can add more for cloud edge effect but my panels are @ 0 degrees so ~85% of face plate + cloud edge?

    Why would having a larger total inverter size be a factor in battery bank sizing if power is limited by the PV array?

    Asked another way, why would the battery bank size be set by the total inverter rating (14kw in my case) and not the solar PV size? (for grid tied operation not backup power)

    When in backup power mode the total draw will be set by the subpanel load. This can be trimmed to match the battery bank size and backed up by a generator if needed. I believe the Radian setup allows me to set a DOD (voltage setting) to limit how much is drawn from the batteries before it disengages.

    Assuming the above is close to being true.

    If I have two sets of EnergyCell 220GH batteries for ~ 440 AH @ 48V (20 hr) this would meet the minimum recommended size by OutBack and could provide ~4kw of power for an hour and draw down less than 50% of bank capacity.

    OK tear me a new one.:blush:
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Question on battery size needed for proper PV grid tied operation

    With grid-tie battery backup, you need an alarm to warn you when the grid fails, so you can shed loads as needed.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Question on battery size needed for proper PV grid tied operation
    Rich V wrote: »
    A couple of thoughts/questions from a guy who knows just enough to get in trouble.

    When grid tied and back feeding the mains isn’t the amount of power back feed determined by how much power is generated by the PV panels? If so then in my case this is in the range of 8KW. We can add more for cloud edge effect but my panels are @ 0 degrees so ~85% of face plate + cloud edge?

    Why would having a larger total inverter size be a factor in battery bank sizing if power is limited by the PV array?

    Asked another way, why would the battery bank size be set by the total inverter rating (14kw in my case) and not the solar PV size? (for grid tied operation not backup power)

    When in backup power mode the total draw will be set by the subpanel load. This can be trimmed to match the battery bank size and backed up by a generator if needed. I believe the Radian setup allows me to set a DOD (voltage setting) to limit how much is drawn from the batteries before it disengages.

    Assuming the above is close to being true.

    If I have two sets of EnergyCell 220GH batteries for ~ 440 AH @ 48V (20 hr) this would meet the minimum recommended size by OutBack and could provide ~4kw of power for an hour and draw down less than 50% of bank capacity.

    OK tear me a new one.:blush:

    No, you're sort of right. Array size does determine how much power is fed back. But so does inverter size. In other words it's array size @ efficiency up to inverter limit. So quite often a larger array is used to keep the inverter output at its max which makes for more efficient system.

    For example:
    4kW array will output on average 3000 Watts. Up it to 5.2 kW array and you get 4000 Watts. With a 4kW inverter you will get no more than that even if there is 8kW of array.

    When this is happening with a hybrid inverter the batteries are acting as a filter, keeping the pulsing of the charge controller (which is how it maintains a fixed Voltage) from sending what is essentially a pseudo-AC signal to the inverter input (AC ripple) which affects its function. Think "big filter capacitor on big DC power supply). The larger the inverter the larger the PV (we hope) and the more filtering is needed.

    Switch to back-up power and things change a bit. For one you can get away with murder on the filtering capacity because you are no longer trying to maintain a particular DC input Voltage to accommodate SELL. But now you have the load demand on the batteries, and they can only take so much current output. With FLA's this is usually a maximum of 25% of capacity, and you don't want to sustain that for very long. AGM's can take a higher current 'hit', but the capacity lasts the same.

    So we're back at the 100 Amp hours per kW of inverter for 48 Volts (double for 24) in order to accommodate full output for grid-tie and full output for back-up, albeit for different reasons.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Question on battery size needed for proper PV grid tied operation

    One other thing: that 4kW array can nail the batteries with 64 Amps of current on a 48 Volt system. For 200 Amp hours of battery that would be a 32% peak charge rate. Kiss your FLA's goodbye.
  • ButchDeal
    ButchDeal Solar Expert Posts: 35
    Re: Question on battery size needed for proper PV grid tied operation
    Rich V wrote: »
    A couple of thoughts/questions from a guy who knows just enough to get in trouble.
    When in backup power mode the total draw will be set by the subpanel load. This can be trimmed to match the battery bank size and backed up by a generator if needed. I believe the Radian setup allows me to set a DOD (voltage setting) to limit how much is drawn from the batteries before it disengages.

    Assuming the above is close to being true.

    If I have two sets of EnergyCell 220GH batteries for ~ 440 AH @ 48V (20 hr) this would meet the minimum recommended size by OutBack and could provide ~4kw of power for an hour and draw down less than 50% of bank capacity.

    OK tear me a new one.:blush:

    Why do you want 16kW of inverter for 8kW of array? You might consider the Flexpower2 which is 7.2kW and would more closely match your array of 8kW, it also comes with two charge controllers and the Mate.
    You then could put just the emergency load you want on it (4kW or so mentioned above) but still be able to feed in 7.2kW...
    http://www.solar-electric.com/outback-flexpower-preassembled-inverter-system-fp2-gvfx3648.html
  • Rich V
    Rich V Registered Users Posts: 7
    Re: Question on battery size needed for proper PV grid tied operation
    ButchDeal wrote: »
    Why do you want 16kW of inverter for 8kW of array? You might consider the Flexpower2 which is 7.2kW and would more closely match your array of 8kW, it also comes with two charge controllers and the Mate.
    You then could put just the emergency load you want on it (4kW or so mentioned above) but still be able to feed in 7.2kW...
    http://www.solar-electric.com/outback-flexpower-preassembled-inverter-system-fp2-gvfx3648.html

    The 8.25KW of panels are at the edge for what one inverter can carry, having two will allow load sharing and keep the units from working at full capacity - good for extending life on any electronic power control. It also allows future expansion of the PV array should I want to.

    Same idea on the backup power side, two inverters allows for load sharing, gives head room for inductive starting loads and allows for future expansion of batteries & load.

    Since Uncle Sam is willing to subsidize this (until 2016 at least) I don't mind going a bit larger. I just don't want to spend >$10k for 2 tons of batteries if I absolutely don't need them.
  • ButchDeal
    ButchDeal Solar Expert Posts: 35
    Re: Question on battery size needed for proper PV grid tied operation
    Rich V wrote: »
    The 8.25KW of panels are at the edge for what one inverter can carry, having two will allow load sharing and keep the units from working at full capacity - good for extending life on any electronic power control. It also allows future expansion of the PV array should I want to.

    Same idea on the backup power side, two inverters allows for load sharing, gives head room for inductive starting loads and allows for future expansion of batteries & load.

    Since Uncle Sam is willing to subsidize this (until 2016 at least) I don't mind going a bit larger. I just don't want to spend >$10k for 2 tons of batteries if I absolutely don't need them.

    the Flexpower 2 has two inverters built in and is stackable for expansion. It has two GVFX3648, which has quite a bit of surge capability.
    http://www.outbackpower.com/downloads/documents/gtfx_gvfx_series/ob_022013_gtfxgvfxseries_specsheet_lr.pdf
    So if you wanted to expand in the future to 16kW you could just add another Flexpower2 or more likely just two more inverters and two more CCs.
    You can expand batteries independently of the inverters with outback as well.

    You would be more efficient working at capacity of the inverter pair instead of have 4 inverters (two each in the radians), In fact the radian solution would have two of them turned off most of the time anyway.

    My suggestion would be on the Flexpower2 and spend the difference on more batteries, but your money.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Question on battery size needed for proper PV grid tied operation

    Grid tie inverters are actually designed to operate at their rated capacity for 3+ hours per day.
    At least the good ones are.
  • Rich V
    Rich V Registered Users Posts: 7
    Re: Question on battery size needed for proper PV grid tied operation

    After speaking with tech help at Outback Power they recommended a minimum of 550AH @ 48V for my proposed system. I'll use 3 sets of the EnergyCell 220GH (660AH) batteries and limit my off grid draw to < 8KW (166 DC amps). I can set the cutoff to ~25% DOD and have ~ 1 hour until I need to start the generator.
    Thanks
    Rich
  • Rich V
    Rich V Registered Users Posts: 7
    Re: Question on battery size needed for proper PV grid tied operation

    Some data on the OutBack EnergyCell 220GH batteries.
    They are re- branded EnerSys PowerSafe SBS190F batteries. They have some impressive capabilities.
    From this PDF www.enersys-asia.com/pdf/EN-SBS-EON-OGH-002_0909.pdf

    1.6 Current Limit
    Inherent low internal resistance allows PowerSafe SBS EON
    Technology battery types to accept in-rush currents up to 600% C10
    amps (e.g. PowerSafe SBS 190F in-rush current up to 1140 Amps).
    For reliable cyclic performance and to achieve maximum cycle life
    potential, the acceptable recharge current limit range is 0.1 to 1C10
    Amps (e.g. for PowerSafe SBS 190F the recharge current range is
    19 Amps to 190 Amps per series string).

    Since I will have 3 strings of these @ 48V my charge current range is 57 - 570 amps :D (The dead short current for these batteries is 3800A, for 3 strings @ 48 V that is 547,200 watts :p )
    I will probably set the limit current to 100 amps and just float them at the 2.28v/cell since these will be used infrequently and I'm not in a hurry to recharge them.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Question on battery size needed for proper PV grid tied operation

    Watch out for what it says in Sec 2.1 about Partial State of Charge! That is a big caution point...
     
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