Several pre-computed off-grid system designs (#PV,12/24/48V,#/type Batts,#BatStrings)

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YehoshuaAgapao
YehoshuaAgapao Solar Expert Posts: 280 ✭✭
in Off Grid Solar & Battery Systems #1
  • Using BB's formula: AH*AbsorbVolts*(1/0.77)*(1/#Hrs)=PVWatts
  • Formula transformed to: AH*AbsorbVolts*(1/0.77)/PVWatts=#Hrs
  • The 0.77 constant is from Solar panels usually delivering about 80% of STC plus 4% losses from the charge controller and wiring.
  • For simplicity, only using 60-cell PV panels, which tend to range fro 225W to 260 W with most being 235, 240, or 245W. 60-cell panels are the most common and are very economical.
  • For simplicity, only using MPPT charge controllers that are max 150 volts open-circuit (Midnite Solar and Xantrex have >150Voc charge controllers available).
  • For simplicity, only Group 31, GC2 Golf Cart, or L16 batteries are considered. Other common battery setups include Group 24, Group 27, Group 8D, Industrial batteries, and forklift batteries.
  • PV KWh/day is computed at 1000W STC = 5KWh/day - somewhat conservative, but assumes a full-sun day and no shading for at least 8 hours per day. My system in Arizona produces 6.25KWh/day per 1000W STC in a full sun day
  • BattKWh is for 100% DoD. Rule of thumb is to plan for 50% DOD. Different brands/kinds of batteries of different DOD peak total-KWh life. Surrette prefers 60-70% DOD. Trojan prefers 20-30% DOD.
  • PV KWh/day and PV charge rate ranges are 225W per panel at the minimum and 260W per panel at the maximum.
  • Systems with more than two PV strings require a combiner box with overcurrent protection on each string. For 60-cell panels in strings of 1, 2, or 3, Midnite Solar MPNV6 with a 15-amp breaker for each string will work good.
  • These designs do not favor high battery reserves (high reserve systems will have a lower PV to AH ratio and have slower charge rates) so most will require or really prefer the presence of a generator or a grid connection or both. PV panels are cheap and getting cheaper and are the longer lived components in the systems. Batteries are expensive and getting more expensive and are among the shorter lived components in the system.

12 Volt Single Charge Controller Systems 
#PV     StrSize #PVStr  PV Watts    PV KWh/day  BatVolt #MPPT CC    Inverter (Min)      Batteries           #BatStr BattAH  BattKWh PV Charge Rate
1       1       1       225-260     1.125-1.300 12      1 (20A)     300W Pure Sine      1x Group 31         1       130     1.56    9.35-10.81 hrs - Nominal
2       1       2       450-520     2.250-2.600 12      1 (45A)     600W Pure Sine      2x Golf Cart GC2    1       225     2.7     8.09-9.35 hrs - Near Cost Effective Maximum
2       1       2       450-520     2.250-2.600 12      1 (45A)     600W Pure Sine      2x Group 31         2       260     3.12    9.35-10.81 hrs - Nominal
3       1 or 3  3 or 1  675-780     3.375-3.900 12      1 (60A)     1000W Pure Sine     2x L16              1       400     4.8     9.59-11.08 hrs - Nominal
3       1 or 3  3 or 1  675-780     3.375-3.900 12      1 (60A)     1000W Pure Sine     4x Golf Cart GC2    2       450     5.4     10.79-12.47 hrs - Low-Nominal
4       1 or 2  4 or 2  900-1040    4.500-5.200 12      1 (80A)     1000W Pure Sine     2x L16              1       400     4.8     8.09-9.35 hrs - Near Cost Effective Maximum
4       1 or 2  4 or 2  900-1040    4.500-5.200 12      1 (80A)     1000W Pure Sine     4x Golf Cart GC2    2       450     5.4     8.09-9.35 hrs - Near Cost Effective Maximum

24 Volt Single Charge Controller Systems 
#PV     StrSize #PVStr  PV Watts    PV KWh/day  BatVolt #MPPT CC    Inverter (Min)      Batteries           #BatStr BattAH  BattKWh PV Charge Rate
2       2       1       450-520     2.250-2.600 24      1 (20A)     600W Pure Sine      2x Group 31         1       130     3.12    9.35-10.81 hrs - Nominal
3       3       1       675-780     3.375-3.900 24      1 (30A)     1000W Pure Sine     4x Golf Cart GC2    1       225     5.4     10.79-12.47 hrs - Low-Nominal
3       3       1       675-780     3.375-3.900 24      1 (30A)     1000W Pure Sine     4x Group 31         2       260     6.24    12.46-14.41 hrs - Low
4       2       2       900-1040    4.500-5.200 24      1 (45A)     1000W Pure Sine     4x Golf Cart GC2    1       225     5.4     8.09-9.35 hrs - Near Cost Effective Maximum
6       2 or 3  3 or 2  1350-1560   6.750-7.800 24      1 (60A)     Outback FX/VFX      4x L16              1       400     9.6     9.59-11.08 hrs - Nominal
6       2 or 3  3 or 2  1350-1560   6.750-7.800 24      1 (60A)     Outback FX/VFX      8x Golf Cart GC2    2       450     10.8    10.79-12.47 hrs - Low-Nominal
8       2       4       1800-2080   9.00-10.400 24      1 (80A)     Outback FX/VFX      4x L16              1       400     9.6     7.19-8.31 hrs - At Cost Effective Maximum
8       2       4       1800-2080   9.00-10.400 24      1 (80A)     Outback FX/VFX      8x Golf Cart GC2    2       450     10.8    8.09-9.35 hrs - Near Cost Effective Maximum

48 Volt Single Charge Controller Systems 
#PV     StrSize #PVStr  PV Watts    PV KWh/day  BatVolt #MPPT CC    Inverter (Min)      Batteries           #BatStr BattAH  BattKWh PV Charge Rate
3       3       1       675-780     3.375-3.900 48      1 (15A)     1000W Pure Sine     4x Group 31         1       130     6.24    12.46-14.41 hrs - Low
6       3       2       1350-1560   3.375-3.900 48      1 (30A)     Outback FX/VFX      8x Golf Cart GC2    1       225     10.8    10.79-12.47 hrs - Low-Nominal
6       3       2       1350-1560   6.750-7.800 48      1 (30A)     Outback FX/VFX      8x Group 31         2       260     12.48   12.46-14.41 hrs - Low
9       3       3       2025-2340   10.125-11.7 48      1 (45A)     Outback FX/VFX      8x Golf Cart GC2    1       225     10.8    7.19-8.31 hrs - At Cost Effective Maximum
9       3       3       2025-2340   10.125-11.7 48      1 (45A)     Outback FX/VFX      8x Group 31         2       260     12.48   8.31-9.60 hrs - Near Cost Effective Maximum
12      3       4       2700-3120   13.50-15.60 48      1 (60A)     Outback FX/VFX      8x L16              1       400     19.2    9.59-11.08 hrs - Nominal
12      3       4       2700-3120   13.50-15.60 48      1 (60A)     Outback FX/VFX      16x Golf Cart GC2   2       450     21.6    10.79-12.47 hrs - Low-Nominal
15      3       5       3375-3900   16.875-19.5 48      1 (80A)     Outback VFX         8x L16              1       400     19.2    7.67-8.87 hrs - At Cost Effective Maximum
15      3       5       3375-3900   16.875-19.5 48      1 (80A)     Outback VFX         16x Golf Cart GC2   2       450     21.6    8.63-9.97 hrs - Near Cost Effective Maximum

Large 48V multiple charge controller systems - These larger systems will be more likely to be grid-tied bi-modal systems (battery backup or mini-grid). 
#PV     StrSize #PVStr  PV Watts    PV KWh/day  BatVolt #MPPT CC    Inverter (Min)      Batteries           #BatStr BattAH  BattKWh PV Charge Rate
24      3       8       5400-6240   27.00-31.20 48      2 (60A)     Outback Radian      16x L16             2       800     38.4    9.59-11.08 hrs - Nominal
                                                                    or Xantrex XW6048
30      3       10      6750-7800   33.75-39.00 48      2 (80A)     Outback Radian      16x L16             2       800     38.4    7.67-8.87 hrs - At Cost Effective Maximum
48      3       16      10800-12480 54.00-62.40 48      4 (60A)     2x Outback Radian   24x 2V L16          1       1200    57.6    7.19-8.31 hrs - At Cost Effective Maximum
                                                                    or 2x Xantrex XW6048
60      3       20      13500-15600 67.50-78.00 48      4 (80A)     2x Outback Radian   24x 2V L16          1       1200    57.6    5.75-6.64 hrs - Grid-Tie Aggressive
96      3       32      21600-24960 108.0-124.8 48      8 (60A)     3x Outback Radian   48x 2V L16          2       2400    115.2   7.19-8.31 hrs - At Cost Effective Maximum
                                                                    or 4x Xantrex XW6048
120     3       40      27000-31200 135.0-156.0 48      8 (80A)     4x Outback Radian   48x 2V L16          2       2400    115.2   5.75-6.64 hrs - Grid-Tie Aggressive
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #2
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    Re: Several pre-computed off-grid system designs (#PV,12/24/48V,#/type Batts,#BatStrings

    somewhat conservative at 8 hours a day? 4-5 hours average, in most locations I have looked up.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #3
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    Re: Several pre-computed off-grid system designs (#PV,12/24/48V,#/type Batts,#BatStrings
    bmet wrote: »
    somewhat conservative at 8 hours a day? 4-5 hours average, in most locations I have looked up.

    Well he is right for AZ for at least 7-8 months out of the year it is at least 6.25 kWh per 1000w STC according to PVwatts.

    Note the second column and the annual average of 6.51 Solar Radiation (kWh/m^2/day):
    "Results from my 12.5 Kw system at 22.6 tilt"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 4.60, 1279, 108.72
    2, 5.63, 1415, 120.28
    3, 6.44, 1726, 146.71
    4, 7.67, 1948, 165.58
    5, 7.99, 2043, 173.66
    6, 7.87, 1880, 159.80
    7, 7.60, 1887, 160.40
    8, 7.40, 1856, 157.76
    9, 7.06, 1719, 146.12
    10, 6.34, 1664, 141.44
    11, 5.08, 1333, 113.31
    12, 4.35, 1220, 103.70
    "Year", 6.51, 19969, 1697.37
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: Several pre-computed off-grid system designs (#PV,12/24/48V,#/type Batts,#BatStrings

    And the point of this exercise is what, exactly?

    Any sort of pre-planned solar design is not likely to be viable because each installation is highly individualistic. This is why I constantly warn against "kits" - and why I spend so much time correcting problems with systems designed and installed by others.

    Are you trying to make more work for me? :p:D
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #5
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    Re: Several pre-computed off-grid system designs (#PV,12/24/48V,#/type Batts,#BatStrings

    It is a nice chart that allows people to quickly get a price on components for a wide range of power options and understand the capabilities of PV solar.

    Not a kit--But does level set expectation of somebody looking for off grid power.

    Will be interesting to see what question get asked... One of the reasons I do the per poster set of calculations is that it allows me to explain the details and assumptions each step of the way in terms of how the question was originally asked. This is a lot of information in one condensed post.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • YehoshuaAgapao
    YehoshuaAgapao Solar Expert Posts: 280 ✭✭
    #6
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    Re: Several pre-computed off-grid system designs (#PV,12/24/48V,#/type Batts,#BatStrings

    Was meaning for them to be fuzzy-kits. Starting templates.

    They are most helpful to people using the most common battery sizes (Group 31, Golf Cart GC2, L16), 150 volt MPPT charge controllers, and 60-cell PV panels. Group 31 and Golf Cart batteries have very good cost per KWh of storage. 60 cell panels are the most common, and 72 cell panels are also common. 60 and 72 cell panels are very economical. 72 cell panels don't fit xantrex controllers very well because they derate a lower voltage but they should fit other 150V MPPT controllers in strings of 3. 80 cell panels don't fit 150V MPPT well. 96 cell panels are a good fit for 150V MPPT controllers in strings of 2 but those can only be found on eBay or chinese trade sites but they do have good pricing. Panels below 60 cells do not have the cost efficiency.

    People using PWM controllers, extra high-voltage MPPT charge controllers (such as 250V Midnite Solar Classic 250 or 600V Xantrex MPPT 80), AC-coupled battery banks, or using PV panels with fewer than 60 cells or panels with more than 72 cells (Xantrex controllers start derating at a lower voltage than MorningStar, Outback, and Midnite so those are 60-cell only) will not find these all that useful.

    People using recycled car/boat batteries that are not Group 31 or GC2, or are using industrial or forklift batteries will need to do battery bank sizing themselves, but the AH totals will still be there and helpful for comparing.

    Also any multiple charge controller system, especially when grid-tied, will need to utilize network communication between charge controller and inverter and brand-matching will become important.
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