My off-grid plan...

johnTjohnT Registered Users Posts: 15 ✭✭
Hello all,

Here is my proposed plan for my off-grid house. I am planning on 4 kwh a day but that is also leaving some "wiggle" room since when I added up my loads and it actually came out to 3.6 kwh per day. After reading a lot of the posts on here I came to the conclusion that over-sizing your system is usually a good thing if you can afford it. So anyway, here it is...


12 - Sharp 235w panels...2,820 watts total
MidNite Solar Prebuilt Magnum System 4,400 Watts 48 VDC...comes with the MidNite Classic 150
8 - Surrette S-530 - 6 Volts, 400 Amp-hours


I know there are more components involved (combiner box, backup generator, etc.) but this will be the "heart" of my system and just wanted to see what you guys thought as far as the solar panels being able to charge the batteries and whether or not said batteries will supply my need, etc. Any and all thoughts/ideas are welcome. I realize the inverter is slightly over-sized going by the rule of 100ah (at 48v) for every 1kw of inverter power, but wasn't sure how exacting that rule was. To be honest, I don't really understand that rule, and can't seem to locate a post or answer of why it needs to be that way. I've only seen it made reference to. This will be going on a house I will be building in Belize, so I can have total control over how the roof faces and will also be designing the house so the distance from array to the MidNite panel will be very short and shading issues shouldn't be a problem. Basically I am starting from scratch and will be designing/building this house with the accommodation and use of solar as the #1 goal.

One last question...how would you recommend wiring the panels for the best results on a 48V system? Here are the specs...and thanks in advance.

Maximum Power (Pmax)*: 235 W
Tolerance of Pmax: +10%/-5%
Type of Cell: Monocrystalline silicon
Cell Configuration: 60 in series
Open Circuit Voltage (Voc): 37.0 V
Maximum Power Voltage (Vpm): 30.0 V
Short Circuit Current (Isc): 8.60 A
Maximum Power Current (Ipm): 7.84 A
Module Efficiency (%): 14.4%
Maximum System (DC) Voltage: 600 V
Series Fuse Rating: 15 A
NOCT: 47.5°C
Temperature Coefficient (Pmax): -0.485%/°C
Temperature Coefficient (Voc): -0.351%/°C
Temperature Coefficient (lsc): 0.053%/°C

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: My off-grid plan...

    For 4 kW hours AC a day it looks good for batteries, controller, panels, and inverter. That "rule of 100 Amp hours for every 1kW of inverter" is for hybrid GT systems which need to filter out the AC ripple. Not applicable to off-grid scenarios. :D

    These panels have a Vmp of 30, which means they will have to be in strings of either three (X4) or four (X3) for a 48 Volt system. The Voc @ 37 means the strings will either be 111 Voc or 148 Voc. That last one could be instant trouble even with a MidNite Classic as cold temps could push the Voc to 192 which is pretty close to the Classic's HyperVoc limit of 198. You'd have to put circuit protection on each string anyway, and the closer to system Voltage the more efficient the controller will work.

    So unless you have some really long wire run from array to controller I'd go with four strings of three in series.
  • MangasMangas Solar Expert Posts: 547 ✭✭✭✭
    Re: My off-grid plan...

    Back up generator?
    Ranch Off Grid System & Custom Home: 2 x pair stacked Schneider XW 5548+ Plus inverters (4), 2 x Schneider MPPT 80-600 Charge Controllers, 2 Xanbus AGS Generator Start and Air Extraction System Controllers, 64 Trojan L16 REB 6v 375 AH Flooded Cel Batteries w/Water Miser Caps, 44 x 185 Sharp Solar Panels, Cummins Onan RS20 KW Propane Water Cooled Genset, ICF Custom House Construction, all appliances, Central A/C, 2 x High Efficiency Variable Speed three ton Central A/C 220v compressors, 2 x Propane furnaces, 2 x Variable Speed Air Handlers, 2 x HD WiFi HVAC Zoned System Controllers
  • BB.BB. Super Moderators, Administrators Posts: 30,497 admin
    Re: My off-grid plan...
    For 4 kW hours AC a day it looks good for batteries, controller, panels, and inverter. That "rule of 100 Amp hours for every 1kW of inverter" is for hybrid GT systems which need to filter out the AC ripple. Not applicable to off-grid scenarios.

    Marc,

    Actually, as I understand the issue--It is still a requirement for off grid systems too (100 AH @ 48 volts per 1,000 watt of inverter capacity).

    The DC Input to a single phase AC inverter has a 120 Hz ripple (to make up the sine wave voltage/current wave form). Also, it turns out that if you assume the inverter can supply ~2x it rated load for surge current--That works out to about the C/2.5 maximum recommended surge current for a flooded cell battery bank too.

    2,000 watts surge * 1/48 volts nominal * 2.5 maximum surge derating for battery bank = 104 AH @ 48 volt minimum for 2kW surge

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: My off-grid plan...

    Bill;

    It's more of a "it all depends" situation off-grid, as most people do not run their inverters anyplace near maximum capacity whereas with grid-tie you are wasting money if you can't achieve peak output of the inverter.

    My 3.5 kW inverter has 232 Amp hours of battery and it works perfectly. If it were in a system where the maximum 3.5 kW was demanded the battery bank would not be able to support the load in that instance. It would need to put out 146 Amps there and then, and since that is more than 50% of the batteries' capacity it wouldn't work. Some types of batteries can take higher discharge rates the same as they can take higher charge rates. Ordinarily you try to keep the two rates the same for average/maximums. A momentary high draw usually won't cause trouble, as in my being able to start 1kW + start-up loads.

    If you use the 100/1kW rule off grid you end up buying more battery than you actually need.

    (Yes, I could have got a smaller inverter, the FX2524T. But the smaller one uses as much power as the larger one and hasn't as much room for load addition expansion.)
  • BB.BB. Super Moderators, Administrators Posts: 30,497 admin
    Re: My off-grid plan...

    In the end--it is always "know your loads", then design the system.

    But, I agree with you Marc that a GT Hybrid system is probably, on average, going to run much closer to its rated power than most off grid folks (that probably only average 1/4 of their inverter's rated output).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: My off-grid plan...

    It certainly makes sense that if you invest in 'X' kW of GT you'd run it close to that as much as possible to improve payback.

    That's a difference between the two types: GT you want as large as possible to maximize your return, OG you want as small as possible to minimize the capital expense. Both benefit from the #1 rule of conservation!
  • johnTjohnT Registered Users Posts: 15 ✭✭
    Re: My off-grid plan...

    Thanks for the feedback guys.

    As Cariboocoot said... "It's more of a "it all depends" situation off-grid, as most people do not run their inverters anyplace near maximum capacity"...that fits my bill as I don't expect to use much more than 1/2 of it's rated capacity most of the time and basically never above 3/4, but decided to go that way in case of possible added loads in the future (none planned for at this time, but who knows...:D) If I do add more panel and battery to expand in the future I was hoping to not have to buy a different inverter. Can you guys see any problems with that for this time? I realize the Magnum pulls a couple more watts than say the Outback VFX3648 (23 vs. 25 running) but that seems negligible to me.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: My off-grid plan...

    Less than 2 Amp hours difference in a day. I wouldn't worry about it.

    Mangas's point about the back-up gen is a good one. The sun does not always shine down upon us.
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