Help with inverter and battery size

AlexusAlexus Registered Users Posts: 1
I have 3 panels total . I have  2.  300 watt.   44Voc and 1 235 watt with 36 Vic
I got all of these second hand so I need some help to figure out what size inverter I need and what size batteries I need. This is not for a whole house more like general purposes lights small appliances TV laptop. Please help thanks I am new to this

Comments

  • EstragonEstragon Registered Users Posts: 1,545 ✭✭✭✭
    I would put the 300w panels in series to an mppt charge controller, and the 235 on a separate pwm controller.

    This should give you something like 20a charging at 24v into 4 6v flooded golf cart batteries wired in series and parallel for ~225 amp-hours at 24v nominal - about 2500 watt-hours of usable capacity.

    The inverter size depends on what sort of small appliances you want to run. Heating type appliances (eg. toaster) take a lot more than a shaver, for example. Things with motors (pump, shopvac) can take a lot of inrush current to start.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators Posts: 26,837 admin
    Welcome to the forum Alexus.

    A few questions... Need to confirm that both sets of panels are Vmp~35-36 volts (Voltage maximum power) or if the panels are Vmp~30 volts or both types. Voc is less interesting (at the moment)--Although, having rest of the numbers (Voc, Vmp, Isc, Imp) would probably save us asking questions later. When mixing and matching solar panels, there are details about the electrical ratings that need to "match" in order to have an efficient and well working system.

    Do you plan on adding more solar panels later?

    Where will you be installing the system--Guessing Atlanta Georgia for the moment.

    Do you need 12 VDC (or other DC voltage)? Or is 120 VAC "good enough"?

    Assuming that all of the panels are "compatible", a "balanced system" would look like using our rules of thumbs. First, sizing the battery bank (flooded cell lead acid batteries) based on the solar array size. 5%-13% rate of charge for battery bank, 5% can work fine for weekend, seasonal, "non-winter" operation. 10%+ rate of charge is "nicer" for full time off grid, less baby sitting of loads vs hours of sun per day/weather, and a longer battery life:
    • 835 Watt array * 0.77 panel+controller deratings * 1/0.05 rate of charge * 1/14.5 volt battery charging = 887 AH @ 12 volts maximum battery bank
    • 835 Watt array * 0.77 panel+controller deratings * 1/0.10 rate of charge * 1/14.5 volt battery charging = 443 AH @ 12 volts nominal battery bank
    • 835 Watt array * 0.77 panel+controller deratings * 1/0.13 rate of charge * 1/14.5 volt battery charging = 341 AH @ 12 volt minimum battery bank
    Based on Atlanta Georgia, the amount of 120 VAC energy the system will generate per day (by month) with a fixed array tilted to 33 degrees from horizontal would be around:
    http://pvwatts.nrel.gov/pvwatts.php
    MonthSolar Radiation
    ( kWh / m2 / day )
    January4.13
    February4.52
    March5.49
    April5.95
    May5.99
    June6.10
    July5.81
    August5.81
    September4.85
    October4.96
    November4.27
    December3.90
    Annual5.15

    Assume February is "break even month" (use genset during winter weather:
    • 835 Watt array * 0.52 AC off grid system efficiency * 4.52 hours of sun per day (Feb) = 1,963 Watt*Hours per day (avg Feb day)
    My humble suggestion would be 2x 6 volt @ ~200 AH "golf cart" batteries in series string * 2 strings in parallel (4x batteries total) for a 12 volt @ ~400 AH battery bank.

    The average "overnight" and "dark day load" assuming 2 days of storage and 50% maximum discharge on the above bank would be:
    • 12 volts * 200 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge =1,020 Watt*Hours per day
    If you wanted to run the TV/appliances for 5 hours per day (from battery bank during bad weather or over night) would be:
    • 1,020 WH per day / 5 hours per day = 204 Watt average load
    The AC inverter for this size bank (12 volt @ 200 AH) would be around 1,000 Watts maximum, and I would suggest that this Morningstar 300 Watt Sine Wave 12 VDC inverter would be a nice unit for this bank/system. Rugged, reliable, and has remote on/off and "sleep mode"--Not many small 12 VDC inverters have these functions:

    https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html

    Note the above numbers are approximate. In reality, +/- 10% is about as accurate as the predictions can be (I used the full numbers so you can track how I use them from calculation to calculation/reduce round-off error) (i.e., a 1,200 Watt inverter vs 1,000 Watt is a "don't care" rounding).

    The above system would be a pretty reliable system that would run LED lights, LED TV, laptop computer, cellphone charging, electric tool charging very nicely and reliably. Just a starting point for discussion.

    What do you think? Any questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 4,253 ✭✭✭✭
    Well first your panels won't work together well, you will need 2 charge controllers to allow them to charge the same battery bank. VOC is not the number you should be looking at, it does tell us you have 2 - 72 cell panels and 1 60 cell panel. VMP is the voltage while the panel is connected and work. VOC is important in an ancillary fashion as you need to protect your electronics/charge controllers from being exposed to the high voltage number. 

    Generally you would start with the amount of load. To start with some panels and what can I use them for is a bit backward, but here goes. Assuming you will use 1 charge controllers and the 2 - 300,  and a cheaper charge controller. This will be up to you, but in a "lets put these to work" sort of deal I would just us the 2 - 300 watt panels. The charge controller for the single 60 cell panel will cost a good bit more and if you buy one just to use it, you may regret it later if you find more 60 cell panels.

    In a system in daily use you would want to charge the battery bank at a rate around a 10-13% of the battery capacity. The 2 - 300 watt panels assuming a VMP of about 35 volts should have an output of about 600/35=17 amps, This puts in in the ball park of 4 - 6 volt golf cart batteries in a 24 volt system. 4 - 6 volt golf cart batteries from Sam's club or Costco will cost about $85 each + a core they will take pretty much any battery as a core, lawn tractor battery will work and worth hunting ddown some dead ones as the core charge is $15-20 a battery. This will give you a battery bank of 215 amp hours at 24 volts or about a 12% charge rate, this is a bit exaggerated as the panels will produce some what less current and voltage under normal operating conditions.

    You could use a simple 24 volt PWM charge controller, Like This one that appears to be made by Y-Solar. I have a friend using a 48 volt newer version from Y-Solar(who makes charge controllers that several people put their names on) and he's happy the newer version(?) has aluminum back and cooling fins not sure if that's just for the 48 volt version.

    https://www.amazon.com/ALLPOWERS-Charger-Controller-Intelligent-Regulator/dp/B01MU0WMGT/ref=pd_sbs_86_1?_encoding=UTF8&psc=1&refRID=M97QQPFCH9SJ6HA0GF8V

    I would suggest something off NAWS web site, but the site is very slow to load today!!!

    What size inverter, how you use the energy will determine what size inverter. you don't have a huge array, but one normal sunny days you can expect about 75 % of panels rating x about 4 hour so around 1.8 Kwhs, energy used during the day. if you are using energy at night realize you will lose about 20% of that to charge the battery or have about 1.45 Kwh.

    Cheap inverters use a modified sine wave, which is inefficient and create extra heat with motors, shortening their lives. They might also not play well with electronics, most will run fine on them. Particularly things with batteries, like laptops and cell phone chargers. Some have reported issues with compact florescent lights, I haven't heard about problems with LED lights but might...

    While I would recommend no continuous loads of more than 400 watts. This system would be large enough to provide some back up incase of a power outage, and motors like the compressor of your fridge will require a larger start up current. So something in the 1500 watt range would be good to have. Something like, this True sine wave Samlex;

    https://www.amazon.com/Samlex-PST-1500-24-Inverter-Continuos-operating/dp/B00AYH68K0/ref=sr_1_1?s=lawn-garden&ie=UTF8&qid=1510677804&sr=1-1&keywords=samlex+1500+watt+24
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • BB.BB. Super Moderators Posts: 26,837 admin
    If you want a "slightly larger" system that could run your refrigerator, a 1,200-1,500 Watt AC inverter (as PhotoWhit suggests). Possibly a 3rd string of Golf Cart batteries (6 batteries total for 12 volts @ 600 AH) would work OK too. Especially for emergency backup power.

    With the size of array you have, it would be close (Energy Star refrigerators run ~1,000 to 2,000+ WH per day). A backup genset (for bad weather) would be handy.

    If you wanted to run a small home/cabin with refrigerator and "near normal" electrical experience (with lots of conservation), a 3.3 kWH (3,300 WH) per day system would be a better fit. Running a fridge/freezer is typically the "division" between a small and a medium size off grid solar power system (and higher costs).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 4,253 ✭✭✭✭
    Please understand that Bill and I are suggesting different system voltages. I think you would be fine with a 24 volt system and 2 panels designed for a 24 volt system. You could put the 60 cell panel on a separate pwm charge controller and likely get some useful charging when the batteries are low. Going this route the charge controllers will be much less expensive, but 24 volt inverters will be somewhat more expensive.
    BB. said:
    If you want a "slightly larger" system that could run your refrigerator, a 1,200-1,500 Watt AC inverter (as PhotoWhit suggests). Possibly a 3rd string of Golf Cart batteries (6 batteries total for 12 volts @ 600 AH) would work OK too. Especially for emergency backup power.
    Going Bills route you would need a MPPT type charge controller that could handle 40+ amps. These will run $480+. But you could use a less expensive inverter like a xantrex 1800 watt XM (about $350)

    https://www.amazon.com/XM-1800-DC-Power-Inverter/dp/B00197TCVQ

    I like higher system voltages particularly when moving into a mid range system. I don't think the reserve would be a problem with 4 golf cart batteries. For a fridge and little else, a few lights. As a back up It should be fine. You will have about 5Kwhs of storage and even with minimal charging should be able to weather a couple day outage, with cloudy days, without a problem. and could go months with single cloudy days.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • PhotowhitPhotowhit Solar Expert Posts: 4,253 ✭✭✭✭
    I perhaps didn't make it clear why I was suggesting the higher wattage inverter. This was only to start those items like a fridge which might pop off due to the high demands when starting. The worries about this are greater with a 12 volt system, due to voltage drop on startup. You will need heavy/correct wiring to avoid inverters shutting down on startup of things like a traditional fridge. 

    I have a friend who is playing with running 2 old freezers on an Exeltech 1100 watt inverter on a 48 volt system, with no problems.

    While running the fridges likely won't draw more than 200 watts and more likely to be around 120-140watts.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • littleharbor2littleharbor2 Solar Expert Posts: 681 ✭✭✭✭
    According to my Kill a Watt meter, My 14 cu. ft. Hotpoint refrigerator freezer draws .3/4 amp while running. 90 watts, not bad.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • BB.BB. Super Moderators Posts: 26,837 admin
    edited November 15 #9
    I agree that for larger battery banks, a higher voltage is usually better.

    However, for smaller a.c. loads, it can be difficult to find lower wattage a.c. inverters at the higher bus voltages.

    For a 3.3kwh per day system, 24 volt battery bus is usually much better than 12 VDC .

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 4,860 ✭✭✭✭
    I agree about going 24Volt, reason is that the 44Voc panels Vmp output will be almost perfect for a 24volt system, as there will be almost no down shifting of the voltage to match a 24 volt batteries needs., also cheaper to wire up than 12volts as you would use smaller sized wire for 24 Volts.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge, Hughes1100 Sat Modem
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • mcgivormcgivor Registered Users Posts: 1,280 ✭✭✭✭
    edited November 15 #11
    +1 on the higher voltage, going the 12V nominal route is akin to painting yourself into a corner for future expansion, the investment in components is, for the most part equal, but once committed, the inverter, which is determining factor, will limit one to the choice made. Additional benefits, as mentioned, will be smaller ballance of system requirements, conductors, breakers, less parallel batteries etcetera, all positives, with no negatives.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • littleharbor2littleharbor2 Solar Expert Posts: 681 ✭✭✭✭
    Not to mention, you can run run twice as much wattage through your controller at 24 volt compared to 12 volt.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

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