Review of quick and inexpensive system to run standard Fridge

prsolarprsolar Registered Users Posts: 1
Hi there, I'm interested in building a system that can drop-in and start running a standard refrigerator, and I've put together a design which I'm hoping your pros can tear apart and make better :) I do know that standard fridges are the most wasteful devices on this planet but this will be for use in houses in Puerto Rico that just need to get back to a normal life for the next few months (to a year) that they will be without power. One suggestion I heard elsewhere was to look at higher wattage panels to save some dollars there. The system as it is (without wiring) is around $2200 using amazon components. My experience so far is "building" a 100W solar-car battery system (which is getting lots of use thank you very much), so any thoughts are welcome. Thanks in advance!!


  • EstragonEstragon Registered Users Posts: 4,496 ✭✭✭✭✭
    I'd highly recommend using 6v flooded deep cycle golf cart batteries in a 4 series, 2 parallel bank for [email protected], or 4x L16s in a single series string for [email protected] 4 parallel strings using 12v could have balance problems, and the batteries may be "marine" hybrid type batteries rather than true deep cycle. If you do end up with more than two strings, you should fuse each string.

    4x100w STC panels will put out maybe 300w in real life, or about 300/28v charging = 10-11a. For a system cycled daily, you really should have more like 30-40a charging for a ~400ah bank to cover daytime loads and charging. You want to be able to recover from a day or two of cloudy weather. At 50%SOC, you need to put 200ah + charging losses back. At 10a, you'll run out of sun even in the sunny south.

    If you can buy pallet quantities of larger panels, that may work better than more small panels.
    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
  • pdhpdh Registered Users Posts: 30 ✭✭
    Be very careful with the cables to that inverter. I have a 1000 watt AIMS inverter that I almost never use because the positive and negative connections to the battery cables are so very close together, instead of being on separate sides of the back panel as is common on other inverters that I've seen. Your picture looks very similar to my unit.

    I fear that it would be incredibly easy to accidentally short those two together, which would be Very Bad. It's good to see that you have a breaker in your diagram, just in case!
  • littleharbor2littleharbor2 Solar Expert Posts: 1,706 ✭✭✭✭
    edited November 2017 #4
    Make the inverter connections first. then the battery connection, and finally insert the fuse. There will be a sharp spark when first connected, know this and don't worry, it's normal. Also the fuse should be as close to the battery as practical. They are there to protect the wiring.  There are fuses that mount on the battery terminal.

    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,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    Agree with @Estragon more PV, larger Ah batteries to avoid parallel strings >2 ,  around 1500W PV would be optimum but would require 2 of the controllers listed, 1000W would be the minimum but even that would require running at almost 100% of the controllers capacity, which is not the best situation, using the 40A version would be better.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • PhotowhitPhotowhit Solar Expert Posts: 5,842 ✭✭✭✭✭
    edited November 2017 #6
    Well you have given us your load goal of 2 Kwhs, Great place to start!

    So you want 1 day autonomy to 50% of capacity. I'm going to figure 2 nights and 1 day without sun, or 36 hours of a 2Kwh load or 3 Kwhs to 50%, so you would need a 6 Kwh battery bank for twice that. So looking to get close to 6 kwhs of battery bank4 golf cart batteries would provide about 6 volts at 220 amp hours for (6vx220amps at 20 hours discharge rate=)1320watt hours. 4 would provide 5280, not ideal but close. If you assume some minimal charging on the cloudy day you are in the right ball park.

    So you will want to be able to recharge for the daily use of 2kwhs. Solar panels under normal conditions produce about 75% of the rating.  We don't have you exact location, but we know Puerto Rico, I'm going to use San Juan, since I can find solar Insolation numbers for there. Looks like you will average about 5 hours of sun with panels angled to your latitude facing south. You might consider +15 degrees, there isn't much difference and the added angle will help rain keep them clean.

    You need about 2 kwh's of charging, there is about 20% waste in charging lead acid batteries, so figure 2400 kwhs incoming. So you will need 500 watts of charging each hour, panels produce 75% of their rating, 500 is 75% of 670, so about 670 watt array.

    Shooting for being in your $2200 budget, and understanding that the new 72 cell panels produce the correct charging voltage for use with inexpensive PWM charge controllers (at the expense of about 10% less current) I would shoot for large 72 cell panels which are relatively inexpensive. If you are shipping out of Miami. Something like these at Sun Electronics. With the 10% less current you will be looking for about 740 watts of array. Something like the Astronergy 310 watt panels (ASM 6612P-310) would work at 55 cents a watt, 3 will be a bit more than your requirements but give you extra cushion;

    Please note, the 250 watt panels at first would look like a better fit, but  they are 60 cell panels and won't work with cheaper charge controllers. These are large panels and NOT easy to ship/move around. If amazon will ship to Puerto Rico, I'd go with 8 - 100 watt panels or 6 - 160 watt panels. Lets say 8 of these 100 watt panels;

    With the panels either the 72 cell large panels or the smaller 36 cell panels wired 2 to a string (creating a string of 72 cells) you will have the correct charging voltage for a 24 volt battery bank with an inexpensive PWM charge controller. You can expect to put these on a 30 amp PWM charge controller, like the Midnite Brat or Something like this;

    We have mostly heard bad reports about Aims inverters. If you want a reliable system for a year, I'd forget Aims. Something like a Samlex 1500 watt pure sine wave;

    $400 Batteries (Costco or Sam's club)
    $900 Solar Array 100 watt amazon panels (cheaper if you can get the larger panels to Puerto Rico $515)
    $30-90 Charge controller 
    $550 Inverter

    You will need some wiring and fuses. For a small temporary system, Particularly if yo do a ground mount no on the roof, I personally would forgo a combiner box. A combiner box does protect the panels if they short out. this is unlikely and there isn't much to catch fire on the solar panel. A combiner box (use a small din rail breaker box) and 4 breakers will run a bit less than $200

    You will want a breaker of the inverter and a breaker from the array is also nice. Breaker don't respond quickly to spikes in wattage, at 24 volts your inverter will draw about 62.5 amps. A 70 amp breaker should be fine. A 30 amps breaker for the incoming amperage from the array and from the array to the battery bank. This should work fine as well. Both would fit in small din rail breaker box. Total <$200 around $100 You may find that the wiring suggested for the inverter is larger than the din rail breakers can handle. In that case you will need to move up to a panel mount breaker.

    You will also need some wiring, use wire designed for UV exposure, USE2 or Solar. Your inverter manual, will tell you the proper size for the distance from battery to the inverter.

    Good Luck!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • PhotowhitPhotowhit Solar Expert Posts: 5,842 ✭✭✭✭✭
    ...All that and I forgot to say, older fridges in hot environments, might use 2 Kwhs all by themselves, maybe more! It would be good to measure with a Kill-A-Watt meter!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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