Specific Instructions for DIY Solar

Lisa
Lisa Registered Users Posts: 1
I am new here but I liked what I saw so I am hoping you have some answers or know where to get them. My site sells portable solar panels and small things so I am a little out of my element when it comes to writing about larger solar energy systems. That hasn't stopped people from asking me and since I plan to get a system myself I have written a beginners article on what to consider when getting a solar panel system.

Then I got this question: What size solar panels would I need to run my refrigerator for 48 hours continuously, rain or shine in a power outtage and what size batteries, converters and other equipment will I need?

This person stated that he had trouble finding specific answers and he had met plenty of others that felt the same way. Since confused people often do nothing I thought I would try to find a computer program or an ap that made it easier. This is for an emergency outtage situation but I would like to be able to give specifics for both emergency and permanent setups. I will write the information down in article form as well.

Any helps and thoughts on this topic would be appreciated and I give credit to my sources, it is only fair (unless you want to be anonymous).

Thanks in advance:D

Comments

  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Specific Instructions for DIY Solar

    Welcome.

    First you will need to be much more specific on what the fridge uses (it's very model specific!) over a 24 hr day and what the peak surge is at start-up after the fridge has cycled a few times, so there is back pressure in the pump.

    That draw is the Achilles heal of a Solar backup system.
     
    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,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Specific Instructions for DIY Solar

    Refrigerators use a lot more power than most people realize... For example, a common Energy Star rated full sized US Refrigerator can use ~365 kWH per year (call it 1.0 kWH per day for our math below).

    In general, an off grid cabin/small home with lights, refrigerator, small well pump, laptop computer can get away with as small as a ~1 kWH (1,000 Watt*Hour) per day system. Add a refrigerator and a clothes washer load ever day or so--closer to 3.3 kWH per day (~100 kWH per month).

    Since you asked specifically about a refrigerator... Lets start with a 1,000 Watt*Hour per day appliance. And a minimum of 4 hours of sun per day (typically in much of the US for ~9 months of the year--The other three months, get a small Honda eu2000i genset or equivalent to make up for bad weather).

    First the size of the battery bank. An "optimum" system will work out to be around 2 days of storage and 50% maximum discharge (for long life) on a flooded cell lead acid battery bank. The math:
    • 1,000 WH per day * 1/0.85 AC Inverter eff * 1/12 volt battery bank * 2 days of "no sun" * 1/0.50 maximum discharge = 392 AH @ 12 volt battery bank

    That would be roughly 4x 6 volt @ 200 AH "golf cart" batteries in series/parallel (two batteries in series for 12 volts, then the two strings in parallel for a 400 AH @ 12 volt battery bank).

    Next the solar array... There are two calculations we like to do... The first is the minimum charging current to keep the battery bank "happy". That ranges from 5% to 13% typical for a solar array. 5% an work for a weekend cabin, short term emergency power. 10%-13%+ is better for full time off grid power (5% is the minimum recommended rate of charge and 10-13% is a healthy rate of charge for flooded cell Lead Acid batteries):
    • 14.5 volts charging * 400 AH battery bank * 1/0.77 panel+controller deratings * 0.05 rate of charge = 284 Watt array minimum
    • 14.5 volts charging * 400 AH battery bank * 1/0.77 panel+controller deratings * 0.10 rate of charge = 569 Watt array nominal
    • 14.5 volts charging * 400 AH battery bank * 1/0.77 panel+controller deratings * 0.13 rate of charge = 739 Watt array "cost effective" maximum

    Next, is sizing the panel for the amount of power you will be using. Again, in much of the US, you can start with ~4 hours a day of sun minimum (long term average) for ~9 months of the year (use a generator and/or larger array for winter power needs):
    • 1,000 WH per day * 1/0.52 end to end system eff * 1/4 hours minimum sun = 481 Watt array minimum

    So, based on our back of the envelope calculations--For somebody that wants "reliable" off grid power for ~9 months of the year, they would be looking at a 481 to 739 Watt solar array on 4 golf cart batteries (12 volt @ 400 AH battery bank).

    AC inverter wise--Probably a minimum of 1,200 to 1,500 Watt 120 VAC (TSW--True Sine Wave inverter) minimum recommended.

    There are some other details to look at that may increase the size of the system a bit (tare losses on AC inverter, does the refrigerator need 120 VAC 24 hours per day * 7 days a week--Most "frost free" models have a timer that needs 120 VAC on all the time).

    And there are other options--Such as the Chest Freezer Conversion that can use about 1/4 the AC power (chest freezer conversions frequently do not make a spouse happy to have to keep moving baskets of food around).

    Anyway--This gives you an idea of what an "absolute" minimum/but reliable Off Grid Refrigerator Sized system could look like.

    -Bill

    Add missing link:

    Chest freezer
    as a chest refrigerator


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Specific Instructions for DIY Solar
    Lisa wrote: »
    Then I got this question: What size solar panels would I need to run my refrigerator for 48 hours continuously, rain or shine in a power outtage and what size batteries, converters and other equipment will I need?

    So you wrote an article and your quizzing us? It's a trick question!!! the answer is in the question, you need none!

    If you question is 'rain or shine' we must assume no sun, hence no panels. In fact the panels will be a net draw since the charge controllers will draw some wattage.

    If you want to know how much battery backup system you need to run the fridge, tell us how much energy the fridge uses when this happens(fridges will draw different amounts of wattage with different ambient air temps.)
    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 backup...lol), 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.
  • AuricTech
    AuricTech Solar Expert Posts: 140 ✭✭
    Re: Specific Instructions for DIY Solar

    When choosing an inverter, make sure that it can handle the power surge needed to start the refrigerator's compressor (typically about 3X the running Watts).

    ETA: An inverter in the power range Bill suggested (1,200 to 1,500 Watt 120 VAC) ought to be able to handle the compressor's power surge for a refrigerator, but it's best to check the specifications to be sure.