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Thread: Off-grid solar system problem: not able to start a fridge. Why?

  1. #1
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    Default Off-grid solar system problem: not able to start a fridge. Why?

    We've just put up this off-grid solar system with an 80W solar panel charging a couple of 12V batteries which are connected in parallel to form a total current of 72amps. And we've got this connected to a 300W (with 600W surge) pure sine wave power inverter.

    Now the fridge (Severin KS 9827) is quite small and needs just 300W per day i.e. the running power is that of 300/24=12.5 W. The manual does not actually give you the surge power it needs to start up but I found out that fridges need about 10 times the running power to start up but we're well covered in that case by the inverter.

    Is it possible that the problem arises from the batteries since they are not new and perhaps there is not enough power? Or could it be because the batteries are connected with a 4mm wire which is too thin in this case? Note that there are 10 batteries in all connected in parallel.

    I appreciate any suggestion. Thanks.

  2. #2
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    Wire, inverter and batteries are all causing the problem

    say the fridge consumes 200W running, and 10x that for start up. That's 2,000 watts for starting. That's 170A at 12v. the wires reduce the voltage, the inverter can't possible manage that, and the batteries voltage will sag under such a load.

    Fridge @ 200W for 8 hours a day run time (33% on cycle) would consume 1600 watt hours. (math is watts x hours = WH)

    And your solar is only 75W for 5 hours a day, you only harvest 375wh each day and need much more to keep the batteries charged.

    The first step is to caculate your loads, then size your batteries, and then size the solar array and backup generator.

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  3. #3
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    Quote Originally Posted by jgau0017 View Post
    We've just put up this off-grid solar system with an 80W solar panel charging a couple of 12V batteries which are connected in parallel to form a total current of 72amps. And we've got this connected to a 300W (with 600W surge) pure sine wave power inverter.

    Now the fridge (Severin KS 9827) is quite small and needs just 300W per day i.e. the running power is that of 300/24=12.5 W. The manual does not actually give you the surge power it needs to start up but I found out that fridges need about 10 times the running power to start up but we're well covered in that case by the inverter.
    OK you have a few things mixed up and deriving usage incorrectly. The unit consumes 112 Kwh/year which works out to an average of 300 watt hours per day (not 300 watts). However it uses 50 watts, not the 12 something you calculated because the compressor does not run 24 hours per day, it cycles on/off.

    Now with that said the specs are very conservative and not real life applications. Under normal use I would expect the fridge to use more like 450 watt hours per day. So armed with that bit of knowledge I can quickly determine the batery capacity needed to be [450 wh x 5 days] / 12 volts = 187.5 Amp Hours @ 12 volts.

    OK now to your big problem, solar panel wattage and location. There is no where in the world an 80 watt solar panel is going generate 450 usable watt hours per day. I suspect you are in Germany and using a PWM charge controller. If using a PWM charge controller th epanels need to generate 900 watt hours per day because th ePWM CC and battery charge efficiency ar at around 50% or less. In Germany winter time you recieve only about 1.5 Sun Hours. So to generate 900 wh in Germany in winter you need a 900 wh / 1.5 h = 600 watt solar panel array to get the job done. You only have 80 watts. An 80 watt panel will not even work in summer. Your battery is dead.

    As for the inverter size we know the fridge uses 50 watts and has a 10X start up. So you would need a minimum 250 watt continuous, 500 watt surge rated inverter. Sounds like you are OK there.

    So if you really want to operate that fridge from solar you need:


    • 600 watt Solar Panel if using a PWM charge controller, or 450 watts if using a MPPT charge controller.
    • PWM charge controller @ 40 amps, or 40 amps using MPPT charge controller.
    • Battery required is an AGM 12 volt @ 200 AH. An AGM is required because of your poor location in Germany requiring such a large panel wattage as it generates a C/5 charge current. Only an AGM battery can handle that high of a charge rate.

    Then as Mike Points out since you use such a low voltage battery you are going to have to be very careful with wire lengths and size to overcome the voltage drop problems running at 12 volts.
    MSEE, PE

  4. #4
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    Thanks a lot for your comments.

    In the meantime I have managed to find out that the running power is about 84 watts and needs 840W starting power so I'm not ok with the inverter. In fact we'll be selling the one we've got and buy a 500W(1000W) pure sine wave inverter.

    Regarding the solar panel, I live in Malta (a tiny island in the Mediterranean just right under Sicily, Italy) and we plan to have the fridge just for summer during which I think we calculated to get around 8 sun hours daily since our summer days are quite long.

    Honestly, we're going to do some trial and error things: check with the electricity meter readings at home to get an average of how much watts does the fridge really consume and check how much Amp hours the batteries actually give out by almost discharging them once. I know it's not that bright(actually it's stupid) but we've made the mistake of having bought everything thinking that we got the concept right when we had not so now it's just manual work and no math.

    And thank you again for your comments. I will read again through them so as to try to grab the whole concept right.

    Thanks,

    Jonathan

  5. #5
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    Quote Originally Posted by jgau0017 View Post
    Regarding the solar panel, I live in Malta (a tiny island in the Mediterranean just right under Sicily, Italy) and we plan to have the fridge just for summer during which I think we calculated to get around 8 sun hours daily since our summer days are quite long.
    Well that is a heck of a lot better than Germany, but 8 sun hours no way. You have to get it right, and you have to use worse case. What I mean if you are there say from the beginning of May, and stay until the end of September you have to use the shortest Sun Hours days which I guess would be around 3 to 4 hours. Now the good news is that brings down the solar panel wattage and charge controller amps to about half of required. Battery remains fixed at 200 AH as there is no way around that.

    Quote Originally Posted by jgau0017 View Post
    Honestly, we're going to do some trial and error things: check with the electricity meter readings at home to get an average of how much watts does the fridge really consume......
    This is what got you into trouble to start with, trial and error has already been done and it is now well known science and simple math. You need to know 2 things.
    Daily Watt Hour Useage
    Location and time of year use.

    Once you got that it is simple to specify what is required.

    Quote Originally Posted by jgau0017 View Post
    I know it's not that bright(actually it's stupid) but we've made the mistake of having bought everything thinking that we got the concept right when we had not so now it's just manual work and no math.
    Well FWIW about 99% of th epeople do the exact same error. They go buy something, and hope it works. 99% of the time that plan fails. Then when they realize what is really needed, they cannot afford it and have to abandon ship.

    I really hate when people come here who have spent $500 they did not have to start with trying to save money on electric bills, only to find out they need another $5,000 to make it work. With off-grid battery you are going to end up spending about 10 times more vs commercial power.
    MSEE, PE

  6. #6
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    I dont think you are going to find a 500w TSW inverter will start the fridge. I have tried many 600w w ones and many times they wont start even little bar fridges. you need at least 800w inverter
    There are ways to modify a fridge to get it to start on a 400 w MSW inverter,but for most people they dont want to be modifying the fridge as it requires the help of a fridge mechanic to do part of the work

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