offgridwannabe: Need off grid power for my temporary home

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in Off Grid Solar & Battery Systems #1
This discussion was created from comments split from: Battery System Voltages and equivalent power.
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  • offgridwannabe
    offgridwannabe Registered Users, Users Awaiting Email Confirmation Posts: 1 ✭
    #2
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    Hello everyone.
     With respect I ask a few humble questions. I've always wanted to get into solar but was daunted by all the amps/power/wattage/volts/draw/peak/controller/inverter/wiring estimations and configurations--Eek! 
      Our house burned down this Spring and I have been living rough since, although we just acquired a used RV to move in to.  So far I have purchased one 12v Marine deep cycle battery and was trying to educate myself on the panels and inverter etc.  It looks like I need approx 800 watts for the rv needs. We are in Texas so average 5+ peak sunshine hours.  With only 1 battery, should I just use 400 watts worth of panels? Can I go ahead and buy a MPPT controller that can handle a bigger system if we add on? Would a 1500 watt inverter be sufficient or should it be matched as closely as possible to the panels? How does a non electrician know what cables and wiring are needed? I'm sure these are dumb questions-- I know nothing lol.  Thanks for any advice!
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #3
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    offgridwannabe said:
    Hello everyone.
     With respect I ask a few humble questions. I've always wanted to get into solar but was daunted by all the amps/power/wattage/volts/draw/peak/controller/inverter/wiring estimations and configurations--Eek! 
      Our house burned down this Spring and I have been living rough since, although we just acquired a used RV to move in to.  So far I have purchased one 12v Marine deep cycle battery and was trying to educate myself on the panels and inverter etc.  It looks like I need approx 800 watts for the rv needs. We are in Texas so average 5+ peak sunshine hours.  With only 1 battery, should I just use 400 watts worth of panels? Can I go ahead and buy a MPPT controller that can handle a bigger system if we add on? Would a 1500 watt inverter be sufficient or should it be matched as closely as possible to the panels? How does a non electrician know what cables and wiring are needed? I'm sure these are dumb questions-- I know nothing lol.  Thanks for any advice!
    We all learn over time...

    Much of it is the nomenclature/words we use. I think every one starts by using 'watts' and watthours' wrong.
    Watts is a term for power used at a moment in time, but is measured as if you ran something for an hour. So a fridge that uses 100watts but only runs for 30 minutes 12 times a day. Might be said to use 100watts (the per hour is inferred) but uses .5 watts per hour for 12 hours a day would use 600 watthours a day.

    So a statement like "need approx 800 watts for the rv" is either speaking to the hourly use or should be 800watthours a day.

    I think you should state a new thread. Bill, or very capable Admin, might split this off into it's own discussion/thread.

    Suggest not buying anything else untill you have a better grasp of your needs.
    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.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    edited October 2022 #4
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    Welcome to the forum OGW, and very sorry to read about the loss of your home.

    As Photowhit says, you (and us) need to better understand your daily energy needs.

    Off grid solar power is not cheap, but for full time off grid with no other source(s) of electricity, it can be a great way to have a somewhat normal "electrical lifestyle" without the issues of running a genset 24x7x365.

    Can you tell us a bit more about hour loads (LED lights, laptop computer, cell phone charging, RV water pump)--Or a bit more such as an electric refrigerator (vs propane or 2/3 way RV fridge). Any genset backup power? etc...

    First terminalogy is the maximum loads you will need. That is Rate of energy usage--Sort of like miles per hour:

    Amps is a rate... You can use 5 amps @ 12 VDC
    Watts is a rate... You can use 60 Watts (notice we do not included "working voltage"--Amps is a partial unit--Watts is a full unit)
    Amps * Voltage = Watts... 5 amps * 12 volts = 60 Watts

    And for total energy used in a day--Sort of like total miles driven:

    Amps * Hours (time)... 5 amps * 5 hours a night (for lights, at home stuff) = 25 Amp*Hours per day @ 12 volts
    Watt * Hours (time)... 60 Watts * 5 hours per night = 300 WH per day
    25 AH per day * 12 volts = 300 WH per day

    For boats, cars, etc... We can stay in Amps and AH of "energy usage" because everything is running at 12 VDC.

    For "modern" off grid power, we typically have the Battery Bank working voltage (12/24/48 VDC) and we may have a 120 VAC inverter (which takes the 12/24/48 VDC and converts to 120 VAC).

    For "mixed voltage systems--We tend to work in Watts and WH as this is less confusing... When somebody says they have a 2 amp load, we always ask is this at 12 VDC or 120 VAC:

    12 VDC * 2 amps = 24 Watts
    120 VAC * 2 amps = 240 Watts

    Both are "2 amp loads", but the difference in "working voltage" (12 vs 120 volts), can mean a 10x difference in energy usage.

    Say today, you are running a small genset (like a Honda eu2200i inverter--generator, quiet, fuel efficient, and not cheap).... You can get a Kill-a-Watt meter and measure your AC power usage for a day or two and get an idea about your daily energy usage:

    https://www.amazon.com/s/ref=nav_bb_sb?field-keywords=amazon+kill-a-watt+meter

    An example for your battery... Say it is a 100 AH @ 12 volt battery:

    100 AH * 12 VDC = 1,200 WH of storage (and that is to "dead"--Should only plan on using 1/2 that per charge cycle).

    Because details matter here... I will stop typing until we better understand your loads/energy needs. The answers for a "small system" (1,000 WH per day) are different vs a 3,300 WH per day (refrigerator, etc.)...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • SamG340
    SamG340 Registered Users Posts: 6 ✭✭
    #5
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    With only 1 battery, should I just use 400 watts worth of panels? Can I go ahead and buy a MPPT controller that can handle a bigger system if we add on? Would a 1500 watt inverter be sufficient or should it be matched as closely as possible to the panels? How does a non electrician know what cables and wiring are needed? I'm sure these are dumb questions-- I know nothing lol.  Thanks for any advice!

    Panel size , battery size & inverter size don't have to be perfectly matched , not exactly anyway.

    It's logical , so....


    ----  A tiny solar panel will take ages to charge up a huge battery

    ------ and a big inverter will flatten a tiny battery really quickly 



    Once you get your head round it, electrics arent all that complicated, it's all just like wiring a plug ... Be safe, do your research, red to red, black to black, do the job right first time.


    You can Google "1500w inverter cable sizes" or what ever size inverter you choose , remember you need a fuse in line for the inverter, (again Google it! ) . 


    You really could do with finding someone local to you that can give you a hand / advice , 

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