System design check PLEASE

momofgcm Solar Expert Posts: 35
Well, after LOTS of info from this board, I put together a summary of one of the houses in Guinea, West Africa. There is no generator.

Can you please check my numbers and correct me where I need it?
It looks to me like the system is too small to run anything major. We are shipping a small freezer over (660W/d) but I'm still not convinced there is much wiggle room in the energy production for it. The freezer consumes 660W/d at 90F I think. We routinely have 100F temps for several months. It looks like the power consumption can go up 30-40% with that 10F change.

One thing I don't get is do I use 12V or 14V for the battery voltage for power calculations? I am using 13V for my calculations.


The system I have is:

Total Daily Load (24 hr period)
550Wh (without fridge or freezer) or
1225Wh (w/ freezer) or
1500Wh (w/ 19cf fridge/freezer)

We plan to insulate the outside of the fridge/freezer to make it run less.

If I am looking at the numbers correctly, we need the following amps to run the 12V system...

Loads/13V = Amps/day to run the system. This comes out to 46, 102, 115 Amps/day respectively.

The fridge will go through a TSW GoPower 1500W inverter.
The other loads will go through a MSW GoPower 1750W inverter.

How do I calculate inverter loss and the like into my numbers?

Battery Bank
It is a 12V system. There are (4) MK 8G4D LTP 12V Gels connected in parallel.
The C/20 capacity is 185

The total capacity = 4x185 = 740Ah

No one has decided what the max DOD of the system will be. Anyone have suggestions? I am leaning towards somewhere between 20-25% as my suggestion. It's the only way to supply the loads for a day. You lose a lot of battery life by cycling that much capacity but I don't know of another alternative (other than a generator!)...

I would have the following energy available based on how much the batteries are discharged:
10% DOD = 74Ah
20% DOD = 148Ah
25% DOD = 185Ah

Assuming fully charged batteries and a fridge/freezer, the system can run for less than 2 days without sun and discharging down 25%..

Solar Panels
There are (4) Sharp 123W ND-123UJF installed directly to roof rails. I don't know the angle or direction.

Vmp = 17.2V
Imp = 7.15A
Voc = 21.8
Isc = 7.99A

The panels are currently connected in parallel.

The ideal max power out would be 492W. Assuming a derate of 33%, that would give me 330W useable.

Assuming the controller makes the PVA input V to 14V, it would give me a max output current of about 23A(not counting controller losses).

The insolation value for that part of Africa is 3.5-4 so the average output/day of the panels would be 4 * 23A = 92Ah/day.

Charge Controller
They are using a BZ500 controller. (YES I KNOW IT'S BAD, but I inherited this!! - I am working on convincing them to hand carry a new controller when they go out this summer).

The max output current is 45A. The max Vip = 500V

Charging Rate
It seems that the charging rate should ideally be between 5%-13%. If I am charging without any load, I would need current between 37-96A.

My max current output from the panels is 23A so the max my charging rate could ever be is 23/740*100= 3.1%.

Is this enough to keep my batteries from sulphating?? What should I be concerned about with this low charge rate?

With any type of fridge, the only way I see this system working is if she uses loads when there is no charging going on and she only charges during the day.

The conclusion I am coming too is I cannot normally run either system for a day without a generator or more panels. Recharging doesn't appear to be fast enough. I feel like I'm panel poor. If we were to add panels to make this sytem work, how much would we have to add?

The current house also has a well pump with 230W of solar panels slated to be totally dedicated to the pump. The panels have not been purchased yet (they are on the list to be bought ASAP but everything is on hold until we work through all this stuff I'm talking about) so I am thinking I want to add them to the house system and utilize their power to supplement the house system. We would let the owner choose between pumping water and the freezer. It seems like a no brainer to me but the guy buying everything is saying 'this (dedicated panels) is the way it's done'. I'm not convinced. The pump is about 30feet from the control panel.

Can anyone recommend a VERY quiet diesel powered portable generator? We are trying to convince the owner to use a generator and she is budging but ONLY if it's VERY quiet. There have been some personal safety issues with single women using generators. What size generator would be needed for this kind of system? QUIET is the key!

I would appreciate all critiques on the numbers and assumptions.



  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    Re: System design check PLEASE

    Your power consumption (loads) needs to be expressed as WattHours. Perhaps you meant to say WH instead of just W. I sure hope so because if not, then the 660W freezer running continuously for 24hrs is almost 16KWH.

    How did you come up with the Watt numbers? Did you use a Kill-a-Watt meter? Why did you do your calculations with 13V instead of 12V?
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: System design check PLEASE

    Kim here is a good string for info on system sizing, inverter derating etc.

    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
  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    Re: System design check PLEASE
    One thing I don't get is do I use 12V or 14V for the battery voltage for power calculations? I am using 13V for my calculations.
    Loads/13V = Amps/day to run the system. This comes out to 46, 102, 115 Amps/day respectively.

    Your figures don't make sense ... the formula is (volts x amps x hours = Watt hours or Wh) So if the refer uses 120 volts and draws 10 amps and runs 6 hrs / day then you have 7200 Wh.

    I'm guessing here but, if your basic load is 550 watts and you run it for four hours a day then; you will need 875 Ah of batteries (50% DOD & 2 days backup). Your 4 panels will need around 6 hours of sun and a charge controller that will handle 50 amps.
    They are using a BZ500 controller. (YES I KNOW IT'S BAD, but I inherited this!! - I am working on convincing them to hand carry a new controller when they go out this summer).

    As far as I am concerned, You don't have a charge controller! The BZ my quit at any time, if it hasn't done so already, and, you will need more panels to make up for it's inefficiency's. Guinea is not the place to look for a new controller!!!
  • momofgcm
    momofgcm Solar Expert Posts: 35
    Re: System design check PLEASE

    Yes, the total daily load unit should read Wh. I will adjust the post.

    They include the wattage x hourly usage to equal daily usage. I labeled them total daily loads. I guess it wasn't clear enough.

    The only loads are CF bulbs (the wattage is listed and I multiplied by hours by the number of bulbs), the fridge is listed with it's energy usage. I looked up the laptop usage. Those are the only loads she has.

    I thoroughly agree about the controller. I am pushing hard to get another controller in there. The powers that be say it appears to be working now. The question is 'define working'. It's hard to convince people that aren't familiar with the subject.

    Any ideas how to convince them (other than reading these posts!)??

    I asked the question about do you use 12, 13, or 14 volts when you calculate?? It's 14+V at the input to the controller, it's 12+ - 14V at the battery when in use. So, what really is the appropriate number to use? I don't know! It doesn't always make sense to use 12V depending on where you are talking within the system.

  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin
    Re: System design check PLEASE

    Well, roughly the batteries when supplying current to your loads are around 12 volts or so... When charging, they are around 14 volts or so...

    So, that is one of the "inefficiencies" of the battery bank... Remember Power=Volts*Amps... So, if you take out:
    • P=V*I= 12 volts * 10 amps = 120 watts
    But when charging, you are:
    • P=V*I= 14 volts * 10 amps = 140 watts
    One of the interesting things about lead acid batteries is that they are nearly 100% efficient with respect to current*time... If you take 100 Amp*Hours from the battery bank, it takes ~100 Amp*Hours to recharge it (but because the voltage is different between charging and discharging--it does take ~20-25% more power/energy to recharge the battery bank).

    Regarding your inverters and many other loads--they tend to be constant power devices... For example a 120 volt inverter supplying 240 watts will take:
    • P=V*I => I=P/V
    • I = P/V = 240 watts / 14 volts = 17 amps
    • I = P/V = 240 watts / 12 volts = 20 amps
    So, as the battery voltage falls, the inverter (and many motors and electronic devices too), will draw more current to make up for the lower input voltage from the battery bank/solar charge controller to keep the total power the same.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: System design check PLEASE

    My concerns would be:

    1). Refrigerators have high start-up current, especially in hot climates. The inverter may be under-sized.
    2). It might be better to have one, higher-capacity pure sine wave inverter. (Not a fan of MSW myself).
    3). BZ controller. I'm sounding like a broken record, I know! They can work without working well, robbing potential charge.
    4). Hot climate derating on panels, components, batteries. This is hard to accurately predict.

    I'd suggest upping the expected maximum current load, total daily Watt/hrs, battery capacity, and panel Wattage - or be prepared to use the back-up generator a lot. When you're far away from utility power and depending on electricity it is better to over-build and be safe than find your refrigerator quit at 10:00 PM and your food is spoiled at breakfast.
  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    Re: System design check PLEASE

    I'm still having problems with your figures ... I think you missed a decimal place.
    Total Daily Load (24 hr period)
    550Wh (without fridge or freezer) or 1225Wh (w/ freezer) or 1500Wh (w/ 19cf fridge/freezer)

    1500 Wh (w/ 19cf fridge/freezer) - 550 Wh (without fridge or freezer) = 950 Wh

    950 Wh / 120 volts = 7.9 amps for 1 hour ???? Assuming your fridge will run at least 6 hours a day in that heat, your fridge only draws 1.31 amps.?

    Is it a 12v fridge, or propane?

    On the controller, If I were you, (and I'm not) I would buy a good controller, hide it in my luggage and, when the BZ craps out (and it will) I would pull it out and offer it up for say 10 x's what I paid and say "I told you so!".

    PS I hope you can use a gun, that is a BAD part of the world!
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: System design check PLEASE

    For a quick and dirty calc, take the name plate rating of your PV, in your case ~492 watt, divide that number in two to account for all cumulative system loses, then multiply that number by ~4 (the average number of hours of "good" sun one can expect (almost never more than 4 on average!) This number will give you a (easy) rough number as to how much solar power you can harvest on an average day.

    492/2*4=984 wh/day (Call it 1 kwh/day) That would translate to ~83 ah out of a 12 volt battery bank.

    From your first post it looks like you will have loads in the 1.5 kwh/day range. In this case you not only have no reserve, but you are setting up the potential to deficit charge by ~50% per day from day one,,This is a recipe for killing the batteries very quickly. Either reduce your loads, increase your PV or both! In this case, double the PV might be a good place to start, but still wouldn't give much reserve.

    Two other observations. My guess is you will suffer significant performance loss due to heat where you are, and your battery life will be shorter due to warm (hot) battery temps. If this is the case, you have to build in larger reserves. If it were me in a hot climate, I wouldn't routinely draw my batteries down more than ~15-20% if I wanted then to last very long.