How do I figure out how many batteries to use?

System
System Posts: 2,511 admin
Complete new guy here, so Im looking for some advice and suggestions.
I would like to set up a small off gird system to run a couple of my appliances and reduce my reliance on the grid.
The house averaged 14kWhours per day last year and I would like to see that reduced as much as possible. (already changed all light bulbs, installed solar water heater and have good conservation practices)

My restrictions are: space for panels, I have 14ft x 14ft of roof space reserved for panels. And money.
I and am thinking about using 6 Kyocera KD185GX panels as advertised on the host site as they are less than 3$ per watt. Does anyone have any reviews on these panels?

Any suggestions for a charge controller, inverter and batteries? Also, is there a way to figure out how many batteries I should use with these panels. I just honestly have no clue about this stuff, someone help! :)

Comments

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: How do I figure out how many batteries to use?

    Why use a battery based system at all? It will come at ~twice the cost with ~ half the efficiency. Unless you have a very unstable grid, consider a expandable grid tie system. Do a search on this forum for hybrid systems, read up a bit, and then begin to work toward your solution,

    Tony

    PS Welcome to the forum,,
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: How do I figure out how many batteries to use?

    in this case tony he may have to do it that way or go with something like the enphase inverters at each pv because the string voltage on the 6 pvs he can fit will not be high enough for many straight gt inverters.

    to the op, kyocera has a good reputation. now do you really have outages that would warrant the backup batteries and other equipment, because in this case i think i'd go with something like an enphase or similar type inverter for each pv.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How do I figure out how many batteries to use?

    Working backwards, 6x185watts=1,110 watts of solar panels.

    Assuming that flooded cell batteries like between 5%-13% rate of charged (rough rules of thumb to start designing your system--certainly can be "violated" for specific requirements and other changes in assumptions):
    • 1,110 watts of panel / 0.05 = 22,200 Watt*Hours of battery (maximum)
    • 1,110 watts of panel / 0.13 = 8,538 Watt*Hours of battery (minimum)
    Assuming a 12 volt battery bank (a 24 volt bank, for example, would by 1/2 the number of Amp*Hours--still storing the same amount of energy):
    • 22,200 WH bank / 12 volts = 1,850 AH @ 12 volt
    • 8,538 WH bank / 12 volts = 712 AH @ 12 volt
    Assuming you are in Malta and have weather similar to Palermo Italy... Using PV Watts program; 1.11 kW of panels, 0.52 derating (including flooded cell battery and inverter losses) and fixed array:
    "Station Identification"
    "City:","Palermo"
    "State:","ITA"
    "Lat (deg N):", 38.18
    "Long (deg W):", 13.10
    "Elev (m): ", 34
    "Weather Data:","IWEC"

    "PV System Specifications"
    "DC Rating:"," 1.1 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 0.6 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 38.2"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 0.2 euro/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (euro)"
    1, 3.16, 53, 0.09
    2, 4.19, 65, 0.11
    3, 4.99, 83, 0.14
    4, 5.94, 95, 0.16
    5, 6.17, 100, 0.17
    6, 6.56, 101, 0.17
    7, 6.79, 107, 0.18
    8, 6.81, 106, 0.17
    9, 5.72, 87, 0.14
    10, 4.93, 80, 0.13
    11, 2.98, 46, 0.08
    12, 2.81, 46, 0.08
    "Year", 5.09, 966, 1.59
    You can run between 46-107 kWH per month worth of usable power (maximum average output--if that makes sense) from your off grid system. Or:
    • 46kW per month / 30 days = 1.53 kW per day = 1,530 WH per day
    • 107 kW per month / 30 days = 3.57 kWH per day = 3,570 WH per day
    Inverter wise--You really need to know how much power you want to use. Here is a Euro version of the Kill-a-Watt meter (great tool for plug-in loads). Even if you don't go solar, it can help you with conservation around your home.

    Kill a Watt - Energy Efficiency We have other posters here from Malta--If I recall correctly, electricity is very expensive. If you could use a Grid Tied inverters (like the Enphase) that connect from solar panels directly to your utility power -- that would be the best return on your investment (assuming you are not looking for backup power). I don't believe that Malta's power company allows net metering / grid tied systems...

    Google Search of "Malta" on this forum.

    Regarding solar charge controllers and inverters--I am not sure which ends up being better for you--Sourcing in Europe or the US...

    I can certainly point you to US sources--perhaps somebody out of your area can recommend sources out of Europe...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • VanDeusen
    VanDeusen Solar Expert Posts: 129 ✭✭✭✭✭
    Re: How do I figure out how many batteries to use?

    you are figuring out the amp hours by the amount of batteries right?

    So in otherwords, if you have 10-6v batteries running 12v and each one is rated at 325ah then that would be 5x325= 1625ah right? If that is the case then how many 125w solar panels would you need?

    Thanx
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How do I figure out how many batteries to use?
    VanDeusen wrote: »
    you are figuring out the amp hours by the amount of batteries right?

    So in otherwords, if you have 10-6v batteries running 12v and each one is rated at 325ah then that would be 5x325= 1625ah right? If that is the case then how many 125w solar panels would you need?
    Yes, but I want to make things clear.

    It depends on how you are connecting the batteries... When you connect batteries in series, their voltage adds, but the AH capacity remains the same. So two 6 volt 325 AH batteries in series become 12 volts @ 325 AH.

    If you now take that 2x 6 volt batteries and put them in parallel with 2 other 6v 325 AH batteries, the voltage remains the same, but the capacity is now 2x325 AH = 650 AH @ 12 volts.

    A rule of thumb is that we take roughly 5% to 13% of the battery bank 20 Hour Rated capacity as the charging current/energy. For solar panels+charge controllers, you can assume that they are about 77% efficient and take that into account too:
    • 1,625 AH * 14.4 volts charging * 0.13 rate of charge * 1/0.77 eff = 3,951 watts of solar panels maximum
    • 1,625 AH * 14.4 volts charging * 0.05 rate of charge * 1/0.77 eff = 1,590 watts of solar panels minimum
    Take the above numbers and divide by 125 watt rated panels:
    • 3,951 watts / 125 watts = 32.6 solar panels
    • 1,590 watts / 125 watts = 12.2 solar panels
    Now, these are rough rules of thumb and you can certainly go above or below these numbers a bit. And if you have other needs, you can go above/below to meet those needs.

    Please note that normally we size the battery bank for the load first, then do the solar panel calculations... If you have light loads, then you probably want to go for the minimum amount of solar panels. And if you have heavy daily loads, you will need a lot of solar panels to supply the needed power (or need generator/utility power to make up for undercharging by the solar panels).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset