System sizing thoughts

I'm having second thoughts on my system sizing and would like some help. I know it would have been smarter to ask for more help before hand. I'm in the final stages of construction on our large 460 sq foot house.

We have been running our genset when using lots of power tools or when our batteries get around 80% charge.

4 - 6v 820ah @ 20 hours rolls
2 - VFX3524 parallel with X240
2 - Flex max 80 with 24 evergreen 200 watt panels 4800 watts
FW1000 AC & DC
Mate Hub 10
18kw diesel genset

We have used a few different load calculators and come out between 4500 and 5000 watts. There are 2 adults and one child that will be living in the house.


  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: System sizing thoughts

    This may be the first time I've ever said this; it looks like you've got more panel than you need.

    820 Amp hours of battery @ 25% DOD = 205 Amp hours @ 24 V = 4920 Watt hours, so that's in line with your estimated usage.

    But it should only require 82 Amps of current to recharge. That's something like this:
    82 A * 28.4 V = 2328 Watts, less 77% efficiency derating: 3024 Watts.

    Ought to yield 9 kW hours per day too. Did you check power potential with PVWatts?

    Nothing wrong with extra panel, but it does cost more money.
  • System2
    System2 Posts: 6,290 admin
    Re: System sizing thoughts

    I have played with pvwatts a few times but there is nothing around around where I'm at and not sure what i'm looking at.

    Between wife, kid, and farm I'm sure something will use the extra power. I think my best day so far was around 8.3KW hours, but the 12 days they have been up we have had rain and cloudy weather.
  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    Re: System sizing thoughts

    As in any system loads must be addressed. If your In North Florida and plan to Air condition your Cabin (I'm proud of you saying 'large' for 3 people!) and want to ba able to servive the high hazy days of summer with reduced production, but plenty of heat, don't want to run your generator, I think your pretty close to perfect.

    If your in the couldy NorthWest with 240 days of drizzle, you might be under sized.

    So load and solar isolation are inprotant in working out your system.
    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, 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.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: System sizing thoughts

    He's in Virginia; should be fairly sunny most of the time. PVWatts doesn't have Roanoke? That's odd (doesn't run on my machine so I can't try it - maybe Bill will help us out here).
  • BB.
    BB. Super Moderators, Administrators Posts: 33,497 admin
    Re: System sizing thoughts

    Using PV Watts website for Roanoke VA, 0.52 system derating (assuming flooded cell and AC inverter), fixed array tilted to latitude, 4.8 kW of panels:
    "Station Identification"
    "Lat (deg N):", 37.32
    "Long (deg W):", 79.97
    "Elev (m): ", 358
    "PV System Specifications"
    "DC Rating:"," 4.8 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 2.5 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 37.3"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 8.0 cents/kWh"

    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.77, 292, 23.36
    2, 4.33, 297, 23.76
    3, 4.89, 354, 28.32
    4, 5.41, 374, 29.92
    5, 5.60, 384, 30.72
    6, 5.58, 365, 29.20
    7, 5.47, 364, 29.12
    8, 5.55, 374, 29.92
    9, 5.11, 338, 27.04
    10, 4.92, 346, 27.68
    11, 3.95, 281, 22.48
    12, 3.40, 255, 20.40
    "Year", 4.83, 4026, 322.08

    1, 3.77, 292, 23.36
    3.77 hours of "full sun" per day
    292 kWH average per Month
    $23.36 worth of power @ 8.0 cents/kWh

    You can do your own power calculation from the "hours of sun" like this:
    • 3.77 Hours * 4.8 kW array * 0.52 derating * 31 days in January = 291.7 kWH per month
    Note that PV Watts is something like a 20 year average in weather patterns and yours may vary by +/- 10% to 20% depending on your local conditions (marine layers, thunder storms can be a big issue).

    PV watts does not account for average air temperature (hot weather lower output, sub freezing higher output with MPPT type controllers).
    Usually, most places have 3 months of the year where solar output falls ... Normally, during this time you are either left with using less power or more generator run time.

    So, the "break even" 4th month for you would be around February (where you may squeak by without running your genset) based on a 4.8 kW array:
    • 297 kWH per Feb * 1/28 days per Feb = 10.6 kWH per day average
    Yes, you will get more power in the sunnier months--but never plan on being able to use 100% of the power from these calculations...

    Typically you can only store ~2-3 days of power (for a reasonable size battery bank)--So there will be days when you have more power than you can store, and other days where you used more power than available (visitors, bad weather, etc.)... Plus, your loads will probably grow somewhat over time too.

    For the other 8-9 months of the year, you should plan on using ~50-75% of the "predicted power" from above (if you don't want to use the genset).

    Anyway, nothing guaranteed here... Just how I would suggest you interpret the data (relatively conservatively).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: System sizing thoughts

    I'm north east of Roanoke up in the woods. Thank you for all the information and it gives me some more stuff to think about.