Need help with PV watts #

D_A_ND_A_N New UserPosts: 8Registered Users
What would the derating number be for this setup:

24 volt panels
sunsaver mppt charge controller
12 volt battery bank

Would be for 12 volt lighting, so no AC inverter in the equation.

thanks in advance



  • BB.BB. Just some guy Posts: 23,755Super Moderators admin
    Re: Need help with PV watts #

    0.77 for panel + charge controller derating
    0.80 for flooded cell battery derating
    0.90 for AGM battery derating

    So, for your system, with flooded cell:
    • 0.77*0.80 = 0.62
    Close enough for a first cut.

    As an example using PV Watts, for Detroit MI, 1,000 watts of panels (if you have 240 watts just simply multiply the numbers by 0.24 for that size array), fixed array titled to latitude:
    "Station Identification"
    "Lat (deg N):", 42.42
    "Long (deg W):", 83.02
    "Elev (m): ", 191
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.620"
    "AC Rating:"," 0.6 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 42.2"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 8.3 cents/kWh"

    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 2.90, 58, 4.81
    2, 3.59, 64, 5.31
    3, 4.13, 78, 6.47
    4, 4.84, 87, 7.22
    5, 5.52, 98, 8.13
    6, 5.58, 93, 7.72
    7, 5.42, 91, 7.55
    8, 5.48, 95, 7.88
    9, 5.18, 89, 7.39
    10, 3.96, 73, 6.06
    11, 2.59, 46, 3.82
    12, 2.15, 41, 3.40
    "Year", 4.28, 913, 75.78

    You would collect ~41-98 kWHR per month:
    • 41,000 watt*hours * 1/30 days per month = 1,367 WH per day in December (long term average)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jcgee88jcgee88 Young at heart Posts: 154Solar Expert ✭✭✭✭
    Re: Need help with PV watts #
    D_A_N wrote: »
    What would the derating number...

    "Perfection" from a practical standpoint is 80% for PV-Watts.

    PV-Watts has a default derating of 77%, which you can see
    replicated in Bill's post.

    If you are not facing due south and/or have shading issues,
    you will want to further reduce the 77% number. For example,
    in my case, I was 30% off due south, have some early
    morning and late evening shading from trees, and one of my
    strings is tilt angled non-optimally (to minimize wind load).
    Because of these factors, my contractor used 72% on his
    initial PV-Watts simulations.

    In my case, under perfect conditions, I have achieved 83%
    for tens of minutes, and typically achieve 75-76% for the
    peak 2-3 hours under excellent conditions.

  • D_A_ND_A_N New User Posts: 8Registered Users
    Re: Need help with PV watts #

    BB, another question, or ten :-)

    This would be for 12 volt led lighting that I want the full output on the shortest days of the year. I calculated the loads at 225 watt hours per day, future expansion may go to 280, so I used 300 whr in my guesstimates.

    Panels would be at 30 degrees incl. pointed due west 270 deg.

    "PVWATTS v.2: AC Energy and Cost Savings"

    "Station Identification"
    "Cell ID:","0251366"
    "Lat (deg N):", 42.58
    "Long (deg W):", -82.88
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.620"
    "AC Rating:"," 0.6 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 30.0"
    "Array Azimuth:","270.0"

    "Energy Specifications"
    "Cost of Electricity:"," 8.6 cents/kWh"

    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 1.60, 28, 2.40
    2, 2.41, 40, 3.44
    3, 3.40, 63, 5.41
    4, 4.45, 78, 6.70
    5, 5.30, 93, 7.99
    6, 5.84, 97, 8.33
    7, 5.70, 96, 8.24
    8, 4.91, 84, 7.21
    9, 3.98, 66, 5.67
    10, 2.58, 44, 3.78
    11, 1.59, 24, 2.06
    12, 1.26, 20, 1.72
    "Year", 3.59, 734, 63.04

    My bad math skills tell me I need 400 watts of PV for December. What do I do with the extra Kw that I produce for five months of the year? If it's not used up in additional loads will the chargers cook the batteries?
  • BB.BB. Just some guy Posts: 23,755Super Moderators admin
    Re: Need help with PV watts #

    300 watt*hours per day, 20 kWH per month in December per 1kW of panels:
    • 300 watts*hours * 30 days * 1/ 20,000 WH per 1kW of panels = 0.45 kW = 450 watts of panels
    Of course, that is for 20 year average weather--And even then, there may be days/week or more of "bad weather" where you will either have to turn off the lights (to protect the battery) or have an alternate power source (generator, extension cord, etc.).

    With Solar Panels, you can turn them on and off without damage. So, basically, when the charge controller needs less than full output from the solar panels during the summer months, it will simply turn the panels off (actually cycle them on and off hundreds of times a second).

    With other power sources--such as Wind and Water Turbines, they need a constant load or they will over-speed (literally, the turbines can tear themselves from too much RPM). In those cases, typically an electric resistance heater is connected to the battery bank (or turbine output) to dump excess energy to keep turbine RPM's stable.

    In your case, you will simply have more energy available in summer (~4x as much) as you have in winter...

    One of the issues with solar--you need more energy for lighting in the winter and have little sun to power it. Forcing you to either install a whole bunch of solar panels or use a fuel driven backup genset.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • D_A_ND_A_N New User Posts: 8Registered Users
    Re: Need help with PV watts #

    What combination of panels / controllers / battery would you suggest?

    Will be 50 feet from panels to charger and then 4 feet to batteries.

    I originally figured 200 watts @ 24 v into mppt to 2 T105's, but you've shown I need more PV. Looking at available panels it is hard to hit 450 watts.

    Looking at more charge controllers I found the Morningstar SunLight that has built in lighting timer. It would simplify my project so I want to use one, but it would need to be paralleled with another charger.
  • icarusicarus Super Moderator Posts: 5,070Super Moderators
    Re: Need help with PV watts #


    We live with 400 watts of PV, into 4 t-105s, ~450 ah of battery (12 vdc) We use, on average ~6-800 wh day. On an average day we generate just about that much, on an ideal day, with cool pv temps and lots of reflection off of snow, we can get between 1-1.5 kwh/day. So in your case, it sounds like ~200 watts of panel might be in the ball park.

  • BB.BB. Just some guy Posts: 23,755Super Moderators admin
    Re: Need help with PV watts #

    Except Dan needs full solar for December--so the reason for more panels is the winter requirements (little sun, long nights).

    Do you want the the lighting to be on from dusk + X hours, or Dusk to Dawn, or programmable by time/day?

    You may be correct--your best bet is a small solar panel + a MorningStar Sunlight solar lighting controller 10-amp 12 volt (or other voltage/current that meets your needs). The one above from our host's site says a minimum of 1 watt solar panel to activate the timers.

    I believe you will still need to put some sort of (not too big) solar panel on the Sunlight controller so it knows when to turn the lighting on and off.

    The optimum current rating for a MPPT type charge controller would be:
    • 450 watts * 1/14.4 volts * 0.77 = ~24 amps
    So a 30 amp charge controller (12 volt bank, or 15 amp for a 24 volt bank) would seem to be a good choice.

    The Morning Star 15 amp MPPT controller would be nice for a 24 volt battery system. The Rogue 30 amp MPPT would be nice for either 12 or 24 volt.

    Using a generic voltage drop calculator, Say we want 30 amps at 17.5 volts and around 1-3% voltage drop. And you want 50 foot one way run:
    • 17.5 volts * 1% = 0.175 volt drop rough min.
    • 17.5 volts * 3% = 0.525 volt drop max.
    • 50 feet of 1 awg copper cable and 30 amps will give you 0.45 volt drop
    Obviously, that is very a expensive solution. But, since we are using MPPT type charge controllers here--We can use a higher voltage array. So lets use 2 panels in series for 35 volt drop:
    • 35volts * 3% = 1.05 volt drop max.
    • 50 feet of 8 awg copper cable and 15 amps will give you 1.14 volt drop
    So, doubling the voltage and 1/2 the current allowed us to use 8 awg wire for the 50 foot run. That is probably the maximum input voltage for the Rogue. The Morning Star 15 amp MPPT with 75 VDC maximum input might be able to take three panels in parallel (and reduce wire gauge again--but you if stick with 12 volt battery bank, you would still need 2x MS 15amp MPPT controllers).

    However, you can look at a Morningstar TriStar 45 amp MPPT solar charge controller (this is not cheap--showing you how to "play the game" with Vmp and controller changes and how it affects your wiring options).

    You can run this at 150 VDC maximum input voltage which works out to a maximum Vmp of ~100 VDC. Or 5x 17.5 Vmp panels in series (Vmp=5x17.5=87.5volts).
    • 24 amps / 5 series panels (instead of 5 parallel panels) = 4.8 amps nominal
    • 87.5 volts * 3% = 2.625 volt drop max.
    • 50 feet of 16 awg copper cable and 4.8 amps will give you 2.34 volt drop
    So, there are lots of options that can reduce the wiring costs, but at the increased expense of using higher end MPPT charge controllers.

    You also have to look at how to series/parallel the array (with XXX watt, YY Vmp panels) to get something that meets all of your needs and cost target.

    Also, perhaps in the summer you can use the excess power to run a small water feature/fish pond aerator/water pump.

    When looking for solar panels, 100+ watt panels tend to be less expensive--So whatever array/panel mix you end up with, take a look at the $$$ per Watt and make sure that a smaller array (made with smaller panels) does not ending up costing you more money.

    Also, something else to think about--24 volt LED strings... This allows you to use smaller (and cheaper) charge controllers (15 amp @ 24 volt will work just fine). And, if you have to string your landscape lighting long distances--running them at 24 VDC (or even 48 VDC) will reduce your voltage drop/wire gauges for wiring them up too.

    Lots of options here--Sorry to be so confusing, but I hope this helps you narrow down the choices that are correct for your application.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hillbillyhillbilly New User Posts: 334Solar Expert ✭✭✭✭✭
    Re: Need help with PV watts #

    A few points:
    Is there any way that you can change the array orientation to face south?
    Why use a 30 degree tilt angle? If you want the maximum energy during the winter months a steeper angle will yield more total kwh (in the summer you'll get less, but most likely you'll still have enough as the long days still produce far more solar energy than the short winter days).

    Try crunching the numbers on pvwatts while tweeking with the array tilt and orientation numbers. Could save you some money.
  • icarusicarus Super Moderator Posts: 5,070Super Moderators
    Re: Need help with PV watts #

    HB makes a good point. A small array of a couple hundred watts is pretty easy to install with an adjustable tilt. Failing that, optimize the tilt for December. Also, consider how many days of autonomy you will need, and take a good look at your average (very localized) December weather. Do you generally have crisp clear days 3 days out of 5? do you generally have fog, flurries and grey 6 days out of ten etc?

    As I suggested in my earlier post, I get much better harvest on an hourly basis in the winter. Additionally because of my odd panel orientation I have fairly short soalr days both winter and summer, so my winter number of hours is not much shorter (on the panels) than mid summer. Cold weather and especially reflection off of snow will dramatically affect over all harvest. I am blessed that my panels look out over a vast expanse of water. If that water is open, I get reflection off the water, if it be frozen (from November to May) I get even better reflection off the snow that sits on the ice.

    Good luck and keep in touch,

  • D_A_ND_A_N New User Posts: 8Registered Users
    Re: Need help with PV watts #

    The 30 degree tilt is the angle of the roof. Due west or east is the only non shaded areas of the roof. I am very close to a lake to the east and regularly have hazy mornings, so I chose the west side. A ground mount is out of the question, it would face south, but would be totally public and sadly I think it would get stolen.

    I'm just outside Detroit, so December is December, sometimes snow or clear or cloudy or rainy and some days all of it. I want 3 days of 'power', another plus of using the SunLight I figure is that it would be easy to shorten up the lighting if the battery was low and the weather uncooperative.

    To another question: I was originally thinking dusk plus 8 hours in December. I was going to use a timer but with the timing options of the SunLight am now thinking dusk plus 6 and dawn minus 2.
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