Is 1.6 to 2.0 days autonomy enough?

Thanks in advance for taking the time to help. It's much appreciated.

My case: RV where power is required, not a luxury (IT working remote). 6 180W panels (3 parallel strings of 2 panels each) work out - according to my math - to keep my bank charged with ~ 3.0+ peak sun hours. I will have backup generator(s), but prefer not to use them unless necessary.

I'm still waffling between AGM & LFP, but with the banks I have envisioned, & liberal estimates of our power use, I've figured on: 2.0 days no-sun autonomy w/ AGM (50% DoD), OR 1.6 days no-sun autonomy w/ LFP @ 80% DoD. If I push the LFP to 100% it's 2.0 days as well. But I see myself using genny before then.

Do you think I'll be happy w/ these numbers? Any problems or concerns?

Comments

  • EstragonEstragon Registered Users Posts: 4,494 ✭✭✭✭✭
    Designing for 2 days autonomy is pretty typical off-grid.

    Panel output will vary with location (altitude, haze, angle and duration of sun, etc), but we generally use 75% of STC panel rating.

    For AC loads, inverter efficiency of 85%, but can be much lower with small loads on big inverters.  Some could use a big chunk of your likely harvest (~2400wh?) Just being on all the time.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • magicbusmagicbus Registered Users Posts: 10 ✭✭
    Thanks Estragon! I used a few modifiers to account for real-world panel output. Not exactly 75% but probably pretty close. And the inverter draw (no-load & otherwise) is already factored in.
  • PhotowhitPhotowhit Solar Expert Posts: 5,575 ✭✭✭✭✭
    I  would count on less when mounted on an RV, very rarely will they be in ideal angle to the sun. The lithiums should give you a bit more efficiency when charging, may make up for somewhat less capacity. Are you experience living on a solar electric system?

    If new to living on solar electric, you might hook up a 'kill-a-watt meter and run the whole system/ live in the RV for a few days to get a feel for the accuracy of your calculations. 
    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.
  • magicbusmagicbus Registered Users Posts: 10 ✭✭
    Thank you, Photowit!

    I'm new to solar living, but have a kill-a-watt meter, and have used it to measure every appliance I own, and looked up the specific draw of every item I don't. There does not exist any electrical device I'm estimating the consumption of. The use of, yes, I'm definitely estimating. But I've done so liberally. I hope my estimates will be on the high side, as that was my intention.

    I'm in the process of constructing this RV (bus) now, so unfortunately, I don't have the luxury of living in it to determine how things work in real life. It's a very small bus as well, where space is critical and places for batteries to go - especially if they're lead - are few & far between. So I'm trying to strike the balance of enough battery to get the job done, but not much more.

    The panels are able to tilt, and are far enough apart to not shade each other in the process. But I get where you're coming from. Most of the times I should have the flexibility to align so I'm facing S, but I'm sure there will be times when that's not true.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    The down side of RV solar is that under most circumstances people prefer to park in the shade to reduce A/C load, which effectively reduces the PV capacity. I agree that most RV installations don’t perform near STC on balance.  I would consider 50% a very good number.

    the “autonomy” number is entirely personal, and determinate about how much you are willing to run a genny, plug into shore power, or drive. Personally, in my travel van, 1 day is plenty since I drive nearly every day.  In my off grid house, I shot for 3 days with NO sun and we almost never have that, which sort of leaves ~a week of weak sun.  Come October/November I may have to run a genny weekly for ~4 hours, by January, when I have snow on the ground usually only need once every couple of weeks, the rest of the year, almost never.

    Tony

  • Marc KurthMarc Kurth Solar Expert Posts: 844 ✭✭✭✭
    Photowhit said:
    I  would count on less when mounted on an RV, very rarely will they be in ideal angle to the sun. The lithiums should give you a bit more efficiency when charging, may make up for somewhat less capacity. Are you experience living on a solar electric system?

    If new to living on solar electric, you might hook up a 'kill-a-watt meter and run the whole system/ live in the RV for a few days to get a feel for the accuracy of your calculations. 

    YES to what Photowhit said!
    And to answer your question:NO, unless you are willing to run a generator. (Which is normal) 
    I always have more questions than answers. That's the nature of life.
  • magicbusmagicbus Registered Users Posts: 10 ✭✭
    Very much appreciate the advice, folks! Thank you so much. I am willing to run a generator. I just wish it to be the exception, & not the norm. And if push comes to shove I can always go somewhere I can plug in, but that's the same scenario. Most of the time I'd hope not too. I'll try to sneak more AH in if I can. Might have to go LPF after all for that reason alone. I'm using as many panels as will fit, so I'm done there., ,
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    magicbus said:
    Very much appreciate the advice, folks! Thank you so much. I am willing to run a generator. I just wish it to be the exception, & not the norm. And if push comes to shove I can always go somewhere I can plug in, but that's the same scenario. Most of the time I'd hope not too. I'll try to sneak more AH in if I can. Might have to go LPF after all for that reason alone. I'm using as many panels as will fit, so I'm done there., ,
    If weight/volume is an issue, lean towards LiFePO4.  Make your array removable so you can put it in the sun when you are somewhere for a long time.  Definitely use an MPPT controller.  A good battery monitor (with accumulator) will help you understand what's happening to your system.
  • BB.BB. Super Moderators, Administrators Posts: 31,680 admin
    edited November 2019 #10
    It does all come down to your loads and sun... Just to put some numbers out:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Scottsdale
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 56° angle from vertical:
    (For best year-round performance)

    JanFebMarAprMayJun
    4.95
     
    5.44
     
    6.48
     
    7.11
     
    7.03
     
    6.77
     
    JulAugSepOctNovDec
    6.21
     
    6.01
     
    6.40
     
    6.03
     
    5.36
     
    4.79
     

    So, 3 hours a day of sun sounds a bit low--Winter is obviously worse for most people. For the above example, lets use 4.79 hours of sun minimum (long term average) for December:
    • 6 * 180 Watt panels * 0.52 typicall off grid AC system eff * 4.79 hours of sun (December average) = 2,690 WH per ave Dec day
    Now, I try to separate between "base loads" (ones that you need all the time) and optional loads (watching TV, fan, etc.--Whatever is "optional" in a small RV) and run 50% or 65% of the daily predicted loads. Lets just use 65% for now:
    • 2,690 WH per day * 0.65 base load fudge factor = 1,749 WH per day base load (you make up with genset if more power needed during bad weather)
    Assume you are a hard working AT guy and need 12 hours per day of power (8-10 hours per day, on-call emergency, a little web browsing/reading):
    • 1,749 WH per day "base" * 1/12 hours per day runtime = 146 Watt average load
    A lap uses something like 30-60 Watts, leaves the rest for printer, networking, LED lighting, radio, or even 2nd laptop.

    So, sounds doable even in winter (guessing your location via IP address). Of course, moving in an RV -- May not pick such sunny climates down the road.

    But does show that running a large desktop computer + screen + networking + heavy printing usage with a laser printer--May be diffult to do (something like 300 Watts).

    Setting up laptop for lower power, auto sleep when not used, etc. all are important.

    Anyway--Just an example of the math and assumptions... Using a small/fuel efficient genset like a Honda eu900i (900 Watt genset, ~720 Watt recommended continuous load--80% of rated capacity). You will get, very roughly 3,000 to 3,500 WH per gallon of fuel of fuel (with reasonable electrical loads).

    Another neat thing about the Honda eu1000i (and others in the eu family) is that you can get a replacement fuel cap and connect a fuel hose that can be dropped into a fuel jug or connected to an outboard tank. There is a small fuel pump in the Honda, and with full genset tank and hose, it can draw fuel directly from a larger fuel tank (extended runtime). Can make those genset days a lot less frustrating.

    https://www.amazon.com/s?k=eu1000i+extended+run

    I got the magnetic tipped oil plug... Worked great on my eu2000i gensets (now old, keep pickled for emergency power needs). I don't know if it will fit the eu1000i or not.

    With a Honda eu2000i, you can run a 20 to 40 Amp @ 12 volt battery charge maximum (depends on the exact charger, also altitude of genset).

    A lot of the food trucks in our area use a Honda eu3000i--But that is probably much bigger than you need or will want to pack around. The eu2200i should be a nice option too, if you need more power (but more fuel, larger/heavier genset, etc.).

    Regarding the numbers above, they are pure guesses... And may or may not reflect your needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • magicbusmagicbus Registered Users Posts: 10 ✭✭
    Really appreciate all the time & effort taken to type that response, Bill. Thank you so much.

    I've already created an extremely detailed spreadsheet which is the culmination of about 3 months of daily rumination, calculations, & figuring. Everything, to my best estimation, is overestimated on purpose. Worst case, I'm expecting to need 2378 Wh/day. When I go through & zero out all the non-essentials, I'm down to 1882 (maybe less if I don't need a cell phone booster). Your 'pure guesses' are spooky accurate! :o:wink:

    Yeah, I'm obviously using WAY more optimistic numbers for the panel array than you & others suggest. I just looked through my calcs & it appears I'm using 90% of the rated wattage. Obviously way high. Revising that now.

    I was planning on 2 parallel-capable inverter generators, but while I love Honda, they're a bit rich for my blood.
  • BB.BB. Super Moderators, Administrators Posts: 31,680 admin
    Glad it was somewhat helpful for your needs (I really like to work with "real numbers" for a poster's needs, rather than just give the same canned answer or two. Saves lots of hand waving and going back and forth about things that "don't fit the situation".

    Regarding the gensets... Inverter generators should be operating at something like a minimum of 25-50% of rated capacity for best fuel consumption.

    Non-inverter generators (gasoline), more or less their minimum fuel flow is pretty much at 50% of rated power. When, for example running at 25% of rated load, those gensets tend to still draw 50% fuel flow.

    Keep your loads and support equipment (like gensets) relatively small... Oversizing will simply just be a waste of money, space, and weight (and fuel).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.