What can I do with excess panels

garynappigarynappi Registered Users Posts: 96 ✭✭
My system is minimal (12 volts 225 watts in 2 panels a single 90Ah battery) powering only exterior lighting, and water features in my yard and I have no plans on grid tie or changing system voltage. 

But, I have access to another 650 watts (12 volt) of panels and 4000/8000 watt inverter at a REALLY LOW cost. 

Even though I have a genset I'm wondering what could I power in the event I really needed to. I guess my basics would be a refrigerator and couple of 6000 BTU window AC units. So other than installing the newly acquired panels and upgrading the basic system of CC and wiring, what would I need in battery upgrades and inverter to run the AC units and side by side refrigerator 24 hours? 


  • BB.BB. Super Moderators, Administrators Posts: 31,613 admin
    Just to do a quick rule of thumb design, using the 650 Watt of panels and FLA battery bank as the starting assumptions:
    • 650 Watt array * 0.77 panel+controller derating * 1/0.10 rate of charge * 1/14.5 volts charging = 354 AH @ 12 volt battery bank nominal
    Using 5% - 13% rate of charge--And 10% as a nominal full time off grid to give you a 354 AH @ 12 volt battery bank.

    Say you use 2x 6 volt @ 200 AH "golf cart" FLA batteries in series for 12 volts time 2 parallel strings for 12v @ 400 AH battery bank (a bit less than 10% rate of charge--But you are in a sunny region).

    Assuming 25% discharge per day for 2 days to 50% discharge for normal operation and longer battery life, the daily (or overnight) battery usage would be:
    • 400 AH * 12 volts * 0.85 AC inverter eff * 1/2 days storage * 1/0.50 max discharge = 1,020 Watt*Hours per day
    That is about the energy usage of a very efficient full size refrigerator's rate of energy usage per day (1,000 to 1,500 WH per day).

    And a 400 AH @ 12 volt FLA battery bank would support a ~500 to 1,000 Watt AC inverter... roughly, for full size 120 VAC refrigerator, a ~1,200-1,500 Watt inverter is suggested... A 1,000 Watt inverter is "close".

    And the amount of energy a 650 Watt array would generate for an off grid system, south facing array, fixed tilt, Orlando FL (guess):

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 62° angle from vertical:
    (For best year-round performance)


    Using December as the worst case month, the average energy harvest would be:
    • 650 Watt array * 0.52 off grid AC system eff * 4.16 Hours of sun per day (Dec average) = 1,406 Watt*Hours per day average December harvest
    It is "close" for running a full size fridge (generally, you want more harvest/larger array for bad weather, running other small loads, etc.).

    So, as it stands, with that array, you really would have a hard time running a full size USA fridge. And you could not really even think of using a 4,000 Watt inverter on a "well balanced system"... Of course, you can make a larger battery bank (still does not change daily harvest), and/or use other battery types (AGM, LiFePO4, etc.) to support higher surge current/peak loads (running water pump, shop tools, etc.)--But does not really help with fridge/AC systems--Which need more panels, larger battery bank to really be reliable.

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • garynappigarynappi Registered Users Posts: 96 ✭✭
    edited June 2020 #3
    WOW! Thanks Bill!  My total system would have 875 watts of panels (225 existing + 650 additional) sorry, but I should have mentioned I'm fairly close to Miami and panels are unobstructed and face directly south. Oh, and the panels are / will be wall mounted and the angle is currently 62 deg but is easily changed for seasonal sunlight capture.  

    Given this, where do I stand?

  • BB.BB. Super Moderators, Administrators Posts: 31,613 admin

    My mistake--I thought you were talking about a second system, not adding to your existing system.

    The "extra" 225 Watt panels to a total of 875 Watt array--Everything would be upsized by approximatly 1.35x vs 650 calcs above. Since I already rounded up on the battery bank AH, the larger array would be ~464 AH @ 12 volts, vs the 400 AH bank above... Not much difference. And if you use Golf Cart batteries, they probably would be around 200 to 220 AH (depending on mfg.)...

    Just for the sake of discussion... How does the system look above? It is it going provide "useful" energy, or it is 1/2 or 1/4 the amount of energy you wish to generate (Fridge, A/C, etc.)? Is a 1,020 to 1,406+ Watt*Hour per day load enough?

    In general, I suggest adding solar panels to increase system output. Just adding more battery storage does not work as well and "too much" battery and "too little" solar panels, you can end up with an under charged battery bank--And shorter life...

    Also, adding more solar makes running day time loads (washing machine, A/C) somewhat easier and cheaper (overnight loads are less, so smaller battery bank could be used)...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • garynappigarynappi Registered Users Posts: 96 ✭✭
    Thanks again Bill, your advice is as always invaluable.

    Unfortunately my available "real estate" on my south facing house wall will only allow one additional 200 watt panel by my wall size estimates which is inexpensive and easy enough to add. As you said the running daytime loads will get help, as would shutting the refrigerator at night in order to keep the small AC units on.

    Also in lieu of the 4000-8000 watt inverter, I come to find that I can substitute a 5000w (running) / 10000w peak/surge model. Considering running (not starting all simultaneously) loads this one should be adequate? Also I'll likely be able to use 4 80Ah Fla deep cycle batteries.

    This is all speculation right now but since it's all so inexpensive I may take a whack at it. 
  • BB.BB. Super Moderators, Administrators Posts: 31,613 admin
    To run a refrigerator--The off grid solar power system moves from "small" to "medium" size... Add other stuff like Air Conditioning, the system is moving towards large. You loads are a lot for a relatively small system.

    What size battery bank do you think you will be running?

    Obviously, I like to size the system to the loads. However, predicting output for a set configuration (such as the solar array) and balancing the battery bank/AC inverter sizing/etc. to match the array and usage works too...

    Are you doing this to save money, or emergency backup power, or???

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • garynappigarynappi Registered Users Posts: 96 ✭✭
    Thanks Bill,

    It's definitely not to save money, nor send power to the grid. I have a genset, but down here gasoline has been historically a panic driven resource after a storm where people (understandably) buy 20-30 gallons of gas just in case. 

    That said as a purely emergency situation (likely between gasoline runs) I'd like to be able to run a couple of small window shaker AC's and / or a refrigerator, not necessarily all at once. Currently I have 4 80Ah Fla batteries and I figure I may ultimately have 6 or 7 (even more easily added no problem) with 1075 watts of panels after I add my last 200 watt panel.

    My south facing wall will be fully populated, and my east west pitched roof has barrel tiles which I do not plan on breaking into in order to support more panels "just in case".  

    I get that most go the route of higher wattage / voltage panels / CC's but my long range plan is to keep it at 12 volts 

  • BB.BB. Super Moderators, Administrators Posts: 31,613 admin
    Gary, I am afraid that you are asking a lot from a "small" solar power system... Let's try this a different way by saying how much power it can generate in an average day with 1,075 Watts of panels, facing south Miami, and you can tilt any time (seasonal tilting, not tracking):

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel where the angle is adjusted each month to get optimum sunlight.


    Assuming hurricane season is June-November, that gives you a range of 5.38 hours through 4.69 hours of sun per day. Presumably the hot summer months are when you use the most power for the A/C... Pick 5.0 hours of sun:
    • 1,075 Watt array * 0.52 off grid system eff * 5.0 hours of sun per day (ave) = 2,795 WH per day
    Let divide the power into a 12 hour cycle:
    • 2,795 WH per day / 12 hours per day usage = 233 Watt average supported load
    If you run the fridge only 1/2 a day at a time, then assume that it takes 120 Watts at 100% duty cycle (warms up at night, cools down during the day):
    • 233 Watt power - 120 Watt running fridge = 113 Watts to run "other loads"
    I am not an A/C guy, but 300 Watts is a pretty low amount of energy usage for an A/C system on low (very efficient inverter type mini-split or inverter window unit) if trying for 12 hour per day runtime. Or,
    • 113 Watts "extra power" * 12 hours = 1,356 WH per day of "extra power"
    • 1,356 WH / 300 Watt A/C load = 4.5 hours of runtime on low
    The above, very rough numbers, and the 300 Watts may be a very low estimate... Get yourself a Kill-a-Watt type meter and unplug the fridge overnight, and plug it in again the next morning and let run for 12 hours--See how many kWH (or WH) it uses in a warm kitchen...

    I know I keep harping on a genset, and you don't want one, but just to compare... A Honda euXX00 family genset will get (very roughly) 3,600 WH per gallon of gasoline. The solar harvest is 2,795 WH per day, or in gasoline:
    • 2,795 WH per day solar / 3,600 WH per gallon of gasoline Honda euXX00 = 0.78 Gallons of fuel
    • 30 gallon fuel supply / 0.78 gallons of fuel (solar harvest) = ~38.5 days worth of fuel (eqv of 1,075 Watt of solar)
    So, if you can keep 30 gallons of gasoline with fuel stabilizer, you can get ~38.5 days of solar equivalent runtime (basically what I do... ~30 gallons of stored fuel+stabilizer, change once a year into cars, always prepared (we don't have "earthquake weather", although, we were hit in urban areas with multi-day precautionary Electricity shutdowns because of high fire hazards/utility caused wild fires late in summer). Ran my in-laws home with 2x fridges and 1x freezer plus a few lights on ~2 gallons of gas per day on an eu2000i with extended runtime tank.

    And lets look at Batteries... How much do they store in gasoline units. 7x 12 volt @ 80 AH batteries or:
    • 12 volts * 560 AH * 0.85 ac inverter eff * 0.80 of battery deep cycling capacity = 4,570 WH of storage
    • 4,570 WH of battery storage / 3,600 WH per gallon of gas = 1.27 Gallons of gasoline equivalent for battery bank
    Roughly $700 of batteries to store the equivalent less than $4.00 of gasoline...

    If you use a Honda eu2200i (new model, I have never seen) or an eu3000 family, you should be able to run:
    • full size refrigerator ~ 2,000 WH per day (~120 Watt average compressor load, ~600 Watt defrost cycle)
    • 2x Small inverter A/C system ~ 2x 350 Watts = 700 Watts
    • total wattage ~ 820 Watts average; 1,300 Watts peak
    • 24 hours per day * 820 Watt average load = 19,680 WH per day (notice: A/C units use A LOT OF ENERGY)
    • 19,680 WH / 3,600 WH per gallon = 5.5 gallons per day
    • 30 gallon storage / 5.5 gallons per day = 5.5 days of runtime on emergency fuel supply
    And that is assuming you run 2x small A/C units 24 hours per day, and your average fridge 24 hours per day.

    If you only run 2x A/C 8 hours per day:
    • 2x 8 hours * 350 Watt A/C = 5,600 WH per day (A/C)
    • 1x fridge = 2,000 WH per day
    • 7,600 WH per day fridge plus some A/C / 3,600 WH per gallon = 2.11 gallons per day
    • 30 gallon storage / 2.11 gpd = 14.2 days of fuel storage
    I really do not think you would be happy with 1,075 Watt array and 7x 12 volt @ 80 AH batteries, if you want to run your fridge and 2x A/C systems. However, a eu2200i Honda should run all your loads (close). And a Honda eu3000 family would easily run + more (lights, fans, computers, etc.). And depending on how much A/C you run, 30 gallons of fuel will run you 5.5 to 14.2 days (during A/C season, a lot more if during cooler weather and fridge only).

    Your 1,075 Watt array would really only run your fridge ~24 hours per day from solar... And your battery bank itself, will run a typical 2,000 WH per day fridge from full to near dead battery bank over ~2 days (4,570 WH battery bank with no sun). Not much (if any) A/C support possible.

    The above numbers are really rough (probably accurate to the first digit)--But it does show how to do more calculations (throwing a Kill-a-Watt type meter on the fridge and A/C systems) to better profile the loads against energy production (solar/genset/etc.).


    Personally, I would really like to run my home on solar+battery, at the very least, during emergencies (was only earthquakes for 1-2 weeks, new "danger" is old utility equipment shut downs to avoid starting fires in summer)--I keep coming back to minimum loads (fridge/freezer/lights/fans/washer+gas drier+laptop+cell phone charging)... I could never justify going full off grid during random/rare emergencies (I have 3.5 kW of GT solar on roof with ~3-6+ hours of sun per day winter-summer).

    A $1,100 or $$2,400 Honda genset + 30 gallons of gas was the best option for me (Other than last year with ~3 day outage for my in-laws, the last multi day outage was close 60-70 days ago in our area of the SF Bay Area).

    If you have multiweek outages, several times a year, a different solution may be better for you. I probably would go with a "small system" for your lights, possibly a fridge. And still use fuel storage and a genset for running your A/C systems when needed.

    I promise, my last "think about generator" post for you...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • garynappigarynappi Registered Users Posts: 96 ✭✭
    edited June 2020 #9
    You missed the part where I already have a genset, it's Honda powered and 8500 watts :-) I've had power outages lasting weeks with shortages of fuel and off days where fuel was not available. Nowadays when a storm is named and heading toward south Florida I try to have 25 gallons of fuel on hand (with conservation that lasts ~6 days) but it's when fuel is unavailable the solar running will be handy. 

    Tossing the contents of a refrigerator for lack of fuel has happened more than once :-(
  • BB.BB. Super Moderators, Administrators Posts: 31,613 admin
    edited June 2020 #10
    Hi Gary,

    I understand love/hate with gensets... I was suggesting that a smaller genset better matched to your loads, and keeping your large load usage low(er) (especially the A/C), can extend your same amount of fuel almost 5x more hours. (trying not to be a generator fanatic on your thread). More or less, a typical gasoline genset fuel flow tends to flatten out to ~50% of max flow at 50% or so. Guessing that you you can run your 8.5 kW genset something like 10-12 hours per day on ~6 gallons of fuel? With a smaller genset matched to your loads, you can run 24 hours per day on 2-4 gallons of fuel (at least your minimum loads of fridge+2x A/C minisplits/inverter widow shakers). The inverter gensets run more fuel efficiently down towards 25% of rated load (at 50% or greater load, the inverter gensets are not as fuel efficient as a standard genset... I am using ~4.6 kWH per gallon for a 50%+ loaded standard genset, and around 3.6 kWH per gallon at ~25% of rated load):

    https://powerequipment.honda.com/generators/models/em6500#Specs (simple genset)
    Honda 5.5 kW -> 6.9 [email protected] rated load, 10.4 hrs. @ 1/2 load (6.2 gallon fuel tank)
    5.5 kW * 0.50 loading * 10.4 hours / 6.2 gallon tank = 4.6 kWH per gallon
    6.2 gallon tank / 10.4 hours running = 0.60 gallons per hour (50% load)

    https://powerequipment.honda.com/generators/models/eu2200i#Specs (inverter based genset)
    Honda 1.8 kW -> 3.2hr @ rated load 8.1 hrs @ 1/4 load (0.95 gallon fuel tank)
    1.8 kW * 0.25 loading * 8.1 hours / 0.95 gallon tank = 3.8 kWH per gallon (first time I have run the new model numbers)
    0.95 gallon tank / 8.1 hours = 0.12 gallons per hour (25% load)

    Your fridge+small A/C is probably closer to 25% loading on the smaller eu2200i genset runs (guessing) something like 5x+ longer runtime on the same gallon of fuel vs the larger genset. I guessed on the larger Honda (your model not made anymore?). Keep the larger 8.5 kW genset for when you need larger loads (such as miserably hot/humid days when you simply need more A/C) and as a backup for the small genset. Run your smaller loads on a smaller genset (better sized to your loads).

    And/or still go with solar to run your base loads, such as a fridge + lighting/cell phone charging/laptop/etc. to have "unlimited" runtime (not constrained by fuel availability).

    Anyway--What do you think of the ability of the 1,075 Watt array calculations. The array really only support (at best) your fridge and maybe a few hours of 1x small AC unit. And even your max battery bank only stores a couple days of fridge usage during cloudy weather (taking bank to near dead). I tried to give solar every advantage (such as changing tilt every month or so for optimum harvest).

    I am guessing, but it looks like the Marine Layer, water haze, and afternoon thunder showers (?) really limit your solar harvest (hours of sun) by 1-2 hours per day in summer. To harvest more power, you really need more sun (it is possible the solar chart is less accurate for your area), or just more array/tracking array (which does not fit your space available).

    You can try PVWATTS, which lets you estimate down to the zip code/address of harvest. For example, Miami with fixed south facing array at 26 degree array tilt from horizontal, and 48% off grid system losses (same as 52% efficiency):

    MonthSolar Radiation
    ( kWh / m2 / day )
    AC Energy
    ( kWh / Month )
    Utility Value
    ( $ / month )
    Annual5.791,022$ 105

    Or still around 2.7 to 3 kWH per day for hurricane season (and I suggest that you have to work very hard to use 100% of predicted energy harvest every day--You should plan on ~50-65% of predicted output for "base/daily required loads").

    Regarding your batteries, I suggest that ~6x series/parallel (2s x 3)p 6 volt @ 200 AH "golf cart" batteries (12 volts @ ~600 AH) may be a better fit/easier maintenance vs 7x 12v @ 80 AH batteries--Unless you can get the 12 volt batteries really cheap.

    I am not against solar, but I think you are asking for more than it can really do for you with that size of array. Of course, a Kill-a-Watt meter measuring your loads would be a big help (i.e., ~2,000 WH vs 1,000-2,500 WH per day your actual refrigerator may consume).

    I tried to be reasonably conservative in your loads--But they are really guesses on my part. And the small inverter A/C watt loads may be pure fantasy on my part (I don't have/use A/C in my home).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • garynappigarynappi Registered Users Posts: 96 ✭✭
    Yup, I get that a smaller genset is more efficient, but I also run my clothes washer / dryer, dish washer,  well pump, TV's fans two AC's and stove with it when needed. I don't like to suffer :-)  I actually can get the batteries pretty cheap. The kill a watt is a good idea, thanks.  
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