How to add a small generator to Conext SW 4024 - DC coupled system

PondWaterPondWater Registered Users Posts: 2
Hi all. 

I have inherited management of rental cabins powered by a small off-grid solar-battery system. I never got an overview of the system so I'm flying pretty blind on this. Not a solar person either, but have basic electrical knowledge. 

My question is about how to hook up a small (and affordable) portable generator to the SW 4024 inverter/charger to help keep the battery levels higher. Right now the system is struggling to meet demand and the batteries are not maintaining good charge. I suspect adding more solar capacity could help, but adding a $600 or so gen seems to be the cheaper solution given the parameters and to ensure reliability for guests regardless of weather.

System details:
Location is Maine, but system is put to bed for winter.
2 280 watt panels – insolation is not superb due to trees which are sort of out of my control. 
8 Rolls S6 GC2-HC batteries in parallel-series for 24v system.
DC coupled using a Tracer AN series MPPT controller (not sure why the installer didn't use the Conext).
The Conext SW 4024 has the Integrated DC switchgear, the integrated AC switchgear, and the System Control Panel.

Load details:
Two rental cabins used for 2-3 nights a week.
LED lights.
Greywater pump for each cabin (I think about 600 watts, a hefty hill for the water to climb).
A few wall plugs with unknown use.

As an aside, I would be interested in an affordable way to monitor energy usage and generation to get a better sense of the renters' usage compared to the generation of the panels. I suspect the panel system is undersized, but I have no clue exactly how much load the renters use. There is no wi-fi or even cell service, so I need a monitor that just logs the data for later review.

Do I need to get the Conext AGS to do this?
I want this to be all automatic for obvious reasons since its rentals. I am not thinking the generator would be intended to directly serve loads so much as to maintain battery levels when weather doesn't cooperate. So the trigger for gen to turn on would presumably be battery charge levels.

Any advice is much appreciated.


  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    Welcome to the forum PondWater,

    Sorry you have not had any replies yet... I do not know much about the details of inverter AGS integration to answer that question... However, you do need to start measuring your daily loads and solar harvest to get a handle on the system. And, some questions:
    • Are you close enough to the cabins to (for example) come by and start a genset every couple of days (as needed during bad weather/heavy usage)? My personal suggestion is to keep systems "simple". Adding genset+AGS adds complexity, more maintenance, and more ways things can go wrong. And finding a "cheap" AGS compatible genset is not usually easy (genset needs low oil, temperature, crank/no start, AC voltage/frequency, etc. alarms), AGS interface (start/stop/starter/alarms/etc.)... Noise/smoke, fuel, fire safety (many a broken fuel line has burned down a generator shed/home).
    • Do you have ability for remote monitoring (Internet/WIFI/Radio/hardwire/land line)?
    • Are you OK with adding solar panels... I will "do some math" below--But you are probably correct that you have way too few solar panels--Plus--Adding solar panels vs adding AGS+Genset can be a good answer.
    • That your cabins are only rented for 2-3 Days a week is probably a saving grace here... That gives your system 4-5 days to recharge--Not a great design, but cost effective.
    • Storing Lead Acid batteries in winter is usually a good thing--Get the batteries fully charged before putting things to bed, turn off master breakers (don't want inverter idling during winter running batteries dead), possibly adding a small vertical array (free of snow) to keep battery bank trickle charged during winter can help too.
    • Could you move array farther from trees? It is certainly doable to move/enlarge an array hundreds of feet from cabins. Solar panels need full sun... Shading and even just a few leaves/dappled shade here and there will kill array output (50% or more losses is easy with even a cord/rope/etc. shadow across array).
    Your batteries are 8x Flooded Cell Lead Acid (FLA) and 6 volts @ 200 AH (20 hour discharge rate). We use rules of thumbs here to quickly get to a "working" system design. The numbers are not carved in stone, and can be deviated from based on your needs--But they are a good way to a reliable system design.

    In your case, you have a "weekend cabin" system. And they are rentals, so it can be very difficult to do any sort of consistent energy conservation plan. For you, LED lighting and a fixed water pumping need--Works for you. Guest come in and plug in hair driers, laptops, computers, kids with games, leave lights on, etc... And the system is supplying way more energy than you may have planned.

    For off grid solar power systems--They are, by design, way over-sized for day to day usage. Yes, they will produced 2-4x more energy than you need in the middle of summer--But towards late fall/early spring (or winter--If full time), little sun/shading from hills/trees/etc.--The system is struggling to provide even the minimum amount of energy needed.

    First, for sizing the solar array based purely on battery bank size/capacity. 5% rate of charge can work for a weekend/sunny weather system. 10% Rate of charge is usually the minimum/nominal required by the battery manufacturers for FLA deep cycle batteries. 13%-20% rate of charge can be OK for more heavily used systems (>13% rate of charge, make sure the solar charger has a "remote battery temperature sensor" on the battery bank for proper/safe charging at higher charging current.
    • your bank is 400 AH @ 24 volts (4 series * 2 parallel strings of batteries)
    • your array is 2x 280 Watts = 560 Watts
    • 400 AH * 29.0 volts charging * 1/0.77 solar panel+charge controller deratings * 0.05 rate of charge = 753 Watt array minimum
    • 400 AH * 29.0 volts charging * 1/0.77 solar panel+charge controller deratings * 0.10 rate of charge = 1,506 Watt array nominal
    • 400 AH * 29.0 volts charging * 1/0.77 solar panel+charge controller deratings * 0.13 rate of charge = 1,958 Watt array "typical" cost effective maximum
    The above numbers are just stakes in the sand... And are rough numbers... Any "number" within 10% of the above is "close enough" to identical for solar--I just carry the full numbers from equation to equation so you can follow my math (and catch any errors/typos I may make :) ).

    Next--Size how much energy per day/over night would the system battery bank be able to supply (i.e., no-sun cloudy/stormy weather). Generally a good starting place is 2 days of stored energy and 50% maximum planned discharge (for longer battery life)... In your case--That would work out to 25% (1/4) of bank capacity per day (2 days = 50% discharge)--And a buffer of another 30% capacity for 50% to 20% state of charge (SoC) for maximum usage (don't take bank below 20% SoC ever--And get recharged back >75% SoC "quickly").
    • 400 AH * 24 Volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 2,040 WH per day
    • 2,040 WH per day / 2 cabins = 1,020 WH per day per cabin
    That is a relatively small amount of capacity... 500-1,000 WH per day is LED lighting, cell phone charging, limited laptop/tablet computer charging, and a little bit of water pumping (your lift pump and even running a 4,000 Watt AC inverter--are pretty heavy loads here).

    24 hour per day loads, even small ones, can be pretty significant on smaller systems. Your AC inverter has an 11 Watt load in "search mode" (off until over something like 10 Watts AC load turned on) and 29 Watts just "on" (no load/tare loss):
    • 11 Watts * 24 hours per day = 260 WH per day "search mode"
    • 260 WH / 2,040 WH = 13% battery draw per day in "search mode"
    • 29 Watts * 24 hours per day = 696 WH per day "on" no load draw
    • 696 WH / 2,040 WH = 34% battery draw per day in "on" no load mode
    In this case, just turning the inverter "off" when cabins are empty is an energy saver here... And if running inverter "on" (vs "search mode") when cabins are occupied--Draws a fair amount of power (1/3rd of your daily battery budget). Over sized AC inverters can be a big energy hog for smaller systems (not saying yours is over sized--But it is largish).

    And for a 400 AH battery bank, the recommended AC inverter maximum size would be around 2,000 Watts for a 400 AH @ 24 volt battery bank. And around 1,000-2,000 Watt AC inverter would "match" the cabilities of your battery bank "better". Your 600 Watt (?) gray water pump (one per cabin) is also a load of "concern" (could you setup drains to one pump to reduce possibility of 2x600 Watt load at same time? Or possibly use relays to allow only one pump to run at a time (of course, more chances for failure).

    Next--Sizing the solar array based on your energy needs... Assuming fixed array tilted south, Augusta Maine:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 46° angle:
    (For best year-round performance)

    Say 3.0 hours of sun "non-winter" day (long term average):
    • 2,040 WH per day (based on battery capacity) * 1/0.52 off grid AC power system eff * 1/3.0 hours of sun per day = 1,308 Watt array "Nov/Jan" break even usage
    You do not have a lot of sun anyway--And add some shading from trees (big issue with trees--They tend to grow/leaf out over time--And a good possibility that shading will grow--So you may need to address trees/shading/t  location/tree trimming at some point).

    I don't know your actual "winter shutdown" dates--Or how much shading you have--But for a rental, you probably would want to only use something like 65% to 50% of the predicted system capacity (bad weather, some shading) --- And/or plan on running a genset:
    • 1,308 Watt array "break even" / 0.65 base load "fudge factor" = 2,012  array adjusted for base load requirement and "some shading"
    Anyway--A lot here. And lots of assumptions. This will give you some ideas of what you can do to reduce loads (turn off inverter/run in search mode)... Possibly switching to 24 VDC LED lights (pretty available these days) so you can run cabin lightning without running AC inverter (save inverter losses). Even simple stuff like using "mechanical twist timers" for lighting (so people don't turn lights on then leave for day and such) can help too.

    Get a Kill-a-Watt type energy meter to better measure your actual loads:

    Or even some sort of energy monitor in cabin/at your place so that everyone is better informed about actual energy usage (you want to keep the battery bank "happy" for longer life).

    After you have gone through all the above stuff--You may still need to visit the "generator solution" and its issues (finding a smallish auto start genset, fuel storage and type--propane vs gasoline vs diesel--And even location--Put generator at "top of hill" or your residence--if applicable and run AC line to cabins/battery system).

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,028 ✭✭✭✭✭
    You can go straight in to CSW L1 with a 120vac genset. Read the manual and you do not need an AGS. Good Luck!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • PondWaterPondWater Registered Users Posts: 2
    edited July 13 #4
    I appreciate the responses.

    BB, thanks for all those thoughts. A lot to digest for sure, and I might have to revisit some of it. I have shifted gears already and am leaning towards more solar for a long term solution since I am confident there is simply not enough charge to keep the batteries happy. Also considering options to ensure the guests don't run larger loads, like putting signs on the few outlets. Then I am thinking a standard pull-start generator might be used if the bank needs a top-off during foul weather.

    Dave, I may end up using the 120v option if the gen I end up with doesn't have 240. I feel I've read conflicting info on the CSW being amendable to that though—somewhere in the manual stating minimum input voltage as 170v? Or maybe I misinterpreted. 

    I will work my way through the manual, but it's a lot for me to digest at this point. In the meantime, any quick recommendation on what modes I should be familiarizing myself with to run AC-generator input simultaneously while the solar DC inputs through the MPPT? The goal of the generator being to top off a hurting bat bank.
    It doesn't look like I'm dealing with the 'AC-Support' modes... Is it really just the 'charger settings' of the SW that I'll need to tweak for charging off the generator? And the DC/solar side will just keep trucking independently using the MPPT or that needs to be shut down?
  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    I am not the guy that can help you much with programming these guys... Load Support is part of the overall operations with these guys.

    More or less, you set the AC input to the maximum output of the genset. For example, if you are using a 2,000 Watt genset with 1,600 Watt output:
    • 1,600 watts * 0.8 derating (don't run genset at full power for hours) = 1,280 Watts
    • 1,280 Watts / 115 VAC = 11.1 amps max continuous AC output. (page 4-24 and following)

    You have the max AC breaker rating... And some inverters will only take 80% of the breaker setting (US circuit breakers may trip at 80% rated current and will trip at 100%+ rated current--eventually).

    So, you could set the breaker at 11 amps, or possibly (11/0.8= ) 14 amps. And set the "load shave" to 11 amps. And the inverter-charger will never draw more AC current than the genset is capable.

    Sometimes too, there is a maximum charging current which limits AC draw indirectly. For example, if you want 1,280 max Watts from genset:
    • 1,280 Watts * 1/29 volts charging = 44 Amps
    So you might set the maximum DC charging current to something like 40 Amps to limit AC current draw.

    But I would really pay attention to the load shave/generator support programming. That will protect the genset from overloading, and allow folks in the cabins to use their power normally (gray water pumps start/stopping, lights and cell/tablet/laptop chargers, etc.). Running the genset will be quite transparent to everyone.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MichaelKMichaelK Registered Users Posts: 215 ✭✭✭
    I have exactly the same inverter in my workshop, and I have mine wired to be charged from a generator.  Here is a pic of the unit, with the location of the AC input/output terminals.  You'll need to take off the left-side panel to access them.  See page 1-3 of the install manual.
    Here is the blowup of that area from page 1.5.
    You simply run wiring from a generator plug to the AC input terminals on the right hand side, and then run an extension cord from the socket of the generator to the wall-mounted generator plug.  Note that the generator plug is male, so that you can use a standard male/female extension cord.  Don't use a female plug, because you would need to make a male to male extension cord, which is considered very unsafe! 
    I wired mine as 240V, because of the amperage requirements.  I believe that 240VAC is the native charging configuration, so I think it best to get a 120/240V capable generator.

    Sadly, none of my pics are showing, so you need to go to the manual to check those pages.  Sorry!

    One other point.  The default charging setting I believe is 90A, which is very high, so you will need Schneider's SCP to alter the charging rate to the level appropriate for your battery.

    System 1) 15 Renogy 300w + 4 250W Astronergy panels,  Midnight 200 CC, 8 Trojan L16 bat., Schneider XW6848 NA inverter, AC-Delco 6000w gen.
    System 2) 8 YingLi 250W panels, Midnight 200CC, three 8V Rolls batteries, Schneider Conext 4024 inverter (workshop)
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