Solar + MPPT Charge Controller + Generator for night. Double check my setup idea?

dmurphdmurph Registered Users Posts: 12 ✭✭


I'm trying to design a power setup that can:

  1. Have Solar which charge batteries, when then power an inverter -> AC power
  2. Have a generator I can turn on to charge the batteries more during the night if they can't last
  3. Have this be as foolproof / maintenance-free as possible (no re-wiring or switching if possible)
  4. Power a constant ~400-500w/h
  5. Basically a double-conversion UPC with solar.

If you know ways to do this efficiently easily, definitely let me know. Below is what my assumptions are, then my current plan, and I would love input on mistakes I've made (bad assumptions) and ways to complete my setup.

After reading a lot of posts here, I've gathered:

  1. I can't just run the DC from my (Honda eu2200i) generator to the solar panel inputs on an MPPT controller, it'll blow the fuse on the geny
  2. I can't run the Honda on Eco-Mode during DC production
  3. If I do the DC (12V 8.3A) from the Honda, I'll have to use a step-up converter to convert it to the correct voltages / amps for my battery bank (ideally only do the bulk charge), then connect it to the batteries in parallel w/ my charge controller.
  4. If I do AC from the Honda, then I can have it in Eco-Mode, and I can use a simpler AC battery charger (again, maybe only bulk mode so I don't waist gas on the absorb mode), and again in parallel w/ the mppt charge controller. AND this shouldn't blow the generator - hopefully the AC charger will have max amps & can shut off?
  5. I basically can't connect the honda to the mppt charge controller directly.
  6. For all of these setups, I can be using the 'load' terminals from the mppt charge controller just fine (even though I maybe be charging from the geny -> AC charger -> batteries

Assuming all of those are correct, I'm guessing my setup would be as follows:

  1. Solar Panels, in series, where the voltage is appropriate for the mppt controller and and the batteries (12V batteries, so >18V from the panels). Calculate # based on watts, and if I can recharge the batteries during the day during the additional load (extra to be safe)..
  2. MPPT charge controller that can handle the amps I need to output (probably 40 or 60).
  3. Lead-acid batteries that last me all or most of the night.
  4. A decent 1000w inverter (for my power load)
  5. A decent AC charger (unknown exactly what to get here) for generator -> batteries
  6. Wiring would be: panels -> mppt -> inverter -> load, geny -> AC charger -> batteries, batteries <-> mppt


  1. Does this sound right?
  2. What might go wrong?
  3. Any product suggestions?
  4. Is it alright that have the load output from the mppt controller, even if solar is generating no power & I'm charging the batteries from the generator?

After finishing this project I'm planning on documenting how I did it for others... definitely a steep learning curve here. I can share my planning doc as well.


  • mike95490mike95490 Solar Expert Posts: 8,613 ✭✭✭✭✭

    > Power a constant ~400-500w/h

    So, that's going to be 12,000 wh daily. Are you sure that is the right number ? it's really large.

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • dmurphdmurph Registered Users Posts: 12 ✭✭

    It's the worst case scenario. Hopefully I can get it to something like 100-200w/h, especially at night.

  • dmurphdmurph Registered Users Posts: 12 ✭✭

    Fore more context, this is for a Burning Man camp. Power is for a 3.5 cuft freezer, small speaker, led lights, and some personal chargers.

    Freezer compressor takes 1.1 to 3amps when it's on... Worst case is 100% duty cycle, although I'm planning on adding a bunch of insulation.

  • dmurphdmurph Registered Users Posts: 12 ✭✭

    Seems like maybe j buy this and hook it up to the batteries directly. Then, when the generator is off the solar battery system will power the inverter, and when the geny is on, it'll just pull from that & charge the batteries is they need it.

  • mcgivormcgivor Solar Expert Posts: 3,274 ✭✭✭✭✭

    Be careful when adding insulation, often the skin metal of the freezer is its condenser, adding insulation in such a cese will result in self destination. The way to determine what it is, is to check if there is a radiator outside the unit itself, or feel the sides for heat whilst operating, if hot don't insulate.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • mcgivormcgivor Solar Expert Posts: 3,274 ✭✭✭✭✭

    Before purchasing anything I would highly suggest a load calculation, once the load values are established then move on to the battery requiments, after which the solar required to not only support the loads during the day, but also to replenish what was taken out the previous night.

    All these calculations are dependent not only on electrical requirements, but also seasonal changes, it needs to function in the worst case scenario. The choice of nominal voltage is important to the equation, 12V should only be used with small systems, a freezer moves it into a higher bracket which is better suited to 24V or 48V.

    What seems possible is often grossly underestimated by a beginner, ultimately ending in dissapointment, best advice is to pause and rewind to the beginning. Starting with loads, accurate calculations, then proceed, doing it any other way is a recipe for disaster.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • BB.BB. Super Moderators, Administrators Posts: 30,052 admin

    To be honest, a solar power system for 2 week project with backup genset and such to run a refrigerator and some other loads--A "reliable" system for such loads would be a "medium" sized system for an energy efficient off grid home.

    The more optimal option would be (just a suggestion), design a battery bank (and solar, if you want) system that runs your small loads (lighting, smaller power system, tablet computer, cell phone charging) overnight ("quiet time") and run the genset during the day to run the fridge (have you thought about a chest freezer, setup to run at refrigerator temperatures--if you don't need ice/freezing--Or two small chest fridge+freezer). You would only need to run the genset about 1/4 time (for fridge) like 3 hours in the morning and 3 hours in the afternoon.

    A small genset (like the Honda euX000i series) sized to your loads would be fuel efficient (maybe 1-2 gallons of gas per day, propane conversion if you have lots of propane is possible) or even a propane power refrigerator/freezer (some folks here have got theirs from RV wrecking yards).

    Roughly, 500-1,000 WH per day for your solar battery system to power your lights and sound is a small solar system.

    To run a refrigerator freezer full on solar in that kind of environment (potentially hot weather, open/closing all the time/putting in new drinks to cool, making ice, etc.)--You could be looking at a 100% duty cycle or something like 120 Watts * 24 hours per day = 2,880 WH per day.

    That, reliably solar, would be a ~640 AH @ 24 volt battery bank, and a 1,200 to 2,400 Watt solar array (support 2-3 days of cloudy weather before genset needs to be started). (very conservative system design for full time off grid living).

    Autostart genset--Can be done, but the issues behind that can be a good size project by itself (genset high temp, low oil, no start, run out fuel, battery state of charge start/stop, generator warm up/cool down, etc.). A manual start genset would be much less complications.

    If you are looking at 2 week a year operation--I would be looking at genset and/or propane refrigerator/chest freezer for heavy loads. And a small battery bank (plus solar if you like) for overnight/quiet time operation.

    If you plan on moving the power installation to your second home/off grid cabin, then spending the time and money on a more complex system would make more sense.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dmurphdmurph Registered Users Posts: 12 ✭✭

    That is really helpful, thank you! Yeah - having separate systems seems cheaper and more reliable. My last full-system attempt was to have both a solar charge controller and an inverter charger connected to my battery bank, and hope that they don't cause problems with each other if they are both charging the batteries (is that a thing?)

    But non-charging inverters are cheaper :/

  • dmurphdmurph Registered Users Posts: 12 ✭✭

    Actually - check this out!!! If I'm reading the spec sheet right, this might handle everything I need.

  • BB.BB. Super Moderators, Administrators Posts: 30,052 admin

    Inverter-Chargers (both AC input, and those that may have DC solar input)--They are not bad, and the good ones are actually very capable. The high end ones can even invert/charge when the genset is running, and can use the battery bank to (short term) boost the genset output (say you have a 3 kWatt load and a 1,600 Watt genset, the inverter-charger with generator support, will actually support the 3 kW load with 1.6 kW from the genest, and 1.3 kW from the battery bank). Once the load drops back below 1.6 kW, the inverter-charger will use the "available extra genset" generator energy and start recharging the battery bank.

    And using several charge controllers on one battery bank is not a big issue itself. You can have a solar panel charger and a genset inverter-charger and/or AC to DC charger connected to the same battery bank.

    What does get "interesting" is to automate all of this (start the genset when battery gets low, turn off the genset when battery is full/work with solar charger, etc.). Automating all of that and automating genset start/stop/safe shutdown/etc. can take a while to get where you have a "fully automatic" system that does not fail when you are not there (i.e., cranking the genset and running the start battery dead while you have an empty fuel tank, etc.).

    In general, having a manual start/stop genset while a human is monitoring the system (and amount of solar harvest) usually works a bit better. And you don't want a still air day and running a genset getting a carbon monoxide in the RV/Living area, or drifting over to the next camper.

    There are some very nice RV gensets that may do what you need:

    This just takes time (and usually money) to get what you are asking for (a fully automated solar power battery system plus genset). Many RVs just don't have the roof space to run a full-out "home" off grid solar power system (space for solar panels and battery bank).

    Propane and gasoline with a small genset and propane or electric refrigerator are just more compact and "energy capable" if you only need 1-2 weeks of dry camping.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dmurphdmurph Registered Users Posts: 12 ✭✭

    That's actually great news, I didn't know I could use an I/C and a solar charger at on the same batteries at the same time. That means I know how to do everything now!

    Actually, for us turning the generator on / off is preferably a human task - it help incentivize our camp to save power & wipe off the panels (no one wants to get up & turn on the generator).

    Now I just have to price everything out. Is there anything I shouldn't skimp on? I guess, between the I/C or the solar controller? I think I can rent batteries, thankfully.

  • BB.BB. Super Moderators, Administrators Posts: 30,052 admin

    That is something you are going to have to figure out the old cost/benefit questions.

    AIMS--Those are usually "lower on the list" of suggested vendors. Some of the less expensive inverters can have 40+ Watts just being turned (people usually choose "big" inverters because inverters are cheap and they think they will power "everything").

    The reality with off grid power, conservation is key to a cost effective system. It is almost always cheaper to conserve energy than to generate power.

    For an RV (excluding a refrigerator), I would be suggesting a 300 Watt TSW (true sine wave) 12 VDC AC inverter that draws ~6 Watts being "on", and much less on "search mode" (inverter stays mostly off until you have over a ~8 Watt 120 AC load).

    There are several methods to design an off grid power system.

    Best, pick loads that are very energy efficient (leave the desktop computer at home, get a small laptop or tablet computer instead).

    Then add the loads up (use a Kill-a-Watt type meter to measure your AC loads. And use a DC Amp*Hour/Watt*Hour meter for your DC loads and figure how much power you need:

    Or... For example: Figure out how many 6 volt @ 200 Amp*Hour batteries you can fit in the vehicle (in 12 volt pairs), and then figure out how much energy they will supply and how many solar panels will be needed to "keep them happy". And figure out if that amount of energy (AH or Watt*Hours per day) is "useful" for your needs (cost effective, space/weight, etc.).

    It is like deciding if a VW beetle, or a Mac Truck+Semi Trailers will meet your needs. They both "work", but only one of them will support your expectations (for a reasonable cost).

    I suggest you do pure paper designs of your solar power system vs your loads--And get that right (battery bank, solar panels, AC inverter sizing).

    Once you have the sizing approximately "right", then you can start picking the hardware (cost vs quality vs functions) (you can only "optimize two of those choices) and run through the specific design issues and costs. Rinse, Repeat.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dmurphdmurph Registered Users Posts: 12 ✭✭
    edited March 2019 #15

    Great! This is all really helpful information, thank you!

    I made a spreadsheet of our usages:

    The first page is all of the totals & configuration possibilities. The second page has the list of power needs (stuff like the LED lights will become more accurate when I get them out of storage & test them).

    My generator is a Honda EU2200i, it's rated for 18.3A/2200W, but I would expect probably 14A max continuous.

    Now it's just choosing a solar controller & inverter charger. So I believe my TODOs are:

    • Choose my MPPT controller amps, by calculate the maximum amps coming from the solar panels.... Or the maximum charging amps? I see good vendors & reviews here, so I'm reasonably confident choosing a controller
    • Choose my Inverter/Charger controller by efficiency, supply watts (probably don't need more than 1000), and charger input amp draw? (can't go above probably 10-14A). Or maybe I calculate that from the converted amps of the charger output?
    • Calculate my wire guage sizes at every step
    • I would LOVE to get bifacial & flexible solar panels that aren't super expensive. Then we could use them for shade! Hopefully I can go in on a bulk order that other camps are getting.

    I'm least confident about the I/C selection, I'm assuming I can google for high efficiency ones.

  • BB.BB. Super Moderators, Administrators Posts: 30,052 admin
    edited March 2019 #16


    I have seen your post--But I will have to take some time to reply...

    Some basics... Watts is a Rate (like miles per hour). Watt*Hours is an amount (like miles driven). Similar for Amps and Amp*Hours:

    • 10 Watts * 6 hours per night (LED light) = 60 Watt*Hours per day
    • 0.5 amps * 6 hours = 3 Amp*Hours (assume 12 volts, but amps/ah is a partial unit, need voltage too)
    • 0.5 amps * 120 Volts = 60 Watt
    • 0.5 amps * 12 volts = 6 Watts

    So, if using Amp*Hours, you need to have the voltage noted. 1 amp at 120 volts is 10x more power than 1 amp at 12 volts. (fix typo--Is 120 VAC, not 12 VAC -Bill)

    Or, conversely, 1 amp * 5 hours = 5 AH at 120 volts would be 10 amps * 5 hours = 60 AH at 12 volts for the same energy:

    • 1 amp * 5 hours * 120 volts = 10 amps * 5 hours * 12 volts = 600 Watt*Hours

    Overall losses for solar... If you want 3,300 WH per day, then the solar array for "break even" would be:

    • 3,300 Watt*Hour daily load * 1/0.52 off grid system eff * 1/6.5 hours of sun per day = 976 Watt array "break even"

    The 6.5 hours of sun per day is a lot (desert in California, you may get that). For a "reliable system, you do not plan on using 100% of your harvest every day (some days better sun, some days you may use more energy). Using 65% of "predicted power" as your base load is usually safer.

    And this is really designing the system "backwards"... I suggest loads drive your battery bank which is the "heart" of your system. Your system has to be able to run 100% from your battery bank for 1-2 days and not take the battery bank "dead" (that will kill your batteries).

    For example, a "conservative" design would be 2 days storage and 50% discharge (longer battery life) would be:

    • 3,300 WH per day * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 647 AH @ 24 volt battery bank

    An a 10% rate of charge (for full time off grid system suggested minimum rate of charge):

    • 647 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,437 Watt nominal array

    Notice that you have lots of sun and a 5% rate of charge is still a ~1,200 Watt array and you "needed" a 976 Watt array to "break even".

    Because of various issues, a full time off grid system (that uses significant power during the day, and allowing for 1-2 days of clouds) a larger array and 2 day storage for your flooded cell lead acid battery bank is highly suggested--Unless you are happy to run the genset to make up for the smaller array and battery bank (FLA batteries cannot really recharge from 50% to 100% State of Charge in one solar day--You need more hours of charging than the sun is in the sky--maybe you can scrape by with less solar in summer).

    For gensets, there are two major classes of battery chargers you can use... Here are examples of the "dumb" dump energy into battery bank reliably (and may need a larger genset because of Power Factor issues--Another discussion). The second is a Power Factor Corrected "smart" power supply. Very nice, but not cheap:

    And don't even bother with the 12 volt output of your eu2x000i genset... Maybe for charging your car battery at the side of the road in emergency... Not useful for a full size deep cycle battery bank.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dmurphdmurph Registered Users Posts: 12 ✭✭

    I think the ship has sailed on running the ENTIRE thing on solar - that seems too expensive, at least for now. Here are my calculations for having a generator on during the day (and powering the battery recharge with the generator & solar):

    Note - since I will have the geny on during the day, and this is in the middle of the desert in Nevada in August, I'm making some assumptions about daylight and that I don't necessarily need sunlight to charge the batteries

    Daytime - Generator ON all day

    4,950 Total HW SHOULD be covered by my generator's 120V AC output. Planning on having the generator hooked up to an I/C, which will both power this as well as charge the battery - OPEN QUESTION: How do I not overload the generator here? What specs from the I/C should I be watching / maybe also from the battery bank specs

    Nighttime - Generator OFF

    6,874 Total WH * 1/.85 Efficiency cost * 1/24 Battery Voltage = 286.42 AH.... 286.42 AH * 1 evening storage * 1/.5 max discharge = 573 AH

    573 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,157.42 Watt nominal array

    2200 / 6.5 daylight hours = 338 Watts of panels

    If these aren't wrong - then I would need ~600Ah of 24V, and ~400 watts of panels.

    I'm assuming the following might be wrong - are either of these true?

    • Maybe because I want only 10% charge per day, I should be upping my batteries to optimize for this.
    • Maybe because I have both solar AND a generator charging the batteries, I need to take this into account for the 10%

    If all of my fears are true, there is a lot of math to do here to make sure I don't overload the generator & make sure I'm at 10% charging per day. If that is the case, maybe I just isolate my systems, and not bother charging the batteries with the generator, and only run the freezer w/ the geny during the day.

    See the spreadsheet above for all of these numbers & other options

  • BB.BB. Super Moderators, Administrators Posts: 30,052 admin

    Regarding the generator output... As you say, derate it by 80%...

    eu2200i is rated 1,800 Watts continuous... 0.80 * 1,800 Watts = 1,440 Watts (or VA) max continuous suggested

    So, roughly, the maximum AC charger is somewhere around:

    • 1,440 Watts * 0.80 rough charge controller eff * 1/29.0 volts charging = 39.7 amp @ 24 volt "maximum" charge controller

    If you want to run other loads (120 Watt refrigerator + 200 Watts sound + 100 watts lighting + whatever else), then subtract that from genset capacity (1,440 Watts - 420 Watts AC loads = 1,020 Watts for battery charging--Then use above equation to figure out max charger output).

    Be a little careful here... The Honda EU family is an "inverter generator"--They are a bit limited in their ability to supply surge current (compressor starting) and short term over wattage output--The inverter is electronic and has very fast and sensitive limits. Old standard gensets can "ride through" large surge currents better than inverter gensets.

    And the various types of AC battery chargers out there--There are not that many Power Factor Corrected units (which are much "nicer" loads for gensets). Many of the older/less expensive ones have "poor power factor" and the 0.80 or so derating factor can be as much as 0.67 (they load the generator wiring more than you would have expected--And therefore the genset can support less charging current to the battery bank).

    Most people use less power at night vs daytime... But if your 6,874 Watt*Hours per night is correct, then the forumula would be:

    • 6,874 WH overnight battery usage * 1/0.52 end to end off grid solar eff * 1/6.5 hours of sun per day = 2,034 Watt array "break even"

    The 0.52 derating is based on:

    • 0.81 solar panel "hot" derating * 0,95 charge controller eff * 0.85 AC inverter eff * 0,80 Lead Acid battery efficiency = 0.52 = 52% overall system efficiency

    Also, the 10% rate of charge is NOT 10% charge per day... It is 10% of battery capacity charging current:

    • 573 Amp*Hour battery (20 hour rating) capacity * 0.10 = 57.3 Amps charging

    So, discharging your battery to 50% state of charge, then 10% rate of charge would recharge the battery back to 100% State of Charge in 5 hours (not really, lead acid batteries will take 10% rate of charge from 50% to ~80% state of charge (3 hours of Bulk stage charging). The last 80-90+% charging takes ~4-6 hours (Absorb stage charging). So the actual time on charge for a 10% rate of charge from 50% to 90% state of charge is ~9 hours minimum (if genset only charging, that is ~60 amp rated charger running for 9+ hours).

    If you are doing both solar and genset charging... You can mix and match charging. Say a 20 amp @ 24 volt battery charger and a 10-40 Amp solar array (more solar, less genset fuel/runtime).

    Generally, when using a genset+solar--Run the genset in the morning (start at 8 or 9am for a few hours, then let the solar finish charging the rest of the day).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 4,451 ✭✭✭✭✭

    For the genny load + charging thing, IMHO you have a couple of options:

    1. A "higher end" inverter/charger, with the ability set limits on both charging current and overall generator draw. To the extent loads are less than overall limit, the I/C should limit charging based on the variable load draw. It should also have a decent PF corrected charger. It's best to limit surge loads as the I/C may take a bit of time to adjust charge current.

    2. Separate AC charger and inverter. The inverter would always supply loads from battery, so there's no issue with charging + loads > genny capacity. The downside is some losses going from genny AC->charger DC->battery->inverter AC-->loads when it could be just genny->loads when running. Good standalone chargers can be a bit spendy as well, but a 3-stage isn't really essential if using solar as well.

    I do it both ways (#1 on a 48v bank, #2 on 12v bank). Both can work well, but in your application I'd probably lean toward #2.

    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
  • dmurphdmurph Registered Users Posts: 12 ✭✭
    edited March 2019 #20

    Alright, after using this info & doing more research, I re-did my spreadsheet calculator. I think it matches all of your calculations, except it has some higher de-ratings:

    I have two input columns in case the person's calculations would require them separate. The highlighted spots are where you can input different numbers to see their result.

    Does this generally check out? Do you mind poking at it and making sure I didn't forget something? I'm planning on publishing this, with lots of context, in a blog post & with some other burning man camps so I want to make sure I'm generally correct. (it's editable)

    Other than those base calculations, the user needs to keep track of

    • Power Factor of chargers
    • Not running the batteries too low (inverter feature maybe)
    • Panel array setup to get the correct voltage
    • Specs of all of their equipment

    Does this generally look right?

  • BB.BB. Super Moderators, Administrators Posts: 30,052 admin

    In the upper left of your spread sheet, I am not sure I understand what it is trying to do...

    You may be confusing Watts with Watt*Hours.

    Watts is a rate--Like miles per hour. And Watt*Hours is an amount, like miles driven.

    So, you are trying to figure out the amount of energy you use in a day, you need to figure out how many hours per day you are running your loads.

    For example, say you want to run 400 Watts of loads for 5 hours per day:

    • 400 Watts * 5 hours per day = 2,000 Watt*Hours per day

    And if you want to size the battery bank for that, assuming this is an AC load:

    • 2,000 WH per day * 1/0.85 AC inverter eff * 2 days of storage * 1/0.50 maximum discharge (longer battery life) * 1/24 volt battery bus = 392 AH @ 24 volt battery bank.

    And a suggestion, if the battery bank AH size is >~800 AH, I would highly suggest that you go up to the next standard voltage:

    • 392 AH @ 24 volts is the same energy storage as 196 AH @ 48 volts
    • 392 AH * 24 volts = 9,408 Watt*Hours of battery storage
    • 196 AH * 48 volts = 9,408 Watt*Hours of battery storage

    The other calculation is the size of your loads in Watts (or AH @ volts)... The larger your loads, the bigger the battery bank needs to be to support those loads. Generally, your average loads should not exceed C/8 discharge rate for flooded cell lead acid batteries (you can go to C/5 for short term heavy loads). Say you have a 1 HP well pump that draws 1,500 Watts running on a 24 volt battery bank:

    • 1,500 Watts * 8 hour discharge rate * 1/24 volt battery bank * 1/0.85 AC inverter eff = 588 AH minimum FLA @ 24 volt battery bank

    The other calculation to look at is surge current... The maximum surge current would be C/2.5 hour discharge rate. Say your pup draws 3,000 Watts surge:

    • 3,000 Watt surge * 2.5 hour discharge rate * 1/24 volt battery bank * 1/0.85 AC inverter eff = 368 AH @ 24 volt battery bank minimum to support surge current

    So, you have to look at battery capacity both in terms of pure energy storage (Watt*Hours or AH @ xx volts) per day, and physically how much surge the battery bank can output.

    And when sizing the solar array, you again have two calculations. One is based on the size of the battery bank--The rate of charge--FLA batteries should be charged at a 5% to 13% rate of charge, with 10%+ being recommended for full time off grid (larger battery bank, larger solar array).

    The second is sizing the solar array on the amount of energy you use per day and hours of sun per day (by season, no shade on array). If you don't have many hours of sun, you will need a larger solar array (larger than the 10% rate of charge suggested above). If you have lots of sun, then this calculation will probably be lower than the 10% rate of charge results from above.

    Ideally, you want the solar array to meet both your Rate of Charge and Hours of sun calcuations.

    And the other fudge factors (your loads may have a minimum per day to run the refrigerator and lights--Base loads. And another, higher WH per day, to run computer, vacuum cleaner, washer, water pump, etc.--The optional loads that you can turn off during bad weather and/or use a genset to support when needed).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dmurphdmurph Registered Users Posts: 12 ✭✭

    Question about that last part - is C total capacity, or is it just the 'top half' of the battery capacity (as I don't want to run the batteries less than %50)

  • BB.BB. Super Moderators, Administrators Posts: 30,052 admin

    C is the "20 hour discharge rate" that we use for our general rules of thumbs:

    • 200 AH battery bank / 20 hour discharge rate = 10 amp discharge (from 100% to 0% state of charge)

    Some vendors, like Surrett/Rolls, (I think) use the 6 hour discharge rate and have "different" rules of thumbs.

    The 20 Hour discharge rate works pretty well for off grid home--For example, 5 hours per night, for 2 nights (no sun/charging during bad weather), to 50% state of charge (i.e., "10 hour" of discharge left). seems to work pretty well for our needs.

    The C/6 discharge rate gives a "lower AH capacity rating". And C/100 hour discharge rate gives a higher discharge rate.

    Flooded Cell Lead Acid batteries have a "larger spread" between C/6, C/20, & C/100.

    AGM batteries, with lower internal resistance (and more "active" physical chemistry), typically have a lower spread (AGM batteries have "lower internal resistance" and higher surge/current ability).

    I try to use "one number" throughout the calculations (i.e, C/20 capacity = 220 AH @ 6 volts for a "golf cart" style battery). If there are "adjustments"... For example, if you want 3,300 WH per day of 120 VAC from a 24 volt battery bank for 2 days of storage and 50% maximum discharge (refrigerator+LED lights+small well pump+washing machine+LED TV+laptop+Cell phone):

    • 3,300 WH per day * 1/0.85 AC inverter eff * 1/24 volts * 2 days storage * 1/0.50 max discharge = 647 AH @ 24 volts (C/20 capacity)

    Vs trying to figure out the size of solar array... The two calculations, first based on 5% to 13% rate of charge (10%+ for full time off grid). The second based on hours of sun for your area (by season):

    • 647 AH * 29.0 volts charging (C/20 hour rage) * 1/0.77 solar panel+charge controller deratings * 0.05 rate of charge = 1,218 Watt array minimum
    • 647 AH * 29.0 volts charging (C/20 hour rage) * 1/0.77 solar panel+charge controller deratings * 0.10 rate of charge = 2,437 Watt array nominal
    • 647 AH * 29.0 volts charging (C/20 hour rage) * 1/0.77 solar panel+charge controller deratings * 0.13 rate of charge = 3,168 Watt array "typical cost effective" maximum

    And based on your 6.5 (Summer) hours of sun:

    • 3,300 Watt*Hour loads * 1/0.52 end to end system eff * 1/6.5 hours of sun = 976 Watt array (break even on 6.5 hours of summer sun)

    Where end to end system derating of 0.52 is:

    • 81% derating of (hot) solar panels * 0.95 typical MPPT derating * 0.85 AC inverter eff * 0.80 FLA batt eff = 0.52 end to end off grid solar efficiency

    Note that this derating of 0.81 panel * 0.95 MPPT controller deraging = 0.77 panel+cntrl deratings works pretty well for a well designed system using PWM controllers too (for entirely different reasons--For MPPT controllers, the solar panel derating approaches 1.0 or even better for sub freezing weather conditions and very cold panels).

    What I did not understand is how you handled the 400-500 Watt load--Originally you wanted 24 hours per day, but that would be too large of load for a 1-2 week a year "solar power system" (very expensive system for short term usage).

    • 400 Watts * 24 hours per day = 9,600 WH per day = 9.6 kWH per day

    Where the above example is 3,300 WH per day (still a "medium sized" off grid power system):

    • 3,300 WH per day / 24 hours per day = 137.5 Watt average load

    Or if just for 5 hours in the evening (TV, lighting, water pump for showers, charging cell phones, cameras, laptops):

    • 3,300 WH per day / 5 hours per day (evening) = 660 Watt load for 5 hours an evening


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DaangeroussDanDaangeroussDan Registered Users Posts: 23 ✭✭
    edited April 2019 #24

    I love this inverter

    I have two of them , powered by 1200 watts in solar pv into a 900 AH AGM battery bank. here is my YouTube page full of videos and tutorials for "off grid". I don't use any generator and have not had the need in a year and a half.

    Click on the "videos" or "uploads" button to see over a dozen helpful and somewhat unique tutorieals on dealing with shade , and line loss issues.

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