Two different inverters

LubyLuby Registered Users Posts: 5

New here and to solar. I bought two used inverters last summer - an older Xantrex and even older ... (can't remember the name, now). They are both 12 V. If I buy panels, which are 24 V each, would it damage any of the components if I hook their output to both inverters in series? Any other solutions to bring down the voltage without too much loss?




  • BB.BB. Super Moderators, Administrators Posts: 32,997 admin
    Re: Two different inverters

    Basically, the inverters are fixed to their designed input voltage... If "12 volts" that would be 10.5-15.5 VDC or so.

    If you have access to "24 volt volt panel"--that is typically solar panels with Vmp of ~35 volts... Then you would use a MPPT type charge controller to take the "high voltage / low current" of the solar panels and efficiently "down convert" the energy to the "low voltage / high current" required to charge the 12 volt battery bank.

    You could hook the "24 volt panels" to a PWM type charge controller and charge a 12 volt battery bank, but you would waste ~1/2 the charging power available from the 24 volt solar panels.

    Your other choice would be PWM or MPPT type charge controller and a 24 volt battery bank. You would then need to obtain new 24 VDC input AC inverters.

    The most flexible charge controller is the MPPT type... Depending on Brand and Model--they can take solar array Vmp from a few volts over Vbatt-charging to nearly Vmp-array-stc of 100 VDC... The high voltage array (and lower voltage battery bank) allows you to run a longer cable from the solar array to the solar charge controller (sort of like running "high voltage" AC and stepping down to low voltage DC with a transformer.

    What is it that you are looking to do (how much energy per day, average/peak power of loads, how far to run cables, etc.)? There are various trade-offs to be made when choosing charge controller types, battery bank voltage, and AC inverter options.

    We can give you an answer better tailored to your needs with a bit more background information.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Two different inverters

    Hello and welcome!

    You're skipping a major component here; batteries. With the exception of grid-tie units (and I assume the ones you have are not, given the Voltage) inverters do not run directly off solar panels. You've got to have batteries. The solar panels recharge the batteries.

    If you could give us some idea of your intended use and expected loads we could help you figure out just how much battery capacity and panels to recharge same are needed.
  • LubyLuby Registered Users Posts: 5
    Re: Two different inverters

    Thank you for your very detailed replies. It is awesome that you made me think and check things better. I thought I could hook up my inverters to DC from the solar panels and now it seems like I cannot. I see only AC input from either the grid or a generator. They can charge 12V battery bank and that's it. Big bummer. Now I have to buy a additional DC to AC converter to "get" the power from the panels to the Xantrex or buy just a DC converter to charge the batteries, which I connect to my inverter, correct? So, this is my question now.

    Below is what I started to write, before I read more and checked better. Now it would be just as a theoretical question. You could ignore it.

    Thanks again,


    I should have explained better what I am asking.

    As I said, I already have those two inverters. One is Xantrex MS2000 Sine Wave, the other one is "regular", old one. They are both 12V, 2000W. I do not have batteries, or anything else, yet. I want to do a very basic set up with two solar panels, for now, so I can just run a freezer and a fridge, plus charge some small appliances. Later I may do it propery.

    If I have a 25V power source, I could apply it to one resistor, as a load. I could replace it with two resistors in series. The voltage on eash resistor would be 12.5V (theoretically). So, I was thinking that maybe it would be possible to have the inverters hooked in series and thus fed by a 25V pannel system. That was my question: Could I see the inverters as loads (since they do not have passive resistance only) or are any other factors that I do not know? Once that hurdle is cleared, then I would ask about batteries (which have to be in two banks of 12V, hooked up separately to each inverter.)

    Here is a model of a solar panel that has 25V output - GEPV-173-MCA-004 173 Watt, 6.90 Imp, 25.10 Vmp.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Two different inverters

    The other missing piece to the puzzle: charge controller!

    The basic wiring is: Solar panels to charge controller, charge controller to batteries, batteries to inverter(s), inverter(s) to loads.

    To do it right you need to get a handle on the loads. Refrigerators and freezers are pretty big power users. Neither will "like" running off the "old" MSW-type inverter.

    One of the other things; your panels. These are "not-quite-24-Volt" panels, in that their Vmp is too low to charge a 24 Volt system's batteries (charge Voltage 28.4). They are also rather high for a 12 Volt system, which would normally have a Vmp of 17.5 for a 14.2 Volt charge. This becomes slightly problematic. You can either use a PWM controller and "waste" a certain amount of available power, or you can spend $$$ and get an MPPT controller. Whether or not it would be worth it would depend on the rest of the system details, such as size of battery bank, site insolation, distance between panels and controller, et cetera.

    You won't be able to connect the two inverters "in series" across a 24 Volt bank. For one thing, you'd need those panels in series feeding an MPPT controller to charge the bank. For another, you would never be able to keep the current draw even across the bank. One inverter will always draw more than the other, dragging half the bank to a lower state of charge. The batteries will fail prematurely because of this.
  • BB.BB. Super Moderators, Administrators Posts: 32,997 admin
    Re: Two different inverters

    If you want to understand why two 12 volt inverters in series to a 24 volt battery will not work... Here are a couple reasons.

    Inverters do not control power flow--it is their AC loads. So, if you have a 100 watt load on one and a 1,000 watt load on the other--they no longer have matched loads so they take different current, and their voltage drop will not be matched (like a pair of matched resistors).

    A second concept to wrap your mind around is the fact that inverters are "negative" resistors... What is that you ask???

    Remember for a resistor, Voltage=Current * Resistance.... Or Current=Voltage/Resistance.

    If voltage drops, then current will drop.

    For an AC Inverter, they are constant power devices... Their equation is Power=Voltage*Current.

    Since Power is fixed by your AC load (100 watts, 1,000 watts, etc.)... Then the equation is:
    • P=V*I
    • I=P/V
    Notice, in this case, as voltage falls, the current actually increases. If you had two matched 12 volt inverters on a 24 volt power source, as soon as something got out of balance, the voltage will fall on one (increasing the current) which would cause voltage to fall more. While the other inverter seeing voltage increase will be decreasing current.

    Basically, this is a positive feed back loop and is exactly how digital electronics/computers work... You want +1 or 0 logic levels (5 volts/0 volts in the old TTL days). You don't want 2.5 volts (0.5 logic level) if you want your digital computer to work correctly.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LubyLuby Registered Users Posts: 5
    Re: Two different inverters

    Wow, wow! And I thought I know something.

    Thank you again, Bill and Marc! You explained it pretty good. I'll need some time to process all the new info and check to see what some of the abbreviations mean. And maybe I should start the whole process from scratch, which I know is usually the best solution.

    I am very happy that I joined this forum. For over a year I have been on and off thinking about doing some solar and never got too deep into it. I even paid someone to come and do estimate/evaluation on the site. So, at least I know where the best place for panels is. Then I started to have more questions than I could find answers for and I would drop it, just to start from the beginning months later. Now I feel more confident that I can get help from people who know how to do it. I was impressed by your mathemtics, Bill. I should know these things and you laid everything nicely for me.

    I will read your suggestions again and will come here with more questions. Thanks for being of such help!

  • LubyLuby Registered Users Posts: 5
    Re: Two different inverters

    Hello Guys,

    After reading and some research, I realized that rather than trying to invent the wheel, I better leave it to the experts - you. I will tell you what I want to have and I am sure you'll be nice to tell me how to do it.

    Due to cash restrains, I have to go with only two panels (I'd like somewhere around 200W each) and limited number of batteries. I'd like to be able to run a well pump (shallow spring), some lights, a laptop, some DC lamps, occasionally charge an electric shaver, some small batteries and, God forbid, watch TV, not at the same time, of course. I have a Xantrex MS2000 charger/inverter, which I think has only AC input. I also have a smaller, 750W inverter from Harbor Freight. I don't think I can use any of these, can't I?

    Reading what I wrote, two panels may not be enough, so I may have to drop some the items, like the TV or even the laptop, if you think that they would consume too much. I live about 60 miles south of Seattle – not much sun here.

    What components do I need? What would be best (AGM batteries) to get and/or most cost effective (Walmart batteries)?

    Thank you very much,
  • BB.BB. Super Moderators, Administrators Posts: 32,997 admin
    Re: Two different inverters

    Just working with some standard rules of thumb... 400 watts of solar panels and 5-13% rule of thumb for sizing a battery bank (5-13% rate of charge for battery bank--under 5%, not enough charging current to properly mix the electrolyte, plus takes a long time to recharge the bank. Over 13% rate of charge is really just a waste of money for solar panels):
    • 400 watts * 0.77 panel/charge controller derating * 1/14.5 volts charging = 21.2 amps nominal max current
    • 21.2 amps * 1/0.13 max rate of charge = 163 AH @ 12 volt battery (20 Hour Rating)
    • 21.2 amps * 1/0.05 max rate of charge = 424 AH @ 12 volt battery (20 Hour Rating)
    So, roughly, around 160-425 AH of 12 volt battery bank can be supported by 400 watts of solar panels (remember, I am showing lots of digits so you can reproduce my math--Anything within 10-20% of the above numbers is OK--but the farther you go from the recommended range, you may trip over other issues).

    Roughly, your recommended rates of power you draw from the battery bank would be (for flooded cell battery bank):
    • C/20--Low current--pretty efficient
    • C/10--Higher current, less efficient (batteries at higher current draw will appear to have less capacity)
    • C/13--Maximum recommended continuous draw (could overheat battery if higher current draw)
    • C/2.5--Maximum surge current (more current and the battery may no longer output "12 volts" for starting surges).
    So, say you choose a 225 AH battery (two 6 volt "golf cart" type batteries):
    • 225 AH * 1/13 max cont. current * 12 volts = 208 watts max continuous load
    • 225 AH * 1/2.5 surge current * 12 volts = 1,080 watts
    So, your "Ideal" inverter would be around 200-500 watts maximum power and ~1,000 watts max surge rating (many inverters are surge rated 2x continuous rated power).

    For a 12 volt system -- The Morning Star 300 Watt TSW inverter (600 watt 10 minute rating) would be about perfect for this setup. It has a low power "search mode" which is not common on smaller inverters--You can "leave it on" and turn your AC loads on and off without wasting as much power (6 watt standby if running with no loads).

    How much power would a 400 watt system supply? Say your weather is similar to Olympia Washington. Using PV Watts website, 1 kWH (1,000 watts--smallest array program accepts) of solar panels, 0.52 system derating (off-grid, flooded cell batteries, AC inverter), and defaults for the rest, we get:
    "Station Identification"
    "Lat (deg N):", 46.97
    "Long (deg W):", 122.90
    "Elev (m): ", 61
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 0.5 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 47.0"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 6.4 cents/kWh"

    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 1.28, 17, 1.09
    2, 2.45, 33, 2.11
    3, 3.52, 52, 3.33
    4, 3.97, 57, 3.65
    5, 4.69, 67, 4.29
    6, 5.19, 71, 4.54
    7, 5.62, 79, 5.06
    8, 5.41, 76, 4.86
    9, 4.66, 65, 4.16
    10, 2.99, 43, 2.75
    11, 1.73, 23, 1.47
    12, 1.40, 19, 1.22
    "Year", 3.58, 601, 38.46
    You get around 17 to 79 kWH per month for 1,000 watts of solar panels. Or Watt*Hours per day (note we are multiplying by 0.4kW of solar panels--400 watts):
    • 17,000 WH * 0.4 kW of panels per month January / 30 days = 227 WH per day in January
    • 79,000 WH * 0.4 kW of panels per month July / 30 days = 1,053 WH per day in July
    So, if you had a 200 watt load, you could run it:
    • 227 WH / 200 watts = 1.1 hours per day in January
    • 1,053 WH / 200 watts = 5.3 hours per day in July
    A 225 AH battery supplying a 200 watt AC load to 50% discharge (maximum recommended day to day discharge for long life):
    • 225 AH * 12 volts * 0.50 max discharge * 1/0.85 inverter eff * 1/200 watts = 5.7 hours
    In an emergency (or if you don't use the system very often--camping/emergency/etc.), you can drain a battery down to about 20% state of charge at the cost of less cycle life:
    • 225 AH * 12 volts * 0.80 max discharge * 1/0.85 inverter eff * 1/200 watts = 9.2 hours
    You can see if you discharge the battery to 20% state of charge in January, it would take almost 8 days to fully recharge it...

    So, you may also want to look at a back up genset and a good quality battery charger for periods of bad weather/heavy loads... Take a look at the Honda eu2000i (1,600 watt) or eu1000i (900 watt) gensets. They will run light loads for a fairly long time on a gallon of gasoline.

    Here are some examples of smaller systems designed for emergency / portable power:

    Emergency Power

    Basically a very long thread that starts from the beginning with a few vague requirements through design and assembly for a "portable" solar RE off-grid power box.

    And here is another example by Mike90045 called the Solar Monolith:



    Update pictures/information here.

    Normally, we design a system for a given set of loads... The above is designed around a given amount of money/solar panels.

    A 400 watt system is certainly capable of running a summer cabin.

    For emergency power to run refrigerator, a few lights, TV during a power outage--A small genset is usually cheaper and more capable. For the Honda, plan on storing about 1-2 gallons of fuel for every day without power--Will give you lights and keep a refrigerator or freezer cold.

    Again, the numbers are based on long term average weather conditions and rules of thumb... Your actual performance can vary by +/-20% or more for various reasons (weather, actual components, how much power you really use, etc.).

    Also, you should never plan on using all 227 WH per day in January--You should plan on 50-75% or so of predicted production... Some days you may use more, some days you may use less. And some times you may go a week or more under a heavy cloud deck and need to use the genset.

    The above system is a nice useful size for emergency use and would be a good system to learn on...

    Batteries, get a pair of 6 volt 225 AH golf cart batteries and plan on them lasting 2-3 years (or less if you mess up somewhere). Good "training batteries".

    AGM are about the "perfect" lead acid battery (more efficient, don't need to check water levels, much cleaner)--But the cost about 2x as much.

    And you should get a Hydrometer (for flooded cell batteries) and a nice DMM (digital multi-meter).

    If you still have some extra cash, a Battery Monitor (Trimetric is a good start) is very nice (and about mandatory for a AGM/Sealed battery bank).

    Questions? Did I mess up the math somewhere? :roll:;)

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LubyLuby Registered Users Posts: 5
    Re: Two different inverters

    Thank you very much, Bill. I have to study all the info before I ask more questions. For now, what comes to mind is:

    Are there solar panels that would work in cloudy sky - no sun? (Yes, I am close to Olympia, WA.) If I get more batteries, that would help, correct? Or maybe not, because then they would require more charging. I will not be using a generator, although I have one. How easy/difficult would it be to later add two more, different type/brand/rating panes?

    You do not know (that is why I am telling you) how much I appreciate your help! This has been bugging me for months and months.

  • BB.BB. Super Moderators, Administrators Posts: 32,997 admin
    Re: Two different inverters

    You are very welcome Luby.

    Pretty much, the output of solar panels is proportional to the amount of sunlight hitting the panels. My Grid Tied System will generate 1/10th or even 1/20th of its capability on a dark and stormy day...

    Simply, solar panels need lots of light to produce useful amounts of energy. Dark weather, shade, shadows from pipes/electric lines/etc. will all dramatically reduce the output output.

    Regarding Batteries, it turns out getting more batteries actually makes for more problems... Usually, shorter battery life because they do not get get quickly recharged (batteries sulfate if stored for days/weeks/months below ~75% state of charge) and do not mix the electrolyte if there is not enough charging current (5% is usually the minimum recommended charging current because of these issues).

    You could go upwards of 400+ AH of batteries and still be OK... But once you use the extra energy the first day or so, it will take 2x as long (and 2x the sun) to recharge the bank...

    That is why we really look at the loads and your needs... We can play with some of the setup (battery bank sizing, solar array, changing over to AGM batteries, using DC appliances instead of an AC inverter)--but that is still, for the most port, dancing around the edges.

    There is no getting around the fact that Off Grid solar power is expensive, batteries are large and heavy for the little amount of power they store, and the batteries will need to be replaced every ~5+/- years, and the electronics every 10 years +/-.

    If the solar is intended for emergency use (a few days/week or so a year)--Then, many times a proper emergency generator with appropriate fuel supply is really the better answer if you have normal household loads (fridge, freezer, large TV, numerous lights, well pump, etc.).

    Off-Grid solar power can make economic sense when you are using 9+ months of the year and your place is a good distance away from existing utility lines. And even then, conservation is still key.

    The off grid solar power will still cost you around 10x the cost of grid power. So to keep the overall costs for solar power reasonable, converting your home/cabin to use a minimum amount of power (energy star appliances, insulation, laptop computer instead of desktop, smaller TV instead of 50" LCD, turning off unused loads, etc.) all can save you a lot of up front cash.

    I would have loved to have a Hybrid Power system for my home (does both Grid Tied and Off-Grid for emergency power)--but the practical answer for me is that I have lived through 1 power outage that lasted a week (otherwise an hour or so every couple years--mostly from a car hitting a power pole)--and that was over 50 years ago. A small Honda eu2000i genset plus 20 gallons of gasoline is just a much more practical investment for my situation.


    PS: Adding panels to an existing system can be done... But it depends on matching the Vmp/Imp of the existing array. If you cannot mix and match the old with the new panels--you may be forced to purchase a second charge controller to properly mate the new panels to the existing battery bank.

    Also, it is very difficult to up size a solar PV system by much more than 2:1 (adding 2x panels later, adding 2x batteries, adding 2x larger inverter, etc.)... The small system components usually need to be swapped out for larger components... Typically, larger systems are 24 or 48 volts, so you need a new inverter. Mixing old and new batteries/upping the bank voltage of an existing system can be costly, etc. If your old panels are no longer in production, the new panels may not match voltage/current ratings so you need a second charge controller, etc...
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Two different inverters

    You won't be able to connect the two inverters "in series" across a 24 Volt bank. For one thing, you'd need those panels in series feeding an MPPT controller to charge the bank. For another, you would never be able to keep the current draw even across the bank. One inverter will always draw more than the other, dragging half the bank to a lower state of charge. The batteries will fail prematurely because of this.
    Another reason is that both inverters have to reference the same ground if they are both feeding the same AC bus. The grounding point of the upper of the two inverters would be offset by the DC voltage of the lower one until you grounded it, which would short out the lower one.
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