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  • Solar inverter as whole house UPS system

    Greetings,

    Not sure this is right section, but let me ask here. After recent trouble with electricity we (my friend and I) decided to install solar inverter as whole house UPS system. We plan to add solar panels later, but main reason is to have power backup for power outage. So we are going to use:
    • Growatt 12KW 48V SPF DVM off grid inverter
    • 300Ah LiFePO4 batteries.
    As this inverter can handle 36KW on spiked this should be enough for a house with average usage of 4-5 KW.

    Now as batteries would provide 3 - 4 hours of whole house operation that should be enough for short time power outage. But for longer time we want to add generator, as this inverter can start it automatically based on battery level. As I understand common way to use generator is to switch main power from grid to generator and use that. Those that is quite common and pretty simple way to do it, it is not optimal, as it requires generator to be able to handle high level of power including spikes.

    So question is, is it possible and viable to connect generator to the one of PV inputs of the inverter? If I am not mistaken this would let us use less powerful generator - it has just to provide power above average so in middle term batteries would be charged, but spikes would be handled by inverter and batteries. So if house has average 4-5 KW with spike let's say 12KW then 8KW generator should be fine assuming inverter would limit max amperage taken from it.

    Another problem is that most of common generators provide AC power but PV input requires DC. So question is does PV requires real DC or AC after rectifier bridge would be fine?

    Thanks in advance

  • #2
    I believe those inverters have an AC input designed to be hooked to any source of AC power.
    A lot of households can live just fine on 12kW of power. Let us know how it works out for you!
    BSEE, R11, NABCEP, Chevy BoltEV, >3000kW installed

    Comment


    • #3
      Originally posted by solarix View Post
      I believe those inverters have an AC input designed to be hooked to any source of AC power.
      I understand. Issue is inverter will drain too much from AC input when connected to generator, and I do not see a way to limit it only for generator. So if I configure inverter to charge batteries with 100A from grid and house usage would spike to 12KW then generator will have to handle 17KW (12KW house and 5KW charging batteries) otherwise it will be overloaded. But if I connect it to PV I can limit it to pull only lets say 8KW then on spikes inverter will use batteries and generator would not be overloaded even if it would be 10KW. Am I wrong here?

      Comment


      • #4
        Sounds like you want to do direct load sharing between solar and generator with power limit. If so, I am not aware of an off-grid inverter that can do it. I doubt feeding rectified AC into the PV input would work well. However, assuming the inverter/battery can provide up to the 12kW surge power you need, you may be able to achieve similar effect by using one or more separate adjustable current limit battery chargers powered by the generator.

        Comment


        • #5
          Originally posted by rGlory View Post
          Greetings,

          .........
          As this inverter can handle 36KW on spiked this should be enough for a house with average usage of 4-5 KW.
          ......
          I would find it easier to manage my house loads when in backup mode than try to cobble together something outside traditional inverter generator designs. I don't have a generator so I am limited to what my inverter can output and how much my solar can charge my batteries.
          9 kW solar, 42kWh LFP storage. EV owner since 2012

          Comment


          • #6
            Originally posted by solardreamer View Post
            I doubt feeding rectified AC into the PV input would work well.
            That is what I want to find out. I think it should as it accepts wind generators and I believe they provide rectified AC, though probably higher frequency. Anyway I can make that rectified AC to proper DC by adding capacitors, but I wonder if that is really necessary.

            Originally posted by solardreamer View Post
            However, assuming the inverter/battery can provide up to the 12kW surge power you need, you may be able to achieve similar effect by using one or more separate adjustable current limit battery chargers powered by the generator.
            I think that would be counter productive, as inverter has 2 PV inputs which are doing exactly that - limiting current and use that to charge batteries. Only question - can I connect output of regular AC generator to that PV input.

            Comment


            • #7
              Originally posted by rGlory View Post
              That is what I want to find out. I think it should as it accepts wind generators and I believe they provide rectified AC, though probably higher frequency. Anyway I can make that rectified AC to proper DC by adding capacitors, but I wonder if that is really necessary.
              The PV input likely has MPPT so rectified AC will likely wreak havoc on it unless you do true AC-DC conversion with low ripple which is likely more expensive than just getting separate charger. Also, rectified AC may damage the inverter if the PV input voltage limit is not high enough.

              Comment


              • #8
                Originally posted by solardreamer View Post
                The PV input likely has MPPT so rectified AC will likely wreak havoc on it unless you do true AC-DC conversion with low ripple which is likely more expensive than just getting separate charger.
                It is strange, I think it supports wind generator as well so it should be fine with ripple, unless I do not understand something. I think I saw somewhere that it supports wind if I am not mistaken, will check.

                Originally posted by solardreamer View Post
                Also, rectified AC may damage the inverter if the PV input voltage limit is not high enough.
                Voltage range for PV input is 60-245VDC so recitified 110V would be definitely withing range.

                https://www.growatt-america.com/show-42-642.html

                Comment


                • #9
                  1) you cannot feed an MPPT input (solar PV, current source) with a battery or generator (voltage sources) . OK, so you "can" but you won't be happy.

                  2) you must create a Critical Loads sub panel, and put only your critical loads on it. Not the 3, 4ton AC units, not the electric water heaters, not the spa pump & heater. Critical loads..Lighting, minimal heat to keep your water pipes from freezing like happened in Texas last winter. Fridge, freezer, your smallest air cond, And you should balance the loads on your CL sub panel, or risk the inverter shutting down from imbalance. The generator will be happier too.

                  3) more advanced inverters have Generator Support. The best ones have <10ms transfer to the battery source and then your generator comes on line, and takes over loads, and the inverter runs backwards to recharge the batteries. If the generator becomes loaded down, battery charging reduces, and if loads climb, Generator Assist uses the battery & inverter to assist the generator .

                  You must understand how Power Factor of your loads will reduce the apparent capacity of the inverter AND the generator.

                  I don't believe Growatt 12KW 48V is a very advanced inverter. It may claim to do a lot of things, but I really doubt it can deliver.
                  Powerfab top of pole PV mount (2) | 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

                  solar: http://tinyurl.com/LMR-Solar
                  gen: http://tinyurl.com/LMR-Lister

                  Comment


                  • #10
                    Originally posted by Mike90250 View Post
                    1) you cannot feed an MPPT input (solar PV, current source) with a battery or generator (voltage sources) . OK, so you "can" but you won't be happy..
                    I would like to understand why. Because it is very strange that I can connect wind turbine generator to it (correct me if i am wrong), but I cannot connect gas powered generator to it. What is the principal difference that will make me unhappy?

                    Originally posted by Mike90250 View Post
                    2) you must create a Critical Loads sub panel
                    Hmm that would be very strange - I choose inverter that can handle whole house including AC but then I have not to use it propelry and eliminate AC. I understand the need of subpanel - when you have limited power for generator/inverter, but when it can cover it all...

                    Originally posted by Mike90250 View Post
                    You must understand how Power Factor of your loads will reduce the apparent capacity of the inverter AND the generator.
                    I do understand it and that's why I am using low frequency inverter which can handle inductive load quite well and this inverter I believe is powerful enough to feed the whole house including central AC without issue.

                    Originally posted by Mike90250 View Post
                    then your generator comes on line, and takes over loads, and the inverter runs backwards to recharge the batteries.
                    Yes this is how it is usually done, but it is too expensive and I am checking if I can avoid that.

                    Comment


                    • #11
                      Originally posted by rGlory View Post
                      It is strange, I think it supports wind generator as well so it should be fine with ripple, unless I do not understand something. I think I saw somewhere that it supports wind if I am not mistaken, will check.


                      Voltage range for PV input is 60-245VDC so recitified 110V would be definitely withing range.

                      https://www.growatt-america.com/show-42-642.html
                      Wind turbine inverters typically take in 3 phase unstable AC and convert to stable DC before feeding to the charge controller. Typical PV inverters don't have the extra hardware for that.

                      If you only use a single 110/120V generator feed in then you will likely need to get a bigger generator at higher cost to meet the 8kW power need you specified.


                      Comment


                      • #12
                        Originally posted by solardreamer View Post
                        Wind turbine inverters typically take in 3 phase unstable AC and convert to stable DC before feeding to the charge controller. Typical PV inverters don't have the extra hardware for that.
                        I believe MPTT controller has DC-DC converter and as voltage range is 65-245 it should not care if voltage is unstable if it is within the range. And gas generator output should be stabilized, otherwise I am not sure how it could work. So lets say if I just convert it to DC using rectifiers and capacitors what could be the issue?

                        Originally posted by solardreamer View Post
                        If you only use a single 110/120V generator feed in then you will likely need to get a bigger generator at higher cost to meet the 8kW power need you specified.
                        I can easily find 10KW gas generator for $1500. But if I would want to feed into AC then it has be 20KW minimum and that is about 10K installed. This generator alone will cost more than inverter and batteries, yes if difference would be 10-20% I would not bother, but order of magnitude...

                        Comment


                        • #13
                          Originally posted by rGlory View Post
                          I believe MPTT controller has DC-DC converter and as voltage range is 65-245 it should not care if voltage is unstable if it is within the range. And gas generator output should be stabilized, otherwise I am not sure how it could work. So lets say if I just convert it to DC using rectifiers and capacitors what could be the issue?
                          Of course, there is DC-DC converter but it's not designed to take rectified AC that has extreme swings continuously at 120Hz. PV or even wind turbine voltages don't continuously vary that fast. In addition, the MPPT tracker will likely get confused hunting for proper voltage operating point likely cause more problems. Just adding a few caps to rectified AC won't help much at your target power level.

                          Comment


                          • #14
                            I mentioned a critical loads sub panel because I thought the battery was limited and you needed a generator to supplement it. A critical loads subpanel helps automate the load shedding need to run longer on batteries. There IS going to be a day when your generator does not start, and your lights shut off at midnight and you have to figure it out in the dark with a flashlight. But if you want to have conspicuous, flagrant consumption of power, help yourself.




                            > 1) you cannot feed an MPPT input (solar PV, current source) with a battery or generator (voltage sources) . OK, so you "can" but you won't be happy..

                            >> I would like to understand why. Because it is very strange that I can connect wind turbine generator to it (correct me if i am wrong), but I cannot connect gas powered generator to it. What is the principal difference that will make me unhappy?

                            You will need to take an electronics course that teaches the difference between a Current source and a Voltage source, Most MPPT controllers can only manage Current sources (PV) Some super fancy ones like my Midnight Classic, can also handle a wind turbine, if a $1,000 Clipper and dump load are added,
                            I seriously doubt your PV input circuit is designed to handle both a Current (PV) and a Voltage ( Wind, rectified generator output) input.


                            So, to save yourself a lot of distress and damaged gear, STOP. Download the manuals for the gear. READ the manuals. Be sure of your plan before you spend a lot of money on things that won't work as you would like them to.


                            A brief review of the growatt specs:
                            -Battery Required to Operate
                            -Built-in 250V 120A MPPT Solar Charge Controller
                            -WIFI remote monitoring is available HERE
                            -Pure Copper Low-Frequency Transformer
                            -Efficiency >85% | Power Factor 1.0
                            -2 YEAR MANUFACTURER WARRANTY

                            Suggests you cannot feed a rectified AC signal into the SOLAR input. it does not have an option listed, to charge batteries from the GRID input
                            With a >85% efficiency, you are going to waste a lot of power, good low freq inverters are in the 92% efficiency & +5 yr warranty.
                            Powerfab top of pole PV mount (2) | 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

                            solar: http://tinyurl.com/LMR-Solar
                            gen: http://tinyurl.com/LMR-Lister

                            Comment


                            • #15
                              Originally posted by solardreamer View Post
                              Just adding a few caps to rectified AC won't help much at your target power level.
                              I wonder then how they managed to feed from AC - they have to change it to DC first. And what did they do to design DC-DC converter to work from AC? They rectified it and smoothed with capacitors. Maybe they added inductive to reduce noise inverter can push back to the grid, but this is not case here. What else?

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