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  • Tiny House Off Grid... Need help w/ system selection!

    Hi Guys,

    I’m new to the forum and the world of off grid, and could use some hints and pointers in system selection for a tiny house with space constraints.

    I’ve picked a few items that I know will fit physically, that are within budget, and should theoretically meet my electricity demands.

    I’ll start with demands…

    Tiny House will be stationary and parked permanently next summer, no grid available. 3hrs north of Toronto. Can see -30C in winter. Currently parked in a place where a long 15amp run serves up about 700 watts before voltage drop is too great.

    Average calculated KW usage per 24 hrs is expected to be somewhere around 2.0 to 2.5 on battery power. I arrived at this:

    -1500watt 6gal water heater. 20 minutes from cold to hot. = 500 watts-ish. One heat cycle per day for shower, well insulated, on a switch. Extravagance = 1000watts
    -5 LED lights on, 10w x 5 = 50watts, say 6 hrs a day as a composite = 300 watts
    -Efficient 4.5 cu ft AC fridge w/ mechanical t-stat = 600 watts/24hrs
    -RV Water pump 80w x 1hr = 80 watts
    -Compost toilet fan 2w x 24hrs = 48 watts
    -Laptop computer 25 watts x 4hrs = 100 watts
    -Radio 10 watts x 5hrs = 50 watts
    -LED TV 40 watts x 2 hrs - 80 watts
    -Incidentals ie vacuum, coffee grinder, various little battery chargers, water pik, etc. 200 watt hours?

    All of the above w/ extravagance showers = 2458 watt hours x1.07 for 93% inverter efficiency = 2630 watts.

    I also don’t intend to pull much more than 2000 watts continuously off the inverter on battery only, and not for long (20 minutes to get hot water + lights and stuff simultaneously).

    The two infrequent appliances I’ve listed below I do not intend to run off battery. If it’s sunny out and I can pull enough juice from the array, then I will use one of the below at a time with the batteries helping out. If it’s winter and I need clean clothes or laundry, then I will use a generator to stuff amps back in the bank while doing dishes or laundry.

    Full 24” Energy efficient new dishwasher
    24” Clothes washer

    Here’s what I’ve picked to start with…

    Phase One Purchase (Very Soon)
    Magnum MSH4024M. 24V Mobile Hybrid inverter with load support, 4000 watt 120v, but I don’t need anywhere past 3000 watts at the absolute most w/ load support, extra capacity will be nice for surge. Has automatic neutral-ground switching for shore power or genset.

    Appropriate 4/0 cabling, battery monitor w/ shunt, fused w/ disconnect, etc.
    Magnum advanced remote
    Proper AC disconnects and breakers
    AC and DC ground to trailer frame
    Midnite Solar SPD wired to shore power and sub panel.

    4x Surrette Rolls 6v 335 AH AGM. The spot under the couch only measures 24.5”w, 20.25”D, and 15”H, so these fit nicely. I think they have a C/4 discharge rate? Should support bursts of 2000 watts for 20 minutes or more, hopefully.
    http://www.rollsbattery.com/wp-conte.../S6-370AGM.pdf

    I could also use 400AH L16’s but would have to run them on their side, with two of them on top of the other two with some spacers of some kind. It would be tight. Thoughts?
    http://www.rollsbattery.com/wp-conte.../S6-460AGM.pdf

    The other option is to buy lead acid and store outside in an insulated enclosure, and have it close to the wall for short DC cabling. But between cold weather and the inconvenience of it, I’d rather not. If I did, perhaps 6v Rolls 375AH flooded x8 would do the trick? (two parallel strings). Or do I run into not enough panel?

    Phase Two Purchase (Late Spring/Early Summer)
    -Honda EU3000is connected through 30 amp shore power plug. (will subsist on shore power until tiny house is moved and solar and genset purchased/installed)

    -Four 250watt-330 watt panels, totalling 1000-1320 watts, ground mount.

    -MPPT Charge Controller 150v. Trying to decide between outback, magnum, and morningstar. Leaning towards morningstar, has no fans, and has ethernet/http hookup for advanced config/monitoring.

    -Midnite SPD on combiner box and one inside tiny house for DC.


    I am prepared and hoping under ideal conditions for two days of autonomy (light usage) on battery without doing dishes or laundry, which is easy for a single guy.

    Any help, thoughts or inputs from the experts would be awesome and much appreciated. I really look forward to hearing from you!

    John



    Last edited by engage1000; 12-06-2016, 11:08 PM. Reason: Trying to stuff as much into one post as possible, won't let me do two posts.

  • #2
    Welcome

    Forget trying to heat water, or cooking with electric. That's what propane is for, Or the generator, You might get away with the occasional use of the vacuum cleaner.

    The SPD's belong outside the house, why invite Lightning indoors.

    Winter is going to be the problem with low sun. You will need more panels I believe.
    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


    • #3
      Thanks Mike!

      I do cook with propane, but the tiny hot water heater was a judgement call I made to get away from propane on demand. All units except the bosch have a danger of freezing when it's cold outside (even though the unit is mounted indoors) due to cold air convecting through the direct vents. They seem to rely on heating elements during the cold weather that burn a couple hundred watts to prevent the exchanger from freezing. The Bosch unit needs no electricity but needed a big B vent thru the roof and uses indoor air for combustion, and I already have a wood stove and a composting toilet using air. Space was a factor too. I'm stuck with the hot water tank, it's already in.

      You see SPD's mounted on e panels indoors and so forth, which is interesting based on what you say. What's the best way to implement an SPD?

      I checked a solar insolation chart for my area... 2.13 hrs for winter and 3.98 for summer. How much bigger would you recommend, Mike?

      Comment


      • #4
        OK I see a few issues you need to think about. First the easy one. SPD you really do not have any use for. SPD aka TVSS in reality can only offer limited protection from the utility for which you are not connected to. They do not offer much or any protection from a direct lightning strike. If you take a direct strike the SPD will be just a source of fuel for the fire you invited inside your home for dinner. You will be on the menu titled roasted engage. If you insist on isnstalling one DO NOT INSTALL IT inside silly. The whole idea is to keep lightning from coming inside on your homes wiring igniting it on fire. I assume you intend to Ground Your System and that significantly increases your chances of being struck by lightning. FWIW a 24 volt system is not required to be a grounded system.

        Do not go crazy with AC Breakers, your Inverter is not capable of delivering enough current to operate a Thermal Breaker. GFCI is your best bet for protection and requires no ground to operate effectively. If I read your material list correctly you have over looked the most important over current protection device you will need. The only thing you have that can deliver extremely high current is the battery. You need Fuses terminated directly on the battery Term Post, one looking toward the Controller, and another feeding your Inverter.

        Now let's talk about your Inverter size vs battery size. You batteries are rated 335 AH @ the 20 hour rate. Yes they can supply up to C/4 @ the C/6 capacity (260 AH / 4 hours = 65 amps). So what that means is your batteries can only support up to a 24 volt x 65 amps = 1560 watts or a maximum of a 1500 watt Inverter reliably. You are pushing your luck with 2000 watts as that will require up to roughly 110 amps of DC current.

        Another note on your battery size 24 volts @ 335 AH only yields you roughly 1.6 Kwh per day usage for maximum battery life. That gets real expensive with AGM batteries. In the long term a AGM battery is 400% higher in cost than FLA. AGM's cost twice as much and last half as long as FLA. You wil end up paying some 10 to 20 times more for power than the POCO would charge you. In the end in battery cost at replacement time, electricity will end up costing you $1 to $2.00 per Kwh where the POCO charges 10 to 15 cents per Kwh. You do know that and what you are asking for right? FLA can bring that cost down to 50 cents.

        I agree with Mike, dump any electric heating, cooling, and cooking.
        MSEE, PE

        Comment


        • #5
          SunKing,

          Thanks for your reply!

          Good to know about the SPD’s, I thought they were advantageous in a lightning strike. I will omit these from the system, but when it comes time to set up some panels and the outdoor combiner box, would it be prudent to have a DC one set up out there?

          As for grounding, I was just going by what the magnum inverter manual calls for, which is to ground the AC and DC systems to the trailer frame (no mention of a ground rod, and had no intention of doing so). Magnum claims this is ideal as it reduces interference, and the unit apparently has an isolating transformer so the inverter supposedly won’t get fried with the common ground. What would you do in this case?

          As for DC safety, my list was not particularly clear, sorry. I am planning on Class T fuse (good to know it should be on the terminal, but ahead of the shunt?), and high amp (400+) marine battery disconnect switch.

          Getting to the batteries, I see your point with that kind of math to support such a load. It is feasible to fit 400amp agm surrettes, which based on your math of C/6 capacity of about 320ah / c4 = 1920 watts, which gets me much closer. But someone else mentioned something about the startup watts of the fridge coming on at the same time as the hot water heater, so I’ll have to check the startup wattage of that and see where this all ends up, it seems like it would stress even the 400ah bank.

          Those batteries would be a tight fit. I would have to stack two on two, horizontally. Do you think they would be ok sitting on their side (the long side), with 3 pieces of 1/2” strapping between the bottom one and the one sitting atop? Would the cases take it? They won't go in any other way.

          You also raise a good point on the wear and tear and economics of the batteries. If I can get two days out of the 400amp bank at 70-75% DOD, which I think I can especially with even a tiny bit of solar coming in on a cloudy day and the above being worst case usage, that will net me 1000 cycles according to surrette.. which is about 5 years of use. I am okay with that scenario, but wiping the batteries out in 2 years or so wouldn't make much financial sense.

          The only other option is to put a large flooded bank outside in an insulated enclosure. I can get the enclosure real close to the wall that the inverter is on to minimize the run, and I could insulate it really really well.

          What do you think of having a flexible insulated pipe going from the house to the box with a thermostatically controlled fan to pump room temperature air into the box when needed? The house will always be warm. I would ensure to have a one way flap in that system and a vent at the top of the battery box. The bank and box setup would need to be able to weather a potential -35C. Has anyone done this before?

          Comment


          • #6
            What about using a DC water heating element, like 600 watts @ 24v? with a new thermostat? or a smaller 1000watt 120v element?

            https://www.amazon.com/dp/B0006IX8AA..._t2_B00KO32AB0
            https://www.amazon.com/Watt-Submersi.../dp/B00KO32AB0

            Comment


            • #7
              Lot of questions, I will try to answer as many as I can with the time I have.

              Let's start with grounding because I do not think you have your head wrapped around it. Not to worry, many electricians do not know a lot about Grounding either. This is what I think might be throwing you off is the term Grounded System and Grounded have two completely different meanings.

              At 24 volts you are not required to have a Grounded System. OK Grounded System means one of the circuit conductors either positive or negative in a DC system is bonded to Earth or something in place of earth like the frame of a Trailer. With a Grounded System only requires you to use only one Over Current Protection Device (OCPD) like a fuse or breaker on the Ungrounded Circuit Conductor. Example you bond the Negative Battery Post to your trailer frame, and use a fuse on the Positive Terminal Post. If this were an AC system the grounded circuit conductor is referred to as Neutral, and the ungrounded circuit conductor as Hot. Same applies here.

              In a Ungrounded System or Floating System as it is sometimes called you do not bond any circuit conductor to ground. However it requires you to use a OCPD device on each polarity. That means one on the positive and negative term post on the battery. What you gain doing this is eliminate unnecessary outages from ground faults on the Hot Conductor, and keeps you from being shocked if you touch any of the circuit conductors. OK that is Grounded Systems.

              However you are still required to Ground your Equipment whether your System is Grounded or NOT. All conductive surfaces must be Bonded. Bonded is a much better word than Grounded because Grounded does not have any real meaning. We use that term to confuse Russians, Chi-Coms, and the dreaded DIY's. It even confuses electricians. Heck I know engineers who do not have a grasp on it. This will confuse the crap out of you. Ground or Earth has no function in a NEC application, and you are a NEC application. It is forbidden for you to use earth as part of a circuit. It is not capable of doing anything for normal operation. Earth is to protect you from what is outside primarily the utility, and secondary is a planned path for Lightning should it strike. You want to divert high voltage utility and lightning to earth to dissipate instead of come inside and finding a path to earth burning everything up in it path like the wires inside your walls or expensive charge controller.

              OK you have a unique situation like that of a RV with very limited tight spaces near steel frames. Batteries do not tplay we ll in tight spaces with steel frames. Thus I recommend you use a Grounded System. Makes OCPD simple and cheap. So you bond your system to the frame of your trailer, and then drive two ground rods 16 feet apart and bond that o your trailer frame rigght next to or as close as you can to where you bonded the System to the Frame. Here is the secret to make it all work.

              Where ever you bring the outside DC cables into the trailer is where you want to make all your ground connections, and if you use a DC SPD gets installed all right there together using the Frame as your Single Ground Point. Keep it all as close together as possible underneath the trailer. Think of it as a Window with a 6-inch Frame Stay inside the window.

              Here is a decent diagram for illustration purposes showing both Floating and Grounded System



              Here is a physical method of how you do that with a Trailer





              MSEE, PE

              Comment


              • #8
                It is generally agreed by most solar professionals that a SPD does provide a fair amount of lightening protection. see test video https://www.youtube.com/watch?v=vkeFYuHxJoU In addition to this video I know for a fact that my Midnight Solar SPDs kept my stuff from frying last year during a lightening strike in the front yard. Several neighbors lost a lot of electronics but I only lost the Dish TV antenna box because it has a separate outside ground. Since this has happened twice in the last few years some of the neighbors have now mounted a SPD or two on their grid tied breaker panels.

                ​I agree that you are a little lite on the batteries and you do want to make sure the fumes vent outside. That combined with the fact that the inverter will sometimes hum pretty loudly makes me wonder if you may be better off mounting a weather tight box on the outside of the house to hold these things. The batteries will produce some heat while cycling so you probably don't need to vent heat from inside the house.

                Comment


                • #9
                  Originally posted by John Galt 1 View Post
                  It is generally agreed by most solar professionals that a SPD does provide a fair amount of lightening protection.
                  Sure they do, added profit. You like all your neighbors are connected to the Grid. There is no SPD that will protect you from a direct strike. If you have damaged electronics from a nearby strike, just means your home is wirign system is imporperly bonded and configured. grounded.

                  The type of SPD that can survive a direct strike are not really available to home owners, they are just to big and expensive for a home owner. A large home unit are average size of 100 joules which is no ta lot of energy in 8 uS. Good enough to protect you from a utility fault, but useless on a direct strike. Commercial units by Telco, Govt, Disp, ect start at 1000 joules. Cel Towers use EMP which is a SPD on steriods and make to survice a nuclear air blast which makes lightning look like a Bic Lighter.

                  SPD's can help with utility faults, distant lightning strikes, and anything that might be generated inside, but are no match for a direct strikes. My bet is you are not connected to a utility, so a SPD is not going to do much for you other than line the pockets of the salesman who sold it to you. However if you do use them at least install it in such a way it actually works. On top of a house with a long downlead just completely renders them useless. A direct strike will will split between the down lead and your 2 circuit conductors going inside your house as they have roughly the same impedance as the down lead going to the same spot.If wired per 2014 NEC code completely disables any SPD's of any size or quality.
                  MSEE, PE

                  Comment


                  • #10
                    Back to your battery size. Technically your batteries can deliver a 1C current of 335 amps for about 30 minutes until dead .Sounds great until you realize your voltage drops from 27 volts to 13 volts. Your Inverter tripped off-line at 23 volts despite the fact your batteries are fully charged. The issue is the battery Internal Resistance, cable resistance, and connection resistance. Since Voltage = Current x Resistance you can clearly see as you increase current you develop high voltage losses. With Low Voltage system you want to keep those losses to 3% or less, pros shoot for 2%. Well 2% of 24 volts is 1/2 of a volt. That 2% includes all losses, Battery Sag, Cable, and Connection. So you design for 2% voltage on the battery and 1% on cabling between Battery and Load. The bad news is for low voltage to get high power means very high currents. Depending on Inverter effficiency and power losses, a 2 Kwh 24 volt Inverter requires 100 to 130 amps. I am not even going to discuss what size cable that requires over 5 feet long. If longer than that you are screwed.

                    OK 100 amps on a 24 volt battery with 1% sag means you need a battery with a Ri = .24 volts / 100 Amps = .0024 Ohms. How big of a battery is that? Depends on the battery. Generically we can say for FLA is C/8, and AGM is C/4 but there are exceptioms as some batteries like those made for UPS and Telecom can do 1C. But not a true deep cycle battery. Quality manufactures will give you Ri either directly or buried in specs in discharge curves you have to calculate. If you do not know how to calculate from discharge curves you are SOL. Rolls AGM is one you can tell as they list it right on the SPEC SHEET. Tell me what it is? Go find it. Or click-drag your mouse below to find the hidden answer.


                    Did you come up with .0016 Ohms?
                    Last edited by Sunking; 12-07-2016, 10:04 PM.
                    MSEE, PE

                    Comment


                    • #11
                      John,

                      I guess I hadn'’t considered that the inverter might make a bunch of noise doing it’s thing. I guess if one were to have an outdoor battery system, it would make sense to have the inverter in the same enclosure (but separated with a divider) and then run AC from there into the sub panel inside. I'’m starting to like this idea. Do you think the magnum would mind the cold?

                      Sunking,

                      I won'’t be on utility shore power for more than a few months, and then that will be it. I guess at this point it would maybe only be worth having one near the future solar array, attached to the combiner box outside?

                      Ok, back to battery. I looked up the spec and for both the 335 ah and 415 ah AGM’s they have an advertised internal resistance of 1.6 m Ohms which works out to .0016 ohms. Ok, so I take it from this that the best I can glean from a 415 amp AGM bank short term is around 2000 watts, but what can it burst for 5 minutes if needed to to help start a fridge?

                      Am I also correct in assuming that eight 6v 375ah flooded batteries would only yield the same scenario, in that the FLA would need 18kw total capacity to vaporize 2000 watts?

                      Alright, if I downsize the heating element in the water tank to below 1000watts, do you think a bank of 415ah AGM’s @ 24 volts would do the trick in my usage scenario with pv and gen support if I was economical with it?

                      What do you think about putting the whole shoot’'n’'match outside, including the inverter and run the AGM's, and then later add on another string of matching AGM’'s 6 months down the road… I'’ll have the space then if I do the outside thing and a C/4 rate around 4000 watts.


                      I really appreciate everyone's replies and help!!
                      Last edited by engage1000; 12-07-2016, 11:17 PM.

                      Comment


                      • #12
                        engage, The Magnum 4024 is rated for temps down to -4F. Since it makes some heat while operating in an enclosure even without insulation it could probably work well at -20. But keep in mind that while the inverter needs to be in a weather resistant enclosure it will need some ventilation for cooling with heavy loads. The batteries are a different matter when it comes to cold weather. FLA batteries aren't hurt by the cold, if anything cooler batteries will last longer than hot batteries but cold batteries can hold less power so it's probably best to try to keep them above 50 degrees to maximize their storage ability during short winter days. see chart here http://cms.cdnrg.com/images/uploads/...re_Effects.pdf

                        ​Up to you but I concider $170 for 2 decient SPDs (one on the AC side and one on the panel side) to be cheap and effective insurance. I agree that nothing is going to protect your system if your home gets blasted by lightening but if that happened you'd have bigger problems than some burnt electronics. With your tiny house you will have no problem putting in a solid fairly straight path to ground SPD. Keep in mind that Sunking works on huge industrial systems and from what he has posted here has little experience actually using a residential solar system.

                        Comment


                        • #13
                          Originally posted by engage1000 View Post
                          I won'’t be on utility shore power for more than a few months, and then that will be it. I guess at this point it would maybe only be worth having one near the future solar array, attached to the combiner box outside?
                          You figgure it out. As an engineer and desiggner I would not waste your time or money on a SPD. Unless you can Install the thing at ground level using buried cables with direct access to the Ground Electrode System, it wil not do anything except fail and need replaced.

                          Originally posted by engage1000 View Post
                          Ok, back to battery. I looked up the spec and for both the 335 ah and 415 ah AGM’s they have an advertised internal resistance of 1.6 m Ohms which works out to .0016 ohms. Ok, so I take it from this that the best I can glean from a 415 amp AGM bank short term is around 2000 watts, but what can it burst for 5 minutes if needed to to help start a fridge?
                          Good for you. You actually looked it up. I also have it in plain text for you. I guess you did not find it, but it is there, just hidden in plain site.

                          Correct .0016 Ohm's. A fridge or any motor does not surge for 5 minutes, more like 5 to 10 cycles or 0.2 Seconds

                          Originally posted by engage1000 View Post
                          Am I also correct in assuming that eight 6v 375ah flooded batteries would only yield the same scenario, in that the FLA would need 18kw total capacity to vaporize 2000 watts?
                          Yes each battery is wired in series, so the resistance adds. If you have 4 batteries in series and each battery has .0016 Ohms' then you have a total of 4 x .0016 = .0064 Ohm's. 100 amps x .0064 Ohms = 0.64 volts sag if you draw 2000 watts

                          Originally posted by engage1000 View Post
                          Alright, if I downsize the heating element in the water tank to below 1000watts, do you think a bank of 415ah AGM’s @ 24 volts would do the trick in my usage scenario with pv and gen support if I was economical with it?

                          What do you think about putting the whole shoot’'n’'match outside, including the inverter and run the AGM's, and then later add on another string of matching AGM’'s 6 months down the road… I'’ll have the space then if I do the outside thing and a C/4 rate around 4000 watts.
                          Never ever mix old and new batteries. End of story. It it gets cold in winter keep in mind battery capacity collapses when cold, does not hurt them but you loose capacity Example at 77 degree is 100% rated capacity, at 0 degrees down to 50%

                          See if this helps. You will not hurt your battery if you draw even say 300 amps. So do not worry about that. Now you rcable may melt, but your battery wil not care about that. Let me give you two scenerios that are very real and you wil experience.

                          1. Say it is morning, sun iis up and your batteries are fully charged up and reading 27.2 volts an the Controller is in Float Mode. You turn on that water heater and Microwave over on at the same time, say 3000 watts. You wil draw 160 amps at 24 battery.. When you turn everything of th evoltage sags to 26 volts, your Controller turns the panels on to full power. Lets say you a 1000 wat panel. The Panels supply 50 amps and the batteries supply 100 amps for 150 amps total. Life is good and everything works well.

                          1. Crap you forget to take a shower come home after dark, your battery is fully charged up, and reading 25 volts. You turn on that heater and microwave oven, voltage sags to 22 volts,you add 1 volt for cable losses, and hit the Inverter with 21 volts. Current is screaming at 170 amps. Your Inverter Trips Off line from low voltage and you get Ice cold shower and frozen coffee. Ironically your battery is still fully charged up and nothing is really wrong other than the system is poorly designed and installed. You might even get 5 minutes run time before the Inverter trips. The Inverter is doing what it is suppose to do, protect itself from over current and save your batteries. As voltage goes down on the Input of the Inverter, the Inverter draws more current to make up for the voltage loss. It can only go so far before it self destructs from over load. So it says screw you I am shutting down.

                          Here is the moro of the story. Everything has to be matched up. It is real easy for us to tell. We see it many times each day. Someone comes here with a 100 watt panel, 10 amp controller, 12 volt 100 AH battery, and a 2000 watt 12 volt Inverter and wonders why it does not work. We laugh a lot and tell them they need 10 more panels, 10 more controllers, and 10 more batteries and they cry and leave. To dang bad. Get over it.

                          Lets work backwards from the Inverter and work our way back to the panels. Lets say you have a 24 volt 2000/4000 wat Inverter. If you use a FLA battery requires a 24 volt 800 AH battery and if AGM can go down to 400 which may not work well depending on the manufacture of the AGM battery.

                          OK a 800 AH battery needs at least 80 amps of charge current, so you need a 80 Amp Controller. To generate 80 amps of charge current takes 2000 watts of panels. Take notice Panel Watts = Inverter Watts.Breaks down like this. I do not care what voltage battery you use being 12, 24, or 48 volts. Pick your poison. The Industry has standards and 80 amps is pretty much max for all charge controllers. That means you hit a wall. It takes a 800 AH battery no matter what the voltage is. So you hit the power limit wall vs battery voltage:

                          12 volts @ 1000 watts. For me I use 500 watts max at 12 volts. 12 volts is a Toy system.
                          24 volts @ 2000 Watts
                          48 volts @ 4000 Watts.

                          Makes no difference when you hit the limit, they all use the exact same battery AH capacity of 800 AH anyway you spin it. Thaty is how I can tell in less than 1 second if your system is right or not. Takes no math, tablets, pencils, laptop or even any thought. Like going 80 mph in a car and you look up and see a concrete wall 50 feet in front of you. No reason to think, hit the brakes, or be scared, you will not feel a thing, you are dead. Case closed.

                          So when I see someone with a 2000 watt 12 volt Inverter, I just laugh and break their hearts. So if you have a 3000 watt Invert, it had better be at least a 48 volt battery. You are walking the line at 2000 watts @ 24 volts. You have no room to grow and need max 800 AH capacity to be fully functional. You might get away with 400 AH using AGM. You chose the lifestyle and have to live with the limitations or as I say the consequences of your actions.

                          Good Luck to you.
                          Last edited by Sunking; 12-08-2016, 04:04 PM.
                          MSEE, PE

                          Comment


                          • #14
                            Thanks very much for your thoughts and inputs, after the last few posts I have a much much better understanding of the theory of sizing, and battery capacities in terms of energy storage, demand and supply.

                            After having re-aligned my expectations and assumptions, and changing what I plan to run, I think I can live with the disadvantages and limitations. I have to keep all of this in perspective as well, as essentially I am living cabin life and I have no illusions about trying to replicate a grid-connected lifestyle. I also have budgetary constraints… a 48v system would be great, and to have that extra “delivery” wattage also costs considerably more in batteries, perhaps inverter, but also in generator as I can’t find a 48v inverter that features load support. Do you know of any?

                            Space constraints are also a huge huge factor, I can’t cram 800ah of anything in that place, it would all have to go outside, inverter, charge controller, batteries, etc. And then a 2kw array vs a planned 1200 or so, it all adds up.

                            That being said, if I put the whole shoot’n’match outside in it’s own “shed”, it would be safer if something bad happened and it burned down. (or inverter maybe can go in a weather proof box mounted to the outside of the building)

                            Does anyone know of an RV style plug disconnect plug that can handle the 60amp line that the inverter calls for if I put it in a separate hut? I know there are two leg 50amp ones. It is a tiny house after all and would like to have the ability to disconnect the “power shed” if need be.

                            So, back to re-aligning expectations and caps on future growth. Load management:

                            -No illusions about running dishwasher or washing machine without having the generator support. They won’t be used much.

                            -Hot water. Have it on a switch, and have command of when it is allowed to turn on. Run a 1000watt AC element, they are readily available. Devise solar collector such as evacuated tubing system w/ antifreeze and tie into the hot water tanks exchanger capability. The tank is designed to be hooked up to a boat engine and cycle coolant through it.

                            Alternative heating element is a 600w or 900w 24v DC element with proper thermostat. This wouldn’t work with the battery bank too far away I guess, but would minimize conversion losses and help with inverter search mode. Any feedback on this would be great.

                            This is the tank I have… http://www.thechandleryonline.com/pr...roducts_id=139


                            -Have plans for generator autostart with the magnum. Does anyone know if there is a consumption set point in the system that will kick the generator on? Ie, inverter only until 1800 watts and then the generator is set to engage?

                            -Fridge running wattage should be minimal, and surge should be momentary. It will be changed out to a DC danfoss fridge sooner rather than later and is a temporary fridge.

                            Updated worst case scenario of running wattage on battery only:

                            Pump - 80w
                            Fridge - 250w (I’m guessing, I haven’t metered it yet)
                            Computer - 25w
                            Lights - 50w
                            Hot water tank - 1000 w

                            Factor in some losses and I guess I’m around 1500watts, depending on the what the fridge turns out to be.

                            Sunking… I need a couple more days to absorb your grounding tutorial, it was informative!

                            Thanks!

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                            • #15
                              Assuming I set up an evacuated tube type collector to heat water, it might end up being practice that the hot water tank gets cycled once a day on electricity if there's no sun using generator load support. I don't think this would end up being too costly with a small honda inverter setup.

                              I like the evacuated solar collector idea.

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