Stageing a PV system, step at a time.

Peterr
Peterr Registered Users Posts: 18
'ello all... first post from a newbie so baaaarr with me..

Ok... future wanna be... grid tied with battery backup. Minimum requirements for existing refer & freezer: 31878 average daily watt hrs. (10.5 amps together)
Making, if I figured correctly, minimum of five, 205 watt panels plus its components. (Charge controller, inverter, shut off.. ect ect) A 2.1k grid tie, which would seem to be the minimum, even for a small 1000sqft home, needs 10 panels. I don't know. Our needs are not that high. We used 3,913 kWh for all of last year.

( Cannot conceive having the ability to produce solar electrical and then not be able to use it when the local power company goes down (APS Sedona Arizona USA) so eventually will want the system grid tied with battery back up. )

That's eventually, but this is now...
What I need is to start a system that I can gradually add onto. I'm handy and been in construction for most of my life and won't need a licensed solar contractor right away because it will be off grid to start up with. When we want to, if it gets to that, and if the rebate system is still in place, we can go official to get it. But now I want to start something that will not be 'temporary', as I have learned that is usually permanent. So even though it will cost more, I want to put the building blocks in place. One at a time and then add capacity.
It makes no sense, but you have to start somewhere, so here is the immediate need. We do a lot of dehydrating peaces, tomatoes, ect in the summer. We like that better than canning. Yet, I just can't stand hearing the fan working and resistance heat ticking the old electric meter as it does it duty, day in day out. Having moved here to my present location in '85', we use to just use mother nature and a humidity of less than 10% on home made drying racks, stacked out in the sun. So dry that when clouds came, we would awe in wonder. ;) Location then was Joshua Tree Ca., southern Mojave Desert. Now, (Sedona), this is not desert.. humidity is higher and trees grow by themselves here. There is even a creek to go to. Running water.. Awesome. ;)

So what I want to do is use solar. But I want to do it in a way that I can add on to it and perhaps build it into a grid tied battery back system as explained. I have now a very inexpensive 700 inverter (likely square wave output) that can charge up my cordless battery power tools, run the laptop ect when away from power. I would like at the start to use that inverter in this beginning situation.

Here at last are the questions.

1) The 400 watt Excalibur dehydrator should be able to take the inexpensive 700 watt inverter (Chicago Inverter from Harbor freight. Peak surge 1800w, continuous 700w, input 10-15vdc) I would think, as all it is, is a resistance elements and a fan, without it destroying it. Correct?

2) A 205 watt panel will give me way more than I need for power but will be the start of the PV system. Hooking up the temporary, all ready purchased inverter, up via one battery, thru a battery controller, should answer the immediate need. Correct?

Now future..

3) Can you add battery controllers in the system later efficiently and easily? Or is it better to get the size for what battery pack you think you will eventually use?
( Of course this depends on capacity, and how long you want your back up to last before it has to be recharged to capacity again, by solar, grid tie or generator)

4) What about imbalance of slowly added panels? The original type may have been discontinued, or the old ones, not as 'vibrant' as the new one added. This will have no effect I believe on the controller, but what of the future inverter and the efficiency of the system?

5) The Enphase Micro Converter would be an ideal solution, for the location of the panels will have to be placed at ground level and on a west facing hillside. The panels will be about 50'-100' from the meter panel. The PV panels of course will be facing south. There will be some shade concerns during the day both summer and winter. Telephone pole, big juniper tree, some elms to the south. But as I understand it, this Micro Converter has to be grid tied from get go. Is there another inverter that can start as off grid, then go grid tied and help with the above concerns?

Sorry for the length and thank you for any help or suggestions given... p.

Comments

  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    Re: Stageing a PV system, step at a time.

    I would look at the XW series inverters or Outback's.

    XW inverters:

    http://store.solar-electric.com/nexaxwseinan.html

    Outback inverters:

    I will let someone better informed that me to point you in the best direction.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Stageing a PV system, step at a time.

    before we get started here on your specific questions i must point out your mistake as your refrigerator and freezer did not consume 31878 average daily watt hrs. it most likely was 3,187.8wh/day. your total consumption by your yearly figure is about 10.72kw/day and that makes the 2 of them about 33% of your electric use. your consumption isn't terrible, but i'm sure some improvements can be made and doing something about the high frig and freezer needs is obvious.
    as to what you can do is have a whole house backups with the battery bank and a sine wave gt capable inverter/charger combo. the battery bank must be large to begin with because you can't add on with more later on years down the road. this will power dedicated circuits through the inverter such as refr/freezer and some lights. later on as monies allow the controller is bought along with some pvs and mounts with any and all respective breakers, combiners, wiring, etc. the pvs can be added as needed as long as the controller can handle the power through it. if too much power from pvs then another controller can be gotten.

    1> that is most likely a modsine inverter and motors and chargers don't like them with some lifespan being destroyed of the loads and a larger power consumption from dissipating the harmonic content of the modsine inverter. simply put, a modsine inverter will draw more power and could destroy what it's running faster with some chargers blowing instantly.
    otherwise it should work.:roll:

    2> it might work for a few hours as the watts in must be a good bit greater than the watts consumed due to efficiencies and other losses. this does vary depending on all of the items being employed in the system.
    btw it isn't a battery controller, but it is a solar charge controller or controller for short and is a more sophisticated regulator circuit.

    3> controllers can be added and some have hubs and other stuff to combine them and make them work in unison, but this isn't a problem in adding more as it can even be of different makes or models at times. the battery bank on the other hand as i've mentioned should not be added to down the road.

    4> with a backups style of a system the bulk can be taken care of by the utility ac and any pvs added will offset that need and could sell back the system excesses.

    5> that is straight gt only and is fine to have some of those if you wish as you put into the grid with them and then take it out later using the grid almost like a battery, but only when the grid is up and running. you indicated you wanted gt with battery backup so we are to assume times without utility power are present.

    i gather you are trying to go ultra cheap right up front and you may get something workable that way, but for future needs it's better to go the route i said for your indicated gt with battery backup.
    hope i covered everything and well enough for you.
  • Peterr
    Peterr Registered Users Posts: 18
    Re: Stageing a PV system, step at a time.

    Ok, thank you very much for the replies.. there is quite a bit to understand. I've only started getting into this concept and can already see changes I'll be making to help conservation of the power to decrease the need.

    The Xantrex XW looks like a good choice for an inverter in the situation that I'm describing.

    I'm going to have to research and understand battery backup better and its components.

    Off the top of my head the understanding I have now, is that you get more capacity work wise (amps) with the batteries in parallel. Higher voltage with them in series, which allows you to send the DC further with less loss, using smaller wire size. A configuration can be made with a mix of both, to give you the capacity and voltage that you want.

    So when you say "...the battery bank must be large to begin with because you can't add on with more later on years down the road." Is that because of the inverter? Because the inverter has to be matched to your choice of configuration, in voltage and capacity and it can't vary? Or is there some other reason that you can give me a clue of, and I'll do some homework. :D

    Also do I understand correctly that your saying that a backup system needs to start with the batteries? You can take one PV panel and charge them. Adding controller, more PV's and the equipment they will need later on as monies allow?

    And yes.. for the curious.. the power reliability here since they put in the new feed lines years ago, is very good. Very rarely do we lose power. But I'm loving having the time to learn something new, while we conserve and become more sustainable.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Stageing a PV system, step at a time.
    Peterr wrote: »
    Off the top of my head the understanding I have now, is that you get more capacity work wise (amps) with the batteries in parallel...

    No. Whether you place 4 batteries in series or 4 batteries in parallel--the amount of work is the same. With batteries in parallel, you get more available current (same voltage). With batteries in series, you get the same available current, but higher voltage.

    From the basic electrical equation:

    P=I*V (power in watts = current times voltage)

    You can see that P is the same if you have 4x the current or 4x the voltage.

    The confusion is working with Amp*Hours... It sounds awfully close to "work" (Power * Time)--but it is not. It is missing the "Voltage" part of the equation.

    Work=P * Time (for electrical work, time is in hours
    Work=P*H=I*V*H=Watt*Hours

    Amp=I*H=Amp*Hours (we always have to ask people what their batteries and battery bank voltage is to know figure out the Watts and Watt*Hours)
    Higher voltage with them in series, which allows you to send the DC further with less loss, using smaller wire size. A configuration can be made with a mix of both, to give you the capacity and voltage that you want.
    Sort of correct. You choose the voltage and current you need to run your loads. Some loads are fixed voltage (like automotive accesories are designed for 12 volts). Other items can be obtained in different voltages (12, 24, 48 volts are common for Solar RE inverters and charge controllers).

    More or less, below 60 volts DC is considered relatively low voltage and "safe" (sort of save--a lot of current behind anything is dangerous).

    For wiring your system up... If you get much above 100 amps--the size of the wires, fuses, switches, and power components become very large and expensive.

    So, if, for example, you need 1,000 watts of 120 VAC; a 12 volt battery bank will only need ~100 amps to supply the power (P=V*I=12V*100A=1,200watts).

    If you need 4,000+ Amps, then you are better off with a 48 volt battery bank... 100 amps at 48 volts will give you 4,800 watts of power.
    So when you say "...the battery bank must be large to begin with because you can't add on with more later on years down the road." Is that because of the inverter? Because the inverter has to be matched to your choice of configuration, in voltage and capacity and it can't vary? Or is there some other reason that you can give me a clue of, and I'll do some homework. :D
    Several issues here... Lead Acid batteries work best if they are charged around 1/20 to 1/10 of their rated load. A 100 amp*hour car battery sized deep cycle storage battery will be very cost supplying ~5-10 amps continuously.

    If you discharge a battery at a very heavy rate (say 1/5th to 1/1 of Capacity)--it will get hot, have low output voltage, and not a very long life (car batteries are made to supply heavy currents for a few tens of seconds to start a car--they are not made to "deep cycle" or provide lower amounts of current for long periods of time).

    So, when you are looking at an XW 6048 Hybrid Inverter (a very nice piece of gear)... The 6,000 watt inverter would need a minimum of 400 Amp*Hours (or more) of a 48 volt battery bank to operate correctly.

    400 Amp*Hours * 48 volts = 19,200 Watt*Hours of storage

    19,200 WH / 6,000 watt inverter load = 3.2 hours of storage

    So, even with a "minimum" battery bank defined for a 6kW inverter (lots of power)--is only a C/3.2 discharge rate--very heavy.

    However, if your home is energy efficient--your peak loads may only be 1,000-2,000 Watts -- You don't have to use the maximum inverter output for your home/emergency power (but it is nice to have a large inverter if you have a well pump or A/C unit that needs large starting power).

    So--battery banks to last a long time (both to supply power for 3 days without sun) and to not cycle too deeply and fail sooner (typically 50% deep maximum after three days of no sun)--means your battery bank should be sized to supply ~6 days worth of load (for long life and cost effective installation). If you draw a lot of power in one day--the bank needs to be 6x larger--which gets surprisingly large to power a whole off-grid house with modern appliances and lighting.

    The other issue is that batteries age (time and cycles). When you place strings of batteries in parallel--they all "see" the same voltage (if the wiring is done correctly). If you have a 5 year old string, and put another new string in parallel--the new batteries will accept much more of the load/charging than the old batteries... So--very roughly--the new bank will quickly "cycle age" until it is roughly in the same condition as the original bank (and the new batteries will fail sooner than you would expect). Mixing batteries in parallel strings of different models/chemistry/ages only makes this aging problem worse.
    Also do I understand correctly that your saying that a backup system needs to start with the batteries? You can take one PV panel and charge them. Adding controller, more PV's and the equipment they will need later on as monies allow?
    You really need to define your needs... Say you really need backup power (ice storms, rural area that gets power restored last after a winter storm or summer brush fire, etc.)--You can sort of do some of the stuff "backwards"...

    You can install the XW 6048 + Batteries + Backup Genset first--your home will be on a "giant" UPS with batteries for "short outages" and a genset for longer outages.

    A while later, you can choose to add banks of solar panels + 60 amp charge controller as your funds allow (~3kWatts at a time per new charge controller).

    Otherwise, you install, for example a solar panel + GT inverter first. Then later, you toss the GT inverter, install a Hybrid inverter + batteries + charge controllers, plus rewire your solar array for different output voltage (typically, panels for GT systems are wired for 200-600 VDC output; off-grid / hybrid systems are wired for ~70-140 VDC -- hence the need for rewiring).

    Basically, anything you try to spread out over the years with solar RE systems--ends up with you getting bit by having to chuck unneeded equipment as you migrate size/functions; deciding what to do with older but working batteries; and what to do for permits and labor (pulling new permits and paying to redo older work).
    And yes.. for the curious.. the power reliability here since they put in the new feed lines years ago, is very good. Very rarely do we lose power. But I'm loving having the time to learn something new, while we conserve and become more sustainable.
    I too would like a XW6048 Hybrid system... But the reality is that my 3 kW GT system is "cheaper" to install and has virtually zero maintenance costs for the next 20-40 years (probably would need 1-2 replacement GT inverters in that time--but they are not that expensive or difficult to replace).

    A battery backed system means monthly battery maintenance (AGMs cost more but need little servicing) and bank replacement every 10-20 years... The last multi-day power outage in our area was 50+ years ago... Just got a simple Honda eu2000i gas genset + 20 gallons of stabilized fuel changed once per year (plus cars with a siphon) to run our emergency loads (the "big one" for us is probably an earthquake). That will keeps us fine for a couple weeks--if needed. After that, we would probably be looking at evacuation if things were that bad.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Stageing a PV system, step at a time.

    peterr,
    basically batteries should not be added later because age differences and differences due to usage even on identical batteries that are years apart will cause the rest of the bunch to be dragged down to the level of the weakest link so to speak. they should be about the same age and be the same make and model numbers. even when you do that some discrepancies can cause some batteries to differ from others. having good battery interconnections using bigger wire for this than is normally used in the largest portion of the entire system helps along with cross placing the charge and load connections. example would be on paralleled batteries (or paralleled strings) that the + of the farthest right battery for the controller + connection while the - of the farthest left battery be used for the - controller connection. the inverter can be on the opposite unused farthest end posts by the far right battery - and the far left battery +.
    do keep in mind these should be quality batteries of fla or agm types, but not mixing those types of course. the xw will have a minimum of 400ah at 48v if i recall correctly and more would be better, but will depend on the loads you present over the day and how much reserve you wish to employ as well. do keep in mind that deep discharging a battery takes away some of its lifespan and so we never recommend (except in an emergency) to exceed 50% depth of discharge or dod so only use up to 1/2 of a battery's capacity.
    there is much you need to know and i ask that you start searching and reading here on the forum to gain further knowledge on the subject for we don't like to tell you what to do, but rather like to advise what we think you should as a guide.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Stageing a PV system, step at a time.

    As to your dehydrator, you can build a tin box, insulate it, and line it with corrugated roof material, painted black. That "warms the air" and you can pipe that into the base of your dehydrator. Using Solar PV to generate electricity, to heat something, is very expensive. Many ideas abound on this principle.
    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

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

  • StevenB
    StevenB Solar Expert Posts: 71 ✭✭✭✭
    Re: Stageing a PV system, step at a time.

    Niel and Bill,
    Is there any condition where a second bank of different batteries could be added?
    Example: I have 16 - 2005 C&D Used & Tested AGM 85 AH Batteries (actually 4 more yet to be added)....all from the same lot....set up as a 48volt bank. Connected to a Xantrex 6048 Hybrid.
    Could I set up a second bank of 2volt/high ah batteries, with a separate set of panels/turbines/controllers .....all hooked to the same inverter.

    Is this a reasonable way to add improved battery strategies, yet keep original setups until they play out?
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Stageing a PV system, step at a time.

    Unless you isolate the two banks,, such that as they feed the inverter so that they won't feed from one another, you will have, in effect, one large battery bank with the requisite problems.

    I suppose that you could, for example use a isolation switch to draw from bank A: for a time then switch to bank B: for some other time. You could probably do this electronically or you could just use a A:B battery isolator switch.

    Tony
  • StevenB
    StevenB Solar Expert Posts: 71 ✭✭✭✭
    Re: Stageing a PV system, step at a time.

    So if the power went out, when I am away...it would only draw from whatever battery bank was activated? No reasonable way to feed two separate banks to a single Inverter?
    Anticipating a No answer....I believe you can "daisy chain" multiple Xantrex inverters through the Xanbus connectors....is that the proper solution to manage multiple battery banks?
    The bank I have now, as far as my limited knowledge can tell, works ok.........but I may want to improve the strategy but don't want to throw away a functioning bank.
  • Peterr
    Peterr Registered Users Posts: 18
    Re: Stageing a PV system, step at a time.
    You really need to define your needs...
    Basically, anything you try to spread out over the years with solar RE systems--ends up with you getting bit by having to chuck unneeded equipment as you migrate size/functions; deciding what to do with older but working batteries; and what to do for permits and labor (pulling new permits and paying to redo older work).
    Hmmm... does look like I'm trying to cross different concepts and force package them into a quick, one thing does it all, solar package with no clue.
    Yepa.. dats ame... dreams like a newbie, talks like a newbie, dinks like a newbie. :blush:

    So developing an efficient PV system, added on to over time, step by step, does not look practical and feasible. It appears you have to have an up front, sophisticated, fairly complete understanding of solar first, then make the commitment. Otherwise, because of the expense of the technology, your gonna shoot yourself in the foot. Not like rainwater collection. ;)
    All I can say is; Nuts!

    Thank you all for the clarity.
    We will study more to refine and define our needs. Look to see if its necessary, or there may be other ways to achieve a similar result. Find out if its affordable, in time, money and commitment.

    Thanks again... this is a great forum.

    p.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Stageing a PV system, step at a time.

    You could draw down one battery bank to a defined voltage,,, have a voltage controlled controller (?) trigger a relay to switch from one bank to the other.

    Say the first bank would run until it was say 12.0 volts under your expected loads. When the voltage dropped below 12.0 for a defined time,, it would trigger the switch. Pretty simple in theory,,, probably not too hard to do in practice.

    Tony
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Stageing a PV system, step at a time.

    Steven,

    The farther the batteries are apart (in age, chemistry, flooded/sealed/agm size, etc.) the more likely they are to have issues with balanced charging and discharging.

    A 1/10th of a volt difference is ~5-10% difference in state of charge...

    Of course, if this is what you have, and the "price is right", and you would not be heart broken if they did not share well or have optimum life... Go a head and try it. The worst you have to loose is you need to recycle some of the batteries earlier than would have been in an optimum setup.

    Make sure you have recommended protection (does anyone here place one large fuse per bank string?) and you can monitor the current with a large DC current clamp/meter to make sure the battery strings are sharing appropriately.

    The reason I ask about per string fuses/breakers/fusible-links is that if the string Amp*Hour ratings are significantly different is that, for example, if you have a 100 AH string and a 1,000 AH string--any shorts on the 100 AH string (battery cell, wiring, etc.) could be extremely dangerous without protection devices (100 AH string short is fed by 1,000 AH string).

    For example, a 100 amp*hour string and a 500 amp*hour string... One would hope to find that the 100 amp*hr string would be supplying 1/6th of the current and the 500 amp*hr string would be supplying 5/6ths of the current to the load/charger.

    If you have two different battery types (flooded cell and AGM)--then you would have to disconnect the AGM's from the string when you do equalization of the flooded cell string every month or so (AGM's should probably be lightly equalized every ~1 year).

    Monitor the state of charge of each string/bank and see how they do track together. You may be pleasantly surprised. You might not get the 100% life from each string--but you may meet your needs with used/old/recycled batteries fairly well.

    Peterr,

    People do "grow their systems" --- For example, a small cabin with small solar to live in while buidling the main home... Keep the small solar for the cabin or sell the system to friend or on EBay.

    The reason I suggest to define your "final" system design--is you can buy the smaller system components which can be migrated to the final system and plan wiring runs/layout to reduce the problems with redoing old work and having to EBay unneeded hardware (may be able to pull permits for the entire wiring run/roof mounts, but add panels and inverters later as cash allows).

    But when you redefine requirements from Grid Tied to Off-Grid/Hybrid--there is little commonality possible (other than the solar panels themselves--if purchased such that they can be rewired to meet both requirements).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Peterr
    Peterr Registered Users Posts: 18
    Re: Stageing a PV system, step at a time.
    But when you redefine requirements from Grid Tied to Off-Grid/Hybrid--there is little commonality possible (other than the solar panels themselves--if purchased such that they can be rewired to meet both requirements).

    As stated earlier I can see no reason to have the expense of PV solar system and not be able to be off grid. Net metering, which is in place now from our local power provider may change, for better or for worse. Earthquakes here, are minimal, a cause of awe, if we feel one at all. Drought and fire, are more of a concern. No system will help if your house is burned down, but the feed lines in could be destroyed for a while.
    ...you can buy the smaller system components which can be migrated to the final system and plan wiring runs/layout to reduce the problems with redoing old work

    Sounds like the step progression I was referring to, but now I understand its more than just one item or two items at a time.

    Can't you have a Off-Grid/Hybrid inverter to start with that was not yet grid tied? I now understand you would have to have your minimum battery set up, so the inverter would run properly. I don't yet understand how many batteries that would be, but when the time came and you where ready to expand, you then could get the full group you needed, and chuck the 'old' ones as a need fulfilled for the time duration used.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Stageing a PV system, step at a time.

    Yes, you can very nicely have an off-grid or Hybrid inverter that is not grid connected (charging/alternate power) or Grid Tied (back-feeding AC power).

    Pick a high end unit... The Xantrex XW 6048 Hybrid Inverter...

    For a minimum system using the XW 6048, you would need 400 Amp*Hours of 48 volt batteries, and an AC input (for charging/supplying power to the protected loads)... It would work just like a "giant" whole home UPS. You might be able to reduce the batteries by somewhat if, for example, you limited your power output to 2kWatts maximum. However, fewer batteries in the bank will mean that they will not last as long.

    Create a protected subpanel where you will wire the circuits where you need emergency backup power (fridge, freezer, some lights, TV, radio, washer/gas drier, etc.).

    Next, you may want an alternative power source for longer power outages--remember that a minimum battery bank will last:

    400 amp*Hours * 48v * 85% invtr efficiency * 1/6,000 kWatt output = 2.75 hours (or less) at with 6kW load (really should run less than 1.5 hours if assuming 50% maximum discharge).

    In reality, you will probably be running 1-2kW average load (fridge, freezer, some lights, TV, washer/gas drier, etc.) -- so you could expect ~5-10 hours with a minimum battery bank.

    Now--the next step is to add backup power to last longer than 5-10 hours... Genset or Solar Panels.

    Minimum Genset--perhaps a Honda eu2000i or Yamaha equvilant with electric start (possibly natural gas or propane)... Will certainly be the least costs (if genset is allowed by your local code--fuel available, etc.). If you use gasoline or propane--you may have supply problems (no electricity at service stations, no fuel deliveries during weather emergencies)--so can you store enough to be of use to you (days/weeks of storage?--keep genset and power usage low to stretch fuel--no A/C for example unless emergency/medical requirements). Natural gas may be a good alternative (earthquakes/landslides could knock natural gas out for us). Works during cloudy weather/no sun (night). Connect genset to AC2 input on Hybrid Inverter. The internal battery charger of the inverter will recharge the battery bank and pass AC power to your loads.

    Minimum Solar Panels--how much "minimum power per day" do you need? Fridge+lights+TV = ~2-3kWhrs per day. Would need, very roughly 1-2 kWatts of solar panels (depending on where you live, summer/winter sun assumptions)... Still may wish to get a minimum sized genset with some backup fuel for true emergency use to charge during the day, run off battery at night.

    Sizing the system up... The two major areas are Battery Bank Capacity and Solar / other DC charging sources.

    If you up-size the system with solar panels (up to ~7kW maximum)--then you could reasonably connect your grid tied back-feed to utility power (if it makes economic sense to you). You may have gotten the original permit / utility company OK to even connect when you added your minimum solar panels with some sort of maximum solar panel (10kW or less) defined. Utilities may "cough" if you try to connect for GT feed-back without solar panels connected (many do not want you to buy cheap power at night and sell it back during the day at an expensive price if you have Time of Use metering).

    If you upsize the battery bank--you may need to replace your first battery bank, plus rewire for larger bank. By itself, the larger bank will be of limited use if you do not have solar panels to drive it (you still will need backup genset/utility power to recharge the bank--you will just have more days of no-gen / no-utility power support. The normal rule-of-thumb around here is to design for 3 days of no power, and 50% maximum discharge = 6x your daily load. As you add battery capacity, you can also shift more of your loads to your protected AC load center.

    You can add more solar panels with out batteries--but that usually only makes sense if you are grid tied... Having lots of panels and a small battery bank means that you will have little space to store your excess solar PV production -- and/or you will be cycling your batteries deeply and wearing them out faster. But the system will still "work".

    One thing that people with TOU metering always wonder is can they buy power at $0.09 per kWhr at night (cheap time), and sell it back during the day at $0.30 per kWhr from the battery bank (with or without solar panels)... Many utilities disallow this load/generation shifting explicitly... The other issue is that you are cycling your batteries (wearing them out faster) by doing this. One time, I did a simple cost analysis that assumed a twenty year system life (battery replacement+inverter+charger) and guessed that to time shift power would cost (very roughly) $0.45 per kWhr--or more than just buying the expensive power during the day (due to system cost and wear & tear).

    Any way, some food for thought...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • StevenB
    StevenB Solar Expert Posts: 71 ✭✭✭✭
    Re: Stageing a PV system, step at a time.

    May I have permission to reprint this on a web site? It's basically what I've done. But I can't imagine it more concisely put.
    We have a local load as our barn where our freezers are....and we have a heavy duty extension cord to connect to the house for any real necessities. We have a line connection too, the cord is just for power outages to run from the barn. But basically we are trying to protect our food supply (garden) when we are away from home.

    As I understand it, at the battery voltage threshold I set the inverter,the Inverter "inverts" to the local load (the barn), and since the freezers are "energy saver" types, the power often surpasses the local load and "sells" to the house grid.
    Does that sound about right? I have no idea how and why Inverting and Selling distinguish themselves from each other. Can you talk about that too?

    By the way, on the East Coast USA most utility companies don't have peak and low rates, so Load Shaving with the Xantrex doesn't work out. But I could never see how you'd save more electricity money than battery degradation/replacement.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Stageing a PV system, step at a time.

    Steven,

    Sure, you can reprint/reformat as you see fit... If you want to be very nice, just point back to the forum or thread as source... Thank you for asking.

    The issue between "Grid Tied" and "Off Grid" inverters...

    The Off Grid inverter works just like an AC genset. As long as the inverter output is "on", it will try and push power out to the loads.

    If you connect an Off-Grid inverter or AC Genset to the Utility Grid directly... They will "fight" with the utility. If the Utility Power is on--you will probably blow breakers/fuses/inverters/wiring. They do not "sync" together.

    If you want a "UPS" (uninterruptable power supply) type function--there is a device called a "transfer switch"... The Transfer Switch may be automatic or manual--but is usually a double pole double throw switch that connects the load to the AC Utility Mains, or to AC alternate source (inverter or AC generator). In no case, will the alternate AC source be able to back feed the utility grid (and kill a line man or cause fires/etc. at nearby homes).

    A Grid Tied Inverter does not contain an internal Transfer Switch... A GT inverter is designed to be synchronous with the AC mains (with tight frequency, voltage, and current specifications). Basically, the AC Utility Grid behaves for all the world like a giant AC Battery. The GT Inverter sync's with that AC Battery and just pushes out current (energy) that follows the AC grid (not to say that is is a "simple" product). If the AC voltage, frequency, or current fall out of specifications for the GT Inverter, the GT inverter simply shuts down and waits for five minutes of "good AC Mains" power before it tries to reconnect and generate power.

    A Hybrid inverter does a bit of both... When the AC power is up--it is in GT mode and will attempt to "sell" power to the grid (may even turn your meter backwards if the GT inverter output is greater than your home's AC loads). If the AC mains fail, an internal AC transfer switch disconnects from the AC grid and the inverter switches to Off Grid mode and supplies power to the "protected" load box.

    The Hybrid Battery Sell function basically monitors the battery bank voltage (say 52 volts DC)... If the bank tries to go up to 52.2 volts (making numbers up here--example assumes battery bus 52+0.00 volts is 0% inverter output, and 52+1.00 volts is 100% inverter AC output) because of solar panels, wind turbine, water turbine, DC generator; the GT inverter says, lets try outputting 20% of our designed load AC load. If the battery bus voltage continues to rise to 53.0 volts--the inverter tries to output 100% of its designed load... The batteries see an average of 52-53 volts (~13.0-13.25 volts for an equivalent 12 volt battery)--so it is getting slightly charged (or "float" stage) and can stay at this voltage forever and the batteries will stay at 100% state of charge.

    When the sun sets, the DC charge current drops to zero amps, the battery voltage will drop to 52 volts--and the GT inverter will stop "selling power". And if the battery voltage continuous to drop--the internal (to inverter) AC to DC battery charger will begin to charge the battery bank itself.

    It, at any time the AC mains fail, the Hybrid inverter will immediately drawing enough power to maintain 120/240 VAC at 60 Hz (as long as the battery bank is above ~42 volts--which would be a "dead" bank).

    And, if you get the autostart option for an AC genset--you can program the XW hybrid inverter to start the genset and use the AC2 power input to both supply charging current to the battery bank and AC power to the AC loads (to make things more complex--I believe that Xantrex HW inverter can also, if the genset frequency/voltage is not real stable, will use 100% of the AC Genset power to charge the batteries and use the DC battery power to make "clean" 120/240 VAC power---I am not completely sure of this function--it may not behave exactly in this manner--but the XW inverter can take variable AC genset power--where other inverter chargers will not accept the out of spec. generator AC power).

    Regarding the whole Time Of Use metering thing (commonly available on East/West costs--I believe)--Watch out for the term "Smart Power", "Smart Meter", and "Smart Grid"... This whole idea is based of "forced" conservation using variable price/availability power plans.

    In our region (Northern California)--We are getting close to even flat rate residential being forced to "Smart Power/Meter/Grid"... Basically, they will drop the price of power a few cents per kWhr--but ~15 weekday days out of the year (2pm-7pm), with 23 hour notice, the utility will jack up power from a minimum cost of ~$0.10 per kWhr to a flat $0.60 (residential) or $0.75 (commercial) per kWhr.

    So, by addressing conservation and possible alternative power sources--you are ahead of the game (for now).

    There are other ways of addressing the GT/Off-Grid issue but it is even more of a complex solution (basically set up an off-grid 4 kW inverter+batteries; the Off-Grid inverter runs the loads--and the GT inverter is connected to the Off-Grid inverter's AC output. The GT inverter will share the loads, or even drive energy back into the Off-Grid inverter and start charging the battery bank--works fine--but issues with safety and charge control).

    Ask if you want to discuss this option too (trying to keep things from getting too confusing).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset