Grid & Battery Bank sizing

stmar
stmar Solar Expert Posts: 370 ✭✭✭
If you have a fairly reliable grid and do not intend to sell power back to the grid, how critical is your battery bank sizing? Batteries will be in FLT state most of the time. My dilemma is what do I spend my $$$$ on: 400 ah ($2440) single string 4 X 6 volt; 500+ah ($2800) parallel series 4 X 12 volt (existing bank); 440ah ($2800) or 600ah ($3600) parallel series 8 X 6 volt. These are AGM batteries and I realize you could save money on other types.

SW4024, 12 X 64 Watt Solarex panels grid tied system. The PVX2580 12 volt batteries replaced a bank of 8 golf cart batteries so the original bank was in the 400 range. I do not know if increasing the ah was beneficial for this application. We are fairly conservative on consumption, our grid power bill runs in the $60 range and $22.50 of that is the base charge. Some of our usage is straight from the grid and does not go through the inverter, I figure about 200KWH per month through the inverter system. Power outages are not of long durations, most of the time just a few hours and when that happens we cut back on consumption even more. I am trying to get as much information as possible to make my choice, battery bank is at an age where it may not be reliable when needed.

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Grid & Battery Bank sizing

    If the only thing you need it for is critical load back-up for occasional brief outages the battery bank needs only to be large enough to handle those loads over the expected time.

    You should not be thinking in terms of kW hours per month but per day, because you are expecting the outages to be less than 24 hours. 200 per month is roughly 6 to 7 kW hours per day, which is rather a lot. So you should reduce the load expectation to just what you need to keep running. This makes for a smaller, less expensive battery bank that will not be such a problem to replace (it is going to die from time rather than use). Keeping a refrigerator and a few other necessities going can be handled with around 2kW hours per day. Power 'going through' the inverter when the grid is active is not relevant to the battery bank size.

    Twelve 64 Watt panels would produce about 24 Amps on a 24 Volt system, which is normally enough for 240 Amp hours but as a maintenance charge it would handle double that. That's probably why the original bank was sized around 400-440 Amp hours. The 768 Watt array could be expected to provide perhaps 1600 Watt hours AC on a good day. At 'full usage' that would fit with the 240 Amp hour bank, but again this is a back-up system.

    It sounds to me as though the 400 Amp hour choice is the best fit here.
  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    Re: Grid & Battery Bank sizing

    Thanks, that is what I was looking for. I am thinking the 400 ah 4 X 6 volt bank is the way to go as you suggest, simple configuration and easy to replace/maintain.

    "Power 'going through' the inverter when the grid is active is not relevant to the battery bank size." So in theory my battery bank may be shot and I really would not know it until an extended outage? Also if the bank is deteriorated the inverter would use more grid power to try and charge, correct?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Grid & Battery Bank sizing
    stmar wrote: »
    Thanks, that is what I was looking for. I am thinking the 400 ah 4 X 6 volt bank is the way to go as you suggest, simple configuration and easy to replace/maintain.

    "Power 'going through' the inverter when the grid is active is not relevant to the battery bank size." So in theory my battery bank may be shot and I really would not know it until an extended outage? Also if the bank is deteriorated the inverter would use more grid power to try and charge, correct?

    Correct. When the grid is up any loads connected to the output of the inverter are powered by the grid. The batteries could indeed be dead, just enough to hold the Voltage without having any real Amp hour capacity, and it wouldn't be noticed until you need them. This is why it is recommended that you test the system regularly by disconnected the grid power and letting the batteries run the loads. You'll be able to tell how long they'll last, and of course once the grid is reconnected they'll charge back up.
  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    Re: Grid & Battery Bank sizing

    My inverter was down for a few months and the bank ran a Sun Frost RF-16, that was the only load on it so they did have some life but I would not trust them in a real emergency. Since the inverter has been repaired we have had very short duration outages so no real test. I have to do some maintenance shortly so will give them a test without too much expectation, lol. Thanks again.
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
    Re: Grid & Battery Bank sizing
    ... If you have a fairly reliable grid and do not intend to sell power back to the grid,

    I do not see the purpose in having solar panels then. Your system could simply be a large UPS. Charge your batteries from the grid.

    When you say that your grid is fairly reliable, what are we discussing? It goes down monthly? or once/year?

    :)
  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    Re: Grid & Battery Bank sizing
    I do not see the purpose in having solar panels then. Your system could simply be a large UPS. Charge your batteries from the grid.

    When you say that your grid is fairly reliable, what are we discussing? It goes down monthly? or once/year?

    :)
    True, we are using it as a UPS. The panels were already installed when we bought the property. They do supply charging in a outage and during the day they supply power rather than the grid unless the demand is too great. The grid goes down a couple of times per year, usually a few hours (1 - 6), but a few years ago it was down for 3 days and we appreciated the UPS system.
  • jebatty
    jebatty Solar Expert Posts: 56
    Re: Grid & Battery Bank sizing

    For uncommon outages, wouldn't a generator sized at least to satisfy the critical loads and a manual transfer switch be the most economical way to handle your situation? For super critical loads, just a small UPS to cover until the generator is fired up. That's what I have on my system.
  • Organic Farmer
    Organic Farmer Solar Expert Posts: 128 ✭✭
    Re: Grid & Battery Bank sizing
    jebatty wrote: »
    For uncommon outages, wouldn't a generator sized at least to satisfy the critical loads and a manual transfer switch be the most economical way to handle your situation? For super critical loads, just a small UPS to cover until the generator is fired up. That's what I have on my system.

    That is a very common lay-out for homes in my township: a manual transfer switch and a generator.

    Some homes have two generators, a small one that can run all day on a gallon of fuel, and one large generator that is capable of running the well and appliances. For those homes, they can keep the lights and radio on all day. Then they run the big generator for one-hour each day, so they can flush the toilet and run laundry.

    I guess it depends on how often you lose power. We rarely lose power more than 4 times a month, but we never go a full month without loss of power.



    In the case of the OP, where he came into it with a previous installed solar array. I can see the purpose in having them to charge a battery-bank.
  • stmar
    stmar Solar Expert Posts: 370 ✭✭✭
    Re: Grid & Battery Bank sizing

    Just to clarify: I don't know how many of you are married but if you are you may understand. When my inverter went down I was of the mindset to install a Generac unit but my wife did not want to rely on gas/propane/fossil fuel (although we have a 1000 gallon propane tank and only use about 250 per year) and thought that solar was "free" and no matter what argument I presented it was not enough, lol. So to keep peace and since with a repaired inverter it was plug and play that is what I did, back to the original setup as when we bought the place. Now I am getting to the end of the battery bank life cycle, hence my original post.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Grid & Battery Bank sizing

    Since he's already got the inverter/controller/panels replacing the batteries is likely cheaper than buying a new generator and wiring it in.

    Besides which the back-up power will be instant and silent, which is often desirable.