Double Check My Numbers

MichiganOffGridMichiganOffGrid Registered Users Posts: 11
A few months ago I had come to you guys to double check some of my numbers. (Here) We had a set back and had to postpone our plans to build. Now that we are ready to move forward again, I am revisting my solar plans. Could you look over my plan and let me know if you see something that I should change? My biggest question is if I have enough panel to recharge this size battery bank. Do I have the right size charge controller? Do you have any recommendations for a size/type of inverter? I havn't purchased anything yet, so any changes are easy to make.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Double Check My Numbers

    Hello again.

    By my estimation it is as you suspect: insufficient solar panels.
    For one thing, your estimated daily AC Watt hours use is 8715 whereas your estimated panel production is only 7560. And that's without any system derating. That 7560 Watt hours will be more like 3780 in practical AC terms (based on the rule-of-thumb nameplate rating * equivalent good sun hours /2).

    For another thing, the array is likely to produce only around 50 Amps @ charging Voltage, and you have a very sizable battery bank. By my calculations it won't meet the 5% minimum recommended charge rate.

    In what may be a first for the forum, you might be over-estimating your usage. If you can, get a Kill-A-Watt and take some actual readings of things, especially the refrigerator and freezer. My 16 cubic foot refrigerator uses about half of your estimate.

    With some reevaluating of use, you could possibly leave out one of those strings of batteries. You would still need more solar panel, however. About 2950 Watts worth in my estimation. At that you'd be running the FM80 at its maximum from time to time. That is not a bad thing.

    If you're going with the Outback charge controller, consider using all Outback equipment. It makes integration of components easier (all OB stuff can be tied together with a Hub and controlled/viewed through a MATE).
  • PhotowhitPhotowhit Solar Expert Posts: 5,022 ✭✭✭✭
    Re: Double Check My Numbers

    I don't think your too bad, Looks like estimating @1050Wh of use and you have 1800+ watts of panels, Your battery bank is over sized, likley if your starting from scratch, a single 48 volt bank would be my prefrence. More later puppy must be walked!...

    Back, Ooops I was reading the bottom as a total, too many years doing estimates...

    Somethings definatly hinky with the freezer numbers unless your running a walkin, freezers typically run 1/2 to a third of what fridges do. check with the company on expected hours, also might see if the electronics can tolerate below freezing temps and place the freezer in the garage, saving cooling in winter when charging is lowest.

    You would still be slightly below the 5% off the top of my head, perhaps invest in 3 more panels. If your willing to grow your system out, this might not be too bad of a starting place.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • MichiganOffGridMichiganOffGrid Registered Users Posts: 11
    Re: Double Check My Numbers

    Thank you guys for the recommendations. My intent at the end of my previous thread was to use 8 T105s. I have a wife and two kids, and I know that electrical use grows, so I thought that I would move up to the next bigger set of batteries. Then I noticed that the Crown 395AH batteries are the same price as the Trojan 325AH. So, thats how I ended up with the bigger Crowns.

    So if I over estimated my batteries and under estimated my panels, I should drop a string of the crowns or go back to the T105s. Eight Crown 395s would give me 9480 usable watts. Twelve T105s would give me 8100 usable. If I add three more 210w panels that should take me up to 10080 watts per day. Am I working these numbers right?

    I am currently living overseas and my wife is living with her mom untill I get home. We don't have any of our appliances yet, so I can't use a kill-a-watt to measure our actual usages. I would rather guess high and end up with too much battery bank, then end up with not enough. But I like hearing that your freezer uses less then what I am predicting. I figured that the fridge and freezer were going to be the biggest loads on our system. The fridge and freezer are going to be in the basement to save space in our kitchen, so they should stay around 60 degrees ambiant temp.

    What size generator should I be looking at to recharge a battery bank this size?
  • PhotowhitPhotowhit Solar Expert Posts: 5,022 ✭✭✭✭
    Re: Double Check My Numbers

    If your planning on gas/solar water heater, yes, fridge and freezer will likely be your next 2 heavy users, I went out and got you a 14 Cu Foot freezer. Just kidding, I did get the numbers though. 14 CuFt I would consider a large freezer. It claims to use @ 357 kWh a year, my guess is at a 72 degree ambient temp, or lets say 1kWh a day.

    I defininetly like over estimating your energy use but 5X one of your heavy users will squew the numbers pretty bad. Also usage tends to go up in the winter months when we're stuck inside due to the weather just when there is less sun light, likely you'll have a solar isolation of 2- 2 1/2 hours in the winter months when, your running the TV more, computer more, more lights, radio (satalite TV?, DSL?)

    With out better numbers, perhaps you've got a reasonable starting system, with a 24v system, you can add panels in reasonable groups. I don't do generators, but others here do.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Double Check My Numbers
    What size generator should I be looking at to recharge a battery bank this size?

    The standard recommendation is to get a generator that can handle the loads and recharging at the same time. Wonderfully vague, isn't it? :roll:

    In reality you don't need gen output equal to inverter output capacity + charge capacity. For one thing, not everything runs at once and some loads are obviously "elective". Also, the panels will produce something even on cloudy days. Many of us get by with really quite small generators compared to system capacity. So unless you're going to have some really big "only with generator" power consumers you can probably manage with a gen about 2X your average load size. In other words if the typical load on the inverter is 1 kW, a 2kW generator will do.

    I'd shrink the battery bank by one string, add PV, and see if the loads can be reduced a tad through judicious use.

    Just my opinion. Others may have a different POV.
  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Re: Double Check My Numbers

    Ideally, you should aim at a 50% load on your generator system for better fuel efficiency... From battery charging only (if that is your primary use) or battery+other AC power (vacuum, well pump to cistern, shop tools, etc.).

    If you are looking at diesel, you are looking at 40-60% minimum loading to keep the diesel running clean and for long life (tend to coke/carbon/glaze cylinder walls at low output).

    And, you may even be looking at two gensets... A small. quiet, fuel efficient one optimized to charge your battery bank and a larger / cheaper unit for the few times you need a lot of AC power to run saws, pumps, as backup to #1, etc....

    Batteries tend to be the wild card in the system... Adding a lot of batteries really does not gain you much, and makes it much harder to quickly recharge them after a few days of bad weather, and a lot of money to replace every 5-15 years. Remember that lead acid batteries do not like to sit below ~75% state of charge for more than a few hours--After a day, you should really get them on a backup charging source so they do not sulphate and loose capacity (another reason that expecting >3 days from your battery bank without recharging is not a good idea).

    Also, I would plan on having savings to replace your major solar electronics every ~10 years (charge controllers, inverters, AC battery chargers, etc.). Things break and after 10 years, with modern electronics, it can be next to impossible to find spare parts or somebody to service the devices.

    Another set of reasons to keep your system on the "small side" and use a generator when you need power for shop tools / bad weather.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MichiganOffGridMichiganOffGrid Registered Users Posts: 11
    Re: Double Check My Numbers

    Ok. So I have taken your advise and cut the battery bank down and added three panels. This takes the output of the panels to 2520 watts. If I run the system at 24v this is 105 amps coming out of the charge controller. I am assuming this is too much for the FLEXmax80, so I will switch to a 48v system.

    I'm torn about which batteries to get. I could stay with one string of eight 6v 395Ah, but if I want to upgrade later I will be stuck buying a bigger second string of batteries then I probly need. If I went with two strings of eight 6v 225Ah batteries, then I could add a third pretty easily, but multiple strings are hard to balance. I know I'm not supposed to mix battery sizes. What are your recommendations?

    I've attached a new drawing with a few more numbers. How does it look? Am I doing my math right?
  • BB.BB. Super Moderators, Administrators Posts: 29,698 admin
    Re: Double Check My Numbers

    Looks good....

    Batteries are a tough choice. You could look at getting "inexpensive" batteries (say starting with two strings of golf cart sized batteries) and run them for 3-5 years, adding a third string if needed and see how it all works out.

    Might be a better choice that investing in an "expensive" battery set from day one.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Double Check My Numbers

    And now that you've gone to 48V you've also had to add a DC-DC converter for the booster pump. These are not very efficient. Might be cheaper and easier to just use an AC pump (if you haven't already bought the DC pump).

    I like the all-series battery configuration, fault finding will be easier.
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