Newbie Designing a System

RobW
RobW Registered Users Posts: 23 ✭✭
Hi all,

I've been lurking around a few weeks but this is my first post. I'm fairly new to solar but I've been fooling around with one of the Harbor Freight 45w kits to familiarize myself a bit. I want to get your inputs on a system design.

I've had a Kill-A-Watt measuring a load I would like to take off-grid. Here's the readings from the Kill-A-Watt:

120 VAC
39.34 kWh
226 hours measured

At the time of the reading the load was drawing 1.7 A and 194 watts. Dividing the 39.34 kWh by the 226 hours looks to be about 175 watts. 39.34 kWh/120 VAC = 328 A. 328 A/226 hours = 1.45 Ah. The load is fairly consistent across time. Of course, all these readings are AC.

Ok, I may be wrong already, but with these next steps I'm really not sure of myself. Here's my stab at it. I may be totally wrong from the get-go.

I'm thinking I'm looking at a system that will supply 2A x 24hours = 48 Ah. Bringing this through an inverter, at 12VDC I'm looking at 480Ah.

Before I go any further, am I looking at this correctly? How bad am I messing up these figures?

Rob
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System

    Welcome to the forum Rob.

    You're looking at a 175 Watt load running for a steady 24 hours? That is 4200 Watt hours per day. On a 12 Volt system roughly 350 Amp hours. This does not include conversion losses or powering the inverter. If you make those adjustments you get about 4800 Watt hours per day or 400 Amp hours @ 12 Volts. So you're not too far off the mark. :D

    But that is power used, not battery capacity. The battery bank would have to be at least twice that.

    And so we see how total Watt hours run up pretty fast even for small loads. Rather than try to construct an 800 Amp hour 12 Volt bank (which presents several problems) you'd do better to go up to 24 Volts for that much stored capacity. Even then you'd be paralleling two strings of four GC2's (440 Amp hours @ 24 Volts).

    Then the thing is that small capacity inverters (you don't need a big one for <200 Watts) are plentiful in 12 Volt, not so much so in 24. One choice: http://www.solar-electric.com/sa300wa24vos.html

    Do you have any direction you want to go with this in the future? A 24 Volt system would be easier to expand to more capacity.

    Oh and those HF 45 Watt panels are not going to come close to recharging this much battery.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Newbie Designing a System

    what coot said it valid, but here's another way to look at it.

    39,340wh/226hr=174.071w for every hour on average. over 24hrs is 4,177.699wh.

    typical max pv hours of operation/24hrs = 5hrs per day meaning the pvs are require to provide 835.54wh for the loads every hour. at an overall efficiency of .52 this means 835.54wh/.52=1,607w stc from pv.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    Yes, the load is a consistent 24 hours a day. Amazing how this small load quickly compounds!

    At this point, with nothing invested, all battery bank options are open. And I would like to keep it as future proof as possible.

    When I arrived at the 480 Ah figure, I wanted to confirm that before going further. That number sounded like money leaving at a higher rate than I anticipated. Just a bit more than my Harbor Freight kit!:D

    The 250w panel amp ratings I've seen run around 8 Impp. I live in an area where 4.5 hours of max sun. 36 Ah per panel pushes me up to 12 panels. With no fudge factor. If that's correct, I may have to start a bit smaller.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    Niel, I came up with those same figures but did not include the efficiency in my numbers. So that 1.6kW, that the required wattage for one 24 hour period. With a 3 day cushion, does that mean I need a pv setup of 4.8kW?
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Newbie Designing a System

    go ahead and play with the harbor freight stuff, but know it does not perform well. in the world of solar that stuff is truly a toy.

    if you were ever serious about a load like that you don't want to use harbor freight solar.

    do understand i was delving into the pv aspect and not the battery aspect. over 24hrs for the batteries you need to provide 4,178wh and so to keep the battery at or above 50% soc it needs to provide at least double that or 8,356wh. for 3 days that would be 25,068wh. to know the amp hours (ah) divide the wh by the battery bank voltage to get the ah capacity you'd minimally need. at 12v i get 2,089ah.

    hope i did this right. coot do you concur?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System

    If you were to go with the 440 Amp hour 24 Volt battery bank you're array would look something like this:

    44 Amps * 24 Volts / 0.77 = 1371 Watts.

    To check that against power usage:

    1371 Watts * 4 hours of equivalent good sun * 0.52 over-all efficiency = 2852 Watt hours AC. As you can see that won't cover the estimated Watt hours. If you had more sun or a bigger array it would be better.

    This is looking like a system much larger than my own which runs the whole cabin's needs every day. It is amazing how much small loads use up over time.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    I understand about the Harbor Freight kit. I didn't get it with any notion of using it for this load. It has helped me get my feet wet with solar.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    You are so right about those small loads adding up. The load I'm discussing consists of a 42" LCD TV, a DirecTv DVR receiver, a BluRay DVD player, and two 60w lamps. The TV is on probably 8-10 hours a day (I know but it's not for me). The DirecTv receiver is always on. The DVD player is not used often. The lamps (which I'm assuming are the biggest power hogs) are on almost all the time. 60w x 2 x 226 hours measured by the Kill A Watt = 27.1 kWh. Oh my! That's 70% of the power consumption. Let's take those lamps off the load and refigure.

    So now we're looking at 12kW / 120 VAC = 100 A. And 100 A / 226 hours = .45 Ah.

    .45Ah x 24 = 10.8 Ah.

    10.8 Ah AC is 108 Ah DC. This maybe a bit easier to work with.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System

    Ever seen this on the forum: Conservation, conservation, conservation! :D

    Couple of easy hints: you can adjust the video on your TV to reduce its power usage significantly. I dial 100 Watts off my 40" Toshiba by changing the settings.

    Lights @ 60 Watts. Ditch the incandescent bulbs. CFL or LED bulbs will reduce that power usage by about 80%.

    If you can get away with it, putting things on a power strip and really turning them off will save a few Watt hours too.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    Yep, conservation. I didn't even realize the load those lamps pulled until I calculated the numbers while writing that reply.

    So, let's say I go with a 24V battery system. I'm looking at roughly 60 Ah, correct? A 200-250 Ah battery bank would suffice? What would I want to use as far as my pv array to match that battery bank?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System
    RobW wrote: »
    Yep, conservation. I didn't even realize the load those lamps pulled until I calculated the numbers while writing that reply.

    So, let's say I go with a 24V battery system. I'm looking at roughly 60 Ah, correct? A 200-250 Ah battery bank would suffice? What would I want to use as far as my pv array to match that battery bank?

    Looking at 60 Amp hours on a 24 Volt system: 1440 Watt hours approximately.
    If you could reduce that a bit further and count on some of the power coming directly from solar during the day you'd be looking at a mimic of my system with its 232 Amp hour batteries.

    Frankly, I get away with a lot because so much of my daily load is during daylight hours that the batteries only have to carry them from charge stop to charge start the next day. :D
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Newbie Designing a System

    be careful that you don't confuse the ac amps and dc amps.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    I think I can reduce and go with a setup similar to yours. 700W of pv, right?

    What's an acceptable controller rating for this? I see you have a 60A MPPT controller. That leaves room for future expansion, I assume.

    175W panels rate around 6 Impp at 35V from what I've found in research. So I could parallel them and use a 30A controller. At this wattage, will the gains with an MPPT controller offset its price versus a PWM controller? I'm sure, for future additions, an MPPT would be the way to build this system for expansion.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    Niel, yes, I think you're right there. Let me look at my numbers again.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System
    RobW wrote: »
    I think I can reduce and go with a setup similar to yours. 700W of pv, right?

    What's an acceptable controller rating for this? I see you have a 60A MPPT controller. That leaves room for future expansion, I assume.

    175W panels rate around 6 Impp at 35V from what I've found in research. So I could parallel them and use a 30A controller. At this wattage, will the gains with an MPPT controller offset its price versus a PWM controller? I'm sure, for future additions, an MPPT would be the way to build this system for expansion.

    My (now discontinued) 175 Watt Sharps are 35 Vmp 5 Imp.

    At this size it makes sense to use an MPPT controller. Yes I bought the MX60 so I could increase capacity without having to replace it.

    The flexibility of array design is the main advantage to MPPT; you can use the "GT" panels which have a Vmp around 30 and cost less per Watt than "standard" panels. Almost all of the larger panels now are GT type. The lower per Watt panel price makes up for the higher cost of the charge controller.

    It also allows you to run the array at higher Voltage/lower current which can ease power loss due to Voltage drop in the wiring. Typically the run from the array to the controller is the longest and most subject to this problem.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    Alright, I think we're making progress. Here's what I'm thinking:

    3 250W GT panels
    30A - 45A MPPT controller
    250 Ah battery bank
    300W PSW inverter

    After doing a solar survey of my property, my best location for panels will be 50-60 feet from controller and batteries. With panels in series I can use #10 or #8 wire to controller. Inverter will be 80-100 feet from load, so #12 will work to 2A load. Much better wiring economy than I thought.

    Do these figures look correct? Anything I'm not including that I should be aware of?

    Thank you so much for your help. I've been running these numbers in my head for a couple of weeks but wasn't sure of myself. A few hours here and things are a bit clearer. I hope one day I can contribute something back.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Newbie Designing a System
    RobW wrote: »
    120 VAC
    39.34 kWh
    226 hours measured

    At the time of the reading the load was drawing 1.7 A and 194 watts. Dividing the 39.34 kWh by the 226 hours looks to be about 175 watts. 39.34 kWh/120 VAC = 328 A. 328 A/226 hours = 1.45 Ah. The load is fairly consistent across time. Of course, all these readings are AC.

    I think this is where you went wrong.

    Yes, you did get 328 Amp*Hours (don't forget the time)--But that was at 120 VAC.

    Most likely, you will be looking at 12/24/48 VDC ... So that 39.34 kWH:

    39,340 Watt*Hours * 1/12 volt battery bank = 3,278 AH @ 12 volts...

    If a pair of 6 volt @ 220 AH into a 12 volt 220 AH battery string:

    3,278 AH (at 12 volts) * 1/220 AH (at 12 volts) = 14.9 ~ 15 parallel * 2 batteries per string = ~30 golf cart batteries for such a load (over 226 hours).

    If you want to store ~20 hours of energy at 12 volts:

    3,278 AH * 1/226 Hours * 20 hours = 290 AH Battery bank (at 12 volts).

    And since we don't want to discharge by more than 50% (longer battery life):

    290 AH * 1/0.50 maximum discharge = 580 AH @ 12 volt battery bank

    Or basically support the load for 10 hours per night for two nights, with 50% reserve.

    Notice, we still have lots of losses, so to account for them, I would be suggesting a larger battery bank...

    Anyway--Before I go into the details--I just wanted to make sure you understand that Amp*Hours are at a specific voltage (typically the battery bank voltage)... So amps at 120 VAC all of a sudden become 10x as large when measured at 12 VDC on the battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    Thanks for the detail, Bill. Niel brought that up and I did not take that into account with my initial numbers. Great explanation. I see the difference.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System
    RobW wrote: »
    Alright, I think we're making progress. Here's what I'm thinking:

    3 250W GT panels
    30A - 45A MPPT controller
    250 Ah battery bank
    300W PSW inverter

    After doing a solar survey of my property, my best location for panels will be 50-60 feet from controller and batteries. With panels in series I can use #10 or #8 wire to controller. Inverter will be 80-100 feet from load, so #12 will work to 2A load. Much better wiring economy than I thought.

    Do these figures look correct? Anything I'm not including that I should be aware of?

    Thank you so much for your help. I've been running these numbers in my head for a couple of weeks but wasn't sure of myself. A few hours here and things are a bit clearer. I hope one day I can contribute something back.

    Well I don't think you're going to find 250 Amp hour batteries. You kind of have to tailor that to what's available and then adjust the array for best recharging accordingly.

    750 Watts of array on a 24 Volt system should provide about 24 Amps, which would be good with the proposed battery size (+/-). You could use the East Penn PS2200 6 Volt batteries at 232 Amp hours, for example, or else you would probably have to go up to a much larger size (I can't keep all battery sizes memorized anymore for some reason :p ).

    One problem with using three 250 Watt panels: they would need to be all in parallel, and the Vmp is likely too low for a 24 Volt system (like 30 instead of 35). If you used four slightly smaller panels you can make two parallel strings of two in series so the Vmp of the array will be high enough to assure proper charging. As a rule, pairs of panels always works out better than odd numbers (although not always).

    If you are looking to expand this later, going with a larger controller now might be a good plan. Otherwise Rogue's new 3048 MPPT controller is out, and MidNite has a 30 Amp unit on the way too.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    The number of panels make sense. I've seen some 185w to 215w panels that would work well. But for the price, four 250w are not much difference, and gives me a bit of headroom, as does a higher amp controller.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System

    That's the spirit! :D

    I've yet to hear anyone complain about having too much recharging capacity. Besides, the good controllers can be current-limited if necessary.
  • simon87
    simon87 Registered Users Posts: 6
    Re: Newbie Designing a System

    Thanks for all the hints and advise. I as another newbie read them with interest and appreciate the tips!
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    After taking a look at those lamps this morning, the incandescent bulbs were not 60W but 75W. 75W x 2 x 226 hours = 33.9 kW. 39.34 kW (kill a watt reading for entire load) - 33.9kW (lamp load) = 5.44kW. Without the lamps I'm now looking at a load of 5.44kW. That's more than half of the 12.1kW load coot and I were discussing.

    5,440W (power over 226 hrs) / 226 hr = 24Wh
    24Wh / 120VAC = 0.2 Ah
    0.2 Ah x 5 (for 24V battery bank) = 1 Ah
    0.2 Ah x 10 (for 12V battery bank) = 2 Ah

    Figures for a 12V system:
    2 Ah x 24 hrs = 48 Ah daily.
    48 Ah x 3 days storage = 144 Ah

    Figures for 24V system:
    1 Ah x 24 hrs = 24 Ah daily
    24 Ah x 3 days storage = 72 Ah

    One 250 W pv panel:
    250W / 30V (Vmp) = 8.3A
    8.3A x 4.5 hrs (max sun hours) = 37.35 Ah
    37.35 Ah x 2 250W panels = 74.7 Ah

    Preliminary System Specs:
    2 250W pv panels
    20A or greater MPPT Charge Controller
    100Ah or greater 24V battery bank
    200W or greater PSW 24V inverter

    Remember all these figures are from a newbie. I'm throwing them out there for your corrections and criticism. I may be off line for a large portion of the day.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System

    Just to straighten out the terminology a bit:

    Two 75 Watt bubs is 150 Watts. Times how many hours per day = Watt hours.
    It's important to work up the consumption number in terms of Watt hours per day.
    In this case you were running the K-A-W for a long time (226 hours) and got 33.9 kW hours for the bulbs in that period, which works back to 150 Watt hours per day. That would mean one hour run time per bulb.

    Now let's replace those with LED's and drop 60 Watts from each. You now have 26 Watts and 26 Watt hours. A lot less than 150. :D

    Your 39.34 kW hours is over 226 hour period, or 174 Watts average per hour. Times 24 = 4.177 kW hours per day. It is best when making this kind of measurement to check the daily average each day, in order to detect peaks that need to be accommodated.

    But it would not be unreasonable, after factoring in conversion efficiency and inverter consumption, to view this as 4.8 kW hours DC per day. Besides it makes the math easier. :D

    On 12 Volts that is 400 Amp hours used, meaning the need for a very big battery bank requiring 80 Amps of peak current to recharge and a lot of cheap batteries in a messy arrangement or some more expensive batteries to reduce the parallel wiring. That is for one day. If you want days of autonomy it will be worse.

    One thing you can skip on is "3 days autonomy" that you will hear brought up in some quadrants. Don't do it; you end up with a much larger system than you will need most of the time and it increases the risk of the batteries spending too much time at a low state of charge, thus speeding up the sulphation which will kill them.

    If you plan the average usage to be 25% DOD then you have another 25% to go before reaching the 50% SOC low end. That gives you two days, especially a there nearly always is some charging and if the weather goes bad you can control the loads to some degree. On day three you start the generator. Designing a system to operate with no generator can be done in some areas, but the more inclement weather you're up against the less practical it becomes.

    The easiest way to store up 4.8 kW hours is on a 48 Volt system (200 Amp hours minimum), but it can be done on 24 as well. The sticking point with this system is that the loads are relatively small (174 Watt average) but because of the run time the Watt hours add up to a lot.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    Thanks, Coot. I think what you're saying is that I'm overestimating the loads the lamps contributed to my readings. To get accurate numbers, I'm going to run the load through the K-A-W again, sans the lamps. Then I'll have some hard numbers to calculate with. Sound like a good idea?

    By the way, the two lamps have been reduced to one. The other was a victim of me moving everything around to get to the receptacle and K-A-W. Wife not happy with K-A-W. Or me.
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    Bill,

    Some math questions for you from your previous response. These questions probably show my great ignorance, but I'm trying to get a grip on designing a system. The questions follow.
    BB. wrote: »
    Yes, you did get 328 Amp*Hours (don't forget the time)--But that was at 120 VAC.
    Most likely, you will be looking at 12/24/48 VDC ... So that 39.34 kWH:

    39,340 Watt*Hours * 1/12 volt battery bank = 3,278 AH @ 12 volts...

    3.934 kWh is 39,340 Wh - got it.
    1/12 volt battery bank =.0833.
    Why are you using this figure? I understand the 12V for 12V system, but how does this fit the equation? (Not questioning its use, just trying to understand why. Probably something simple I'm missing) What would you use in calculating a 24V or 48V system? 1/24?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System
    RobW wrote: »
    Bill,

    Some math questions for you from your previous response. These questions probably show my great ignorance, but I'm trying to get a grip on designing a system. The questions follow.



    3.934 kWh is 39,340 Wh - got it.
    1/12 volt battery bank =.0833.
    Why are you using this figure? I understand the 12V for 12V system, but how does this fit the equation? (Not questioning its use, just trying to understand why. Probably something simple I'm missing) What would you use in calculating a 24V or 48V system? 1/24?

    Watt hours divided by system Voltage = Amp hours. So 39340 Watt hours divided by 12 Volts equals 3,278 Amp hours.

    Bill used multiplying (* symbol in algebra) by the division reciprocal (1/12) instead of dividing by 12. The result is the same.

    On 24 Volts it would be half that: 39,340 / 24 = 1,639 Amp hours.
    On 48 Volts it would be one quarter the 12 Volt amount: 39,340 / 48 = 819 Amp hours.

    Note there is actually a slight improvement in system efficiency as the Voltage goes up: the more the power is delivered by Voltage the less delivered by current which equates to less becoming heat (waste).
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    I see. My algebra days are far behind me.

    I've scaled back a bit. Here's the numbers I'm working with.

    Load: 120 watts per hour at a bit over 1 amp. (nice numbers for 120VAC) Usage: approximately 6 hours per day.
    720Wh and 6Ah, all at 120VAC.

    I want to run the numbers on both a 12V and 24V battery bank.

    Can you guys walk me through designing this system, from solar panels to charge controller to battery bank to inverter?

    I'm going to forget everything I think I know and follow along with all of you.:blush:

    Thanks!
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Newbie Designing a System

    So the new spec is 720 Watt hours on 120 VAC.

    Now here's where it gets interesting. To provide this from batteries requires an inverter which has both a conversion factor and self-consumption rating. These numbers are dependent on the particular inverter. But since the load average is small (120 Watts) let's use the numbers for a small (300 Watt) inverter: 90% conversion efficiency and 6 Watt power draw.

    So the 720 Watt hours AC becomes (720 / .90) 800 Watt hours DC, and the 6 Watts for 6 hours to run the inverter adds another 36. Total: 836 Watt hours DC.

    Divide by 12 nominal and you get 69.6 Amp hours. Call it 70. So for 25% DOD average you'd go with a battery bank 4X that size: 280 Amp hours. And that's when you say "darn, there are no 280 Amp hour batteries!" There are smaller ones, which means you'd have to scale back power usage even more, or there are bigger ones, which means you'd have slight over-capacity.

    So you might choose Crown 260 Amp hours http://www.solar-electric.com/cr260am6vode.html and discharge a bit deeper (27%). Or you might choose Crown 305 Amp hours http://www.solar-electric.com/cr305am6vode.html and have more reserve. (Just as an example; not a specific recommendation.) If some of the load will be used during daylight hours you can make use of power from the panels that would otherwise go unrealized. If it is all nighttime use such as lights this isn't possible; all power most be stored first.

    But let's say you go with the 260's. Now again if it's all night use you can actually go for a smaller array because there is no need to supply power to loads concurrent with charging. But usually you try for 10% peak current on a good day, which just happens to be a short cut to a viable system (not that you will see that much current always or even ever).

    So then you multiple 26 Amps times ...

    17.5 Vmp of "12 Volt" panels for use with a PWM controller. You get an array of 455 Watts. You have to adjust this number, preferably up, to available panel size. For example four 130 Watt Solartech http://www.solar-electric.com/solartech-spm130p-s-n-130-watt-multicrystalline-solar-module.html panels.

    12 Volts nominal (preferred lowest system Voltage) for use with an MPPT controller to get 312, then apply the standard derating of 77% (divide by 0.77) to get the array size of 405 Watts. (Notice the small difference in array size between the two controller types; this is due to the MPPT being able to make use of 'Voltage overhead' to provide additional charging power. It is less than 10% difference.) Again you want to "round up" to panels available. For example two 215 Watt Kyoceras http://www.solar-electric.com/kyocera-kd215gx-lfbs-215-watt-polycrystalline-solar-panel.html

    Note that you will need a 30 Amp charge controller in either case. The PWM type are readily available (Morningstar, Xantrex), the MPPT in this size are not so easy to come by (the Rogue is available, MidNite's version has not been released yet).

    At that point you start thinking about component placement, wire runs (sized for current and Voltage over distance), and fuses et cetera.

    So there's the basic process. Confused yet?
  • RobW
    RobW Registered Users Posts: 23 ✭✭
    Re: Newbie Designing a System

    After following the discussion here:
    http://forum.solar-electric.com/showthread.php?21269-24v-battery-charging-with-12v-system

    And revising the load I'm wanting to supply, I have this: a daily load of 870Wh. Using a K-A-W, I've got 174W * 5 hrs. Those 5 hours will be primarily at night. Now, I want to assume I go with a 24v battery bank with a couple of the Wal Mart 27DC deep cycle batteries, rated at 109Ah. Please check my figures.

    In the discussion linked to above, BB shared this formula:
    Battery Ah * charging voltage * 1/0.77 * 1/0.05 = minimum watt array

    BB gave 14.5 for charging voltage of 12v system. I assume (?) I can double that for a 24v system.
    109 * 29 * 1/0.77 * 1/0.05 = 205 min watt array
    So that's the minimum watts required to charge the 24v battery bank at a standard 5% rate.

    Another of BB's formulas:
    Minimum array watts * 0.52 efficiency * min. hours of sun = min. watts of 120 VAC per day

    Plugging my daily load of 870W into this and doing some basic algebra:
    Minimum array watts = 870 watts / 0.52 * 4 (min. hours of sun)
    Minimum array watts = 418 watts

    So my need is roughly twice the minimum required for my batteries.

    Using another formula BB used in the linked-to discussion:
    Battery Ah * battery voltage * 0.85 typical inverter eff. x 1/2 days storage x 0.50 max discharge = Wh of 120 VAC battery power per day

    109Ah * 24v * 0.85 * 1/2 * 0.50 = 556 Wh of 120 VAC battery power per day

    Now, if these numbers are accurate, with a 400w array feeding a 24v battery bank consisting of 2 109Ah 12v batteries, I can power my load of 174w for 5 hours a day. In amp hours that would be:
    174w/120 VAC * 5 = 7.25Ah AC * 5 = 36.25Ah DC @ 24v

    How close am I with these numbers?