Upgrading, now what?

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bmet
bmet Solar Expert Posts: 630 ✭✭
in Solar Beginners Corner #1
I jumped on the Evergreen frenzy, finally got a completed bill of sale, and expect delivery before Christmas.

How to implement 8 225W panels? If actually receive what I paid for, the specs would be:

Voc - 36.1
Vmp- 29.5
Isc - 8.6A
Imp - 7.6A

Not sure if I want to use them all for a water well conversion, emergency power for the home, or piece them around the ranch for spot electricity. Would there be enough wattage to support a battery-based 30 amp circuit? What do you think?

If these sound like stupid questions, remember I am still operating on my first small system. I want to approach a serious upgrade with realistic expectations.

Thank you
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Upgrading, now what?

    Since I deal mainly with off-grid apps I'll give you my perspective from that POV.

    Eight 225 Watt panels = 1800 Watts. Pretty good size array, suitable for a 24 Volt system.
    The Vmp of 29.5 means these can only be used with an MPPT type charge controller (obviously they were meant for a GT system).

    As such, you could reasonably expect:
    1800 Watts @ 77% = 1386 / 24 Volts = 57 Amps, enough to support 570 Amp hours of battery (+/-).
    1800 Watts * 4 hours equivalent good sun * 50% over-all efficiency = 3.6 kW hours AC.

    Pretty good power, actually.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
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    Re: Upgrading, now what?

    i think this may be a first in someone saying, i bought them because of the price and now what do i do with them?

    remember that your loads in wh will determine the size of the battery bank and what in pv will suffice to charge them. you are going fire, ready, aim.
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  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #4
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    Re: Upgrading, now what?
    Cariboocoot wrote: »
    Since I deal mainly with off-grid apps I'll give you my perspective from that POV.

    Eight 225 Watt panels = 1800 Watts. Pretty good size array, suitable for a 24 Volt system.
    The Vmp of 29.5 means these can only be used with an MPPT type charge controller (obviously they were meant for a GT system).

    As such, you could reasonably expect:
    1800 Watts @ 77% = 1386 / 24 Volts = 57 Amps, enough to support 570 Amp hours of battery (+/-).
    1800 Watts * 4 hours equivalent good sun * 50% over-all efficiency = 3.6 kW hours AC.

    Pretty good power, actually.
    Coot,
    Which is a more realistic voltage number to use in your equation, the actual battery voltage, or the charging voltage? And why? I see it written both ways and it can be a big difference.
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  • Seven
    Seven Solar Expert Posts: 292 ✭✭
    #5
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    Re: Upgrading, now what?
    niel wrote: »
    i think this may be a first in someone saying, i bought them because of the price and now what do i do with them?

    you are going fire, ready, aim.

    I have been dealing with it for several days on other, non-solar, boards. Some don't have near Bmet's level of knowledge.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: Upgrading, now what?
    Seven wrote: »
    Coot,
    Which is a more realistic voltage number to use in your equation, the actual battery voltage, or the charging voltage? And why? I see it written both ways and it can be a big difference.

    When your looking for the potential maximum current that will occur when the panels are at their peak output and the battery is at its minimum Voltage (greatest Voltage difference = most current).

    Now, this will probably never occur as the panels' production will be at its best mid-day, which means the batteries will already have been charging for a few hours. But you never know. :roll:

    Since nominal system Voltage is roughly 50% DOD (don't shoot me for saying that) and is a good point to stop draining the batteries (LVD). As the Voltage comes up the current will go down.

    The worst part of doing this with rules-of-thumb and generalizations is that absolutely every system is site-specific but too often people take the basic ballpark figures and think they're done. We need a better way of emphasizing this. I was one of those who used to write it the other way as a means to a short-cut to a properly balanced system. I've stopped doing that and am trying to make the whole process more step-specific.
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #7
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    Re: Upgrading, now what?

    Seven, Thank you for the vote of confidence, I'm reviewing the FAQs on basics, and trying to find BB's equations for various estimates. I don't want to go too complicated with a grid-tie, was thinking more of an indEpendant back-up run from a Tuff Shed next to the house. Perhaps the system could run refridge or deep freeze during a brown-out, or reducing the bill by simply getting a couple of appliances off of the house power. I'm in unfamiliar territory .

    Besides, I took all of your advise, buy as much panel as I could afford!
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
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    Re: Upgrading, now what?

    bmet,
    that would suffice for a backup to a frig and possibly much more. 8 x 225w = 1800w stc. derate at 52% (for off grid system and inverter) = 936w after all is said and done through all losses. multiply that by your hours of typical sunshine and that's what you will get on a daily basis in watt hours. you still need to figure it by the loads if this will fall into what you need and for how long you'll need it.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
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    Re: Upgrading, now what?

    bmet;

    3.6 kWh per day is more than I run the cabin on, including refrigerator, computer & satellite modem, water pumps, microwave, lights, et cetera. It could provide you with quite substantial back-up power, once you buy all the other pieces needed (charge controller, batteries, inverter, misc.) That's the drawback; there's always something more to buy! :cry:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #10
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    Re: Upgrading, now what?

    The problem with rules of thumb is that they "bury" and "blur" the details of the calculations.

    On the other hand, when you are talking about +/- 10-20% accuracy, the difference between /14.5 or /12.0 is less important.

    Also, it does depend on the controller too... PWM is purely Imp current--there is no /14.5 or /12 (the Vmp is at ~ 17.5 volts).

    For MPPT controller where they are "more or less" constant power:
    • Icurrent= Watts STC / Vbatt-charging (ignoring losses)
    It does make a difference... But then we have to look a the entire range of battery charging voltage--From ~12.0 when the battery is 50% discharged and charging--To ~14.5 volts when the battery is 80-90% full and under heavy charging current.

    Most people, for most of the time, are going to be operating in the 80-100% range of battery charging--which means they will be somewhere around the ~14.0 to 14.5 volt battery charging range.

    The 14.5 volt charging and 12.5 volts discharging (rough numbers) account for much of the battery "losses" (~80-90% efficient battery bank).

    For the most part, if there is a "choice to be made"--I tend to error on the conservative side. So I use the "worst case" assumptions/numbers--And, hopefully, the system will perform at least as well as predicted, if not a little bit better (+5-10%)...

    But, again, I try to "show my" work... And with that information, you are more than welcome to "adjust it" (and/or ask questions) to fit it for "your" needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #11
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    Re: Upgrading, now what?

    You know the most effective use would be grid tie. Harvest all you can with no battery loses. Least cost to build as well.
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #12
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    Re: Upgrading, now what?

    Except that when the grid goes, so do the panels.
    I've always wanted to have emergency power, whether it is in support of
    Infrastructure, prolonging perishables, or keeping a light going in the dark.
    Water is key, just not sure if a day well would support two homes, plus two gardens.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #13
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    Re: Upgrading, now what?
    bmet wrote: »
    Except that when the grid goes, so do the panels.
    I've always wanted to have emergency power, whether it is in support of
    Infrastructure, prolonging perishables, or keeping a light going in the dark.
    Water is key, just not sure if a day well would support two homes, plus two gardens.

    the light going in the dark should be dc and that will be fairly easy for you to do. the real small white leds are great for night lights imho, but you have to watch that the current doesn't run away when the heat builds on it. this is a different subject anyway.

    if you just want to back things up you need to determine how much power for how long so you can size a battery bank. so that it keeps with the 50% dod point the battery capacity will be x2 of what you figured you'd need. odds are you'd want enough to be sure you have something for that rainy day so it should be capable of going 2 days.

    i'll give my crappy 'frige for an example. it's about 1500wh/day. going 2 days makes it 3000wh. doubling it for 50% dod reserve capacity makes it 6000wh. at 12v this represents 500ah capacity. 24v is 250ah capacity and 48v is 125ah capacity. doable i think if this example would fit what you want to run and for that long. for the record, i have 400ah of capacity for mine, but i didn't figure on going 2 full days as i'd implement other measures to extend the time if need be. if you add more items to be backed up then you must figure those loads into it. adding more time too means much higher capacity as well. you have the pv to recoup quickly though and that's nice as you may be able to keep the reserve capacity to about 2 days as many would go 3. you could probably even run a frig off grid with the example i gave and with the 936 reliable watts from those 8 pvs i suspect it would be fine to run the frig and maybe a few small lights off grid with only 2 hours of full sun per day.

    this is only my example given so that you have something to start your figuring with. ultimately you need to decide what to get for the setup as you need other major and minor things like the controller (mppt), inverter (sine wave), combiner, fuses/circuit breakers, wire, etc. etc. do get a battery temp sensor and a battery monitor as these are often forgotten.
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #14
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    Re: Upgrading, now what?

    Thank you everyone for your input. Since I have a KillaWat meter, I am running it on both the fridge and freezer each for 3 days to see what % they contribute to the utility bill.
    Also, I have read the specs for a couple of deep well pumps with a range of voltages and flow rate that 8 panels could support, but my information on the well is 20 years old, so I'm trying to get a contractor out that could confirm or update it's specifics.
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