System design math check needed for 100' distant 24V system concept

[SolarSteve]

Where are you located? The price for 300 watt panels seems pretty high. They are less than half that in my area (So. Cal.)

Just did super quick Google search:

https://www.thepowerstore.com/phono-solar-300w-monocrystalline-perc-solar-module-60-cell-silver-frame?gclid=EAIaIQobChMIt92w3_zM1QIVByhpCh12LgTFEAQYCiABEgKUEvD_BwE

Searches came up with better pricing but only if you bought a minimum of 4 or more.

I am predisposed to name brand. I do believe "you get what you pay for" when it comes to this stuff. For example, the 30A controller that came with my 100W panel is junk. I bought it because I wanted to do a real world test on the roof and to do that as cheaply as possible before making a decision what to do.

Steve

[sensij]

Hmm. There appears to be something wrong with their calculator. I entered 0.4kW and it spits out the same values as 4kW. You'd think they would have accounted for that by either not ignoring the decimal or having the system sized in W instead of kW. Oh well!

Sounds like there is still some operator error here. The *first* column in the output is the daily average insolation that is being modeled each month. It will not change based on array size, because it is normalized to area (notice the units include area... kWh/m2/day). The 2nd column is the modeled AC output, but note, this is only intended for *grid-tie* systems, for a variety of reasons, battery systems will produce less.

If you take some time to read the help files, and look closely at the hourly output that can be downloaded, there is lots to learn. The takeaway, though, is that the average daily generation potential of a fixed tilt 400 W array, in the summer, is no more than 1800 Wh, and it would be safer to use a number somewhat less than that. Single axis tracking bumps that 1800 up to 2000 Wh.

If you increase your array to 795 W, that basically doubles these numbers, and is a much better choice for supporting the load you've described.

Now knowing this, I entered the W output of the new 3x265W option and divided by 10. It comes out to be roughly 470W-650W per day, depending on the month. If my worst case usage for the fridge is accurate (and I deliberately made it inaccurate on the high side) it will consume 1800W on the worst of worst days. This means for the worst of worst days I'll need 2-3 additional days to keep it running. Given that it can be shut off for 4-5 days in a row, that seems to be doable.

The question is if 100AH storage is going to store 2-3 days worth of juice. Any thoughts on that?

24 V * 100 Ah = 2400 Wh of storage. You estimated your daily consumption to be 2400 Wh. So no, 100 Ah is not enough to support that load.

Roughly, if you are using FLA batteries, 795 W could provide 33 A of current at STC conditions and 100% controller efficiency. That suggests a battery size of 8x that, or at least 264 Ah.

[Mike90250]

Now that's sounding a bit more reasonable

4 x 100W = $520 with ~76V output

400W

3 x 265W = $550 with ~93V output

795W 9A @ 93V and then 31A @26V

3 x 300W = $750 with ~96V output

900W
Are these voltages Vmp or Voc ? Voc is the voltage that kills charge controllers

Batteries, are those 100ah batteries rated to charge at up to 31A ? You may be OK, as the battery charges in the morning, it starts off slow, then as solar builds, amps can increase. When batteries are partially charged, their amp input will start to reduce. Or the Kid may have a way to limit the output amps with some programming.

And don't forget the fuses and such you need to protect things and easily shut down stuff. I use the Midnight switch duty rated DC breakers instead of fuses, and just use the Blue Seas MRBF holder & fuse at the battery terminal. https://www.bluesea.com/products/519...k_-_30_to_300A

[Sunking]

Here's the system I'm thinking of. Please tell me if I'm on the right track:

1. Four 100W panels wired in series
2. Two ~100AH batteries wired in series
3. PWM controller that can handle at least 70V input and has 24V output
4. 100' of 10 AGW

The 400W system wired in series puts out roughly 64V @ 6A max (I am presuming actual max output per panel is 16V @ 6A). If this is put into 10 AWG and pushed 100' the voltage drops to 62.8V by the time it hits the controller, which then dumps into a 24V battery bank consisting of 2 x 100AH batteries in series. The refrigerator and lights can handle either 12V or 24V as is, so no problems there.

Not even in the same Train Station. None of this is going to work.

First if your load demands 1800 watt hours or 1.8 Kwh, then the minimum Battery Reserve Capacity must be 9000 or 9 Kwh. At 24 volts is 9000 wh /24 volts = 375 Amp Hours. You are not even remotely close that.

Next there is no way you can use a PWM Controller. If you wired the 4 panels in series would mean you turn you 400 watts of panels into 150 watts. @ 24 volt battery. With PWM you must run low voltage and everything in parallel. Do that and you have 400 watts of panels and at best 300 watts.

With PWM Output Current = Input Current. The Panels put out 6 amps. 6 amps x 24 volts = 144 watts my friend. With MPPT Output Current = Panel Wattage / Battery Voltage. 400 watts / 24 volts = 16.7 amps.

OK a 24 volt 375 AH battery, minimum charge requirement is 37 amps, and to generate 37 amps of charge current takes a minimum 37 amps x 26 volts = 884 atts, let's just call it 1000 watts so you can have a light on. Problem is your location and shade issues. 1000 watts i snot likely enough from September to May. However let's just pretend 1000 watt sis enough. Herre is what you are really looking at.

Panel Wattage = 1000 Watts. $1500
40 Amp MPPT Charge Controller , $400 to $500
24 volt 375 AH battery $1300

Total $3000 to $3500

Assuming you use 250 watt panels with panels configured 2S2P running a Vmp of 72 volts @ 14 amps 100 feet 1-way wil requires 6 AWG copper wire. Do not forget you must have a generator and AC charger of 40 amps.

[SolarSteve]

If you take some time to read the help files, and look closely at the hourly output that can be downloaded, there is lots to learn.

Yeah. I'm trying to do too many things all at one time. The results are... suboptimal!

The takeaway, though, is that the average daily generation potential of a fixed tilt 400 W array, in the summer, is no more than 1800 Wh, and it would be safer to use a number somewhat less than that. Single axis tracking bumps that 1800 up to 2000 Wh.

If you increase your array to 795 W, that basically doubles these numbers, and is a much better choice for supporting the load you've described.

Thanks for the calculations. I think one thing that's abundantly clear to me so far is that a 400 W system is absolutely not the right way to go.

24 V * 100 Ah = 2400 Wh of storage. You estimated your daily consumption to be 2400 Wh. So no, 100 Ah is not enough to support that load.

Roughly, if you are using FLA batteries, 795 W could provide 33 A of current at STC conditions and 100% controller efficiency. That suggests a battery size of 8x that, or at least 264 Ah.

I do have a piece of info that I didn't mention before. I had a 105AH battery hooked up to the fridge only. It started off at a 99% charge and went down to about 46% in roughly 2 days (I wasn't there so it's rough info from family members). This seems to indicate that daily consumption was around 30AH. How does that information help with the calculations?

What I need to do is sharpen my pencil about what the estimated load is for a 24 hour period. I do have a meter that tracks cumulative usage from the battery in Watts. I should fire it up with the meter reset and use a generator to make sure I can keep it going long enough to establish a reasonable hourly average.

Steve

[SolarSteve]

Not even in the same Train Station. None of this is going to work.

Yup, I think you guys have made an excellent case for that

Assuming you use 250 watt panels with panels configured 2S2P running a Vmp of 72 volts @ 14 amps 100 feet 1-way wil requires 6 AWG copper wire. Do not forget you must have a generator and AC charger of 40 amps.

I'm confused by this. According to the specs with 3 x 265W I'd be pushing 93V at ~9A to the MTTP controller along 100' run. I don't see much difference in drop off between 10 AGW and 6 AGW. In any case, I'd be hitting the 24V battery bank at roughly 30A from the controller. Good point about going with a 40A controller to give a margin of error/safety.

Thanks!

Steve

[SolarSteve]

Thanks to all of your help, I think I'm homing in on what I should be looking at:

3 x 265W panels ($550)
Morningstar PS-MPPT-40 ProStar MPPT 40A Controller ($425)
2 x 105AH Duracel Ultra Deep Cycle (flooded) batteries ($220)
100' 10AWG two strand direct burial 600V rated wire ($174)

~$1350 for all this plus shipping and taxes if applicable


It seems there's agreement about most everything except for the storage. I dug into things a bit more and came up with:

- From anecdotal observations of meters and estimates it appears that worst case I'm going to draw 3A for roughly 5 minutes about 5 times an hour for 16 hours. For the remaining 8 hours it will cycle half as often because the door won't be opened while people are sleeping. If I've done my math right, that should be somewhat under 30Ah per 24 hour worst case scenario.

- I seemed to confirm the above because in a ~2 day period with no solar hooked up the fridge on its own consumed ~60Ah.

- Primarily the fridge will be on for 2 days, then no load on the system for 5 days. Therefore, I only need about 60Ah of storage for worst case usage (kids and others who think electricity grows on trees) and worst case charging (none).

Based on this it seems that with flooded 60Ah capacity I can go one full day from 100% charge to 50% without any solar activity and worst case use pattern. If I have a flooded 105Ah setup I could go almost 2 days from 100% charge to 50% without any solar activity with worst case use pattern. Obviously this will decrease over time as the batteries age, but it's where I'd start at.

I also have the following backup plans in the event we have really bad luck on something like a 3 day weekend of miserable weather for the entire week prior:

- We've found that the fridge can be turned off at night and be 45 deg in the morning. Shutting it off at night will shave off ~5Ah of drain. In fact, if this becomes a routine need I can get a timer to automate it.

- I have a 20A generator which, though not ideal, is enough to bring the two 105Ah batteries from 50% to a near full charge without any solar involved in about 8 hours.

- The 20A generator can easily run the fridge periodically to bring it back down to temp without any solar activity at all *and* dump about 35 minutes @ 20A and 25 minutes @ 17A into the batteries for each hour run.

- We have coolers and access to ice, so if everything goes to maximum Hell we can shut everything down and still have cold beer and cheese, which is really all one needs to survive a weekend at camp


Thoughts?

Steve

[ButchDeal]

- From anecdotal observations of meters and estimates it appears that worst case I'm going to draw 3A for roughly 5 minutes about 5 times an hour for 16 hours. For the remaining 8 hours it will cycle half as often because the door won't be opened while people are sleeping. If I've done my math right, that should be somewhat under 30Ah per 24 hour worst case scenario.


Thoughts?

Just one.....What VOLTAGE were those amps measured for the fridge?
Did you convert it to battery voltage?

You may find it simpler to do the calculations in watts and watt-hours and then convert to ah at the end

[SolarSteve]

Just one.....What VOLTAGE were those amps measured for the fridge?
Did you convert it to battery voltage?

The fridge is DC and is capable of running between just under 10v (with a setting for low cut off) and something like 26v, depending on what it senses for supply. Since it draws whatever voltage is available I discarded voltage because it seemed irrelevant. Is that true or did I mess up logic here?

You may find it simpler to do the calculations in watts and watt-hours and then convert to ah at the end

I had been doing it that way, but for whatever reason I switched it around because it seemed easier and in this circumstance reasonably accurate. But I could be wrong

Steve

[littleharbor]

The fridge is DC and is capable of running between just under 10v (with a setting for low cut off) and something like 26v, depending on what it senses for supply. Since it draws whatever voltage is available I discarded voltage because it seemed irrelevant. Is that true or did I mess up logic here?



I had been doing it that way, but for whatever reason I switched it around because it seemed easier and in this circumstance reasonably accurate. But I could be wrong

Steve

That 26 volt max. will be a problem with a 24 volt nominal battery voltage. Were you going to use a step down module?

[Sunking]

I'm confused by this. According to the specs with 3 x 265W I'd be pushing 93V at ~9A to the MTTP controller along 100' run. I don't see much difference in drop off between 10 AGW and 6 AGW.

There is a heck of a big difference between 9 and 14 amps.

[neweclipse]

Keep the voltage at it's highest till you travel the 100' will make voltage drop less dramatic. Then lower it nearer to battery location.

6 volt golf cart batteries would be best "dollar for dollar" in the long run.
Just my 2 cents.

[SolarSteve]

Quick follow ups...

Littleharbor, this is a Novo Kool conversion system I'm using. It is designed for battery based systems. I checked the literature and it only talks about the low cut off options and how it senses whether to run in 12v or 24v mode by voltage sensing. There's no stated max voltage (I thought there was), but this is all irrelevant really because it has no bearing on designing the system. If I have a 24v battery system it will work fine.

Sunking, the thing that's confusing me is I don't know where you are getting the 14A from. The 265W panels I noted the specs for don't put out that much amperage, so where is your 14A figure coming from?

Neweclipse, yup keeping the voltage up is definitely the priority. As for golf batteries, I'm not sure that's the best solution for my particular needs as the system is only used periodically for less than half the year. But I'm always open to taking in advice.

Steve

[littleharbor]

Quick follow ups...

Littleharbor, this is a Novo Kool conversion system I'm using. It is designed for battery based systems. I checked the literature and it only talks about the low cut off options and how it senses whether to run in 12v or 24v mode by voltage sensing. There's no stated max voltage (I thought there was), but this is all irrelevant really because it has no bearing on designing the system. If I have a 24v battery system it will work fine.


Steve

OK I just noted your mention of 10 to 26 volts. If it says it's suitable for 12 or 24 volt systems it should tolerate the, up to 30 volts it would see from charging and equalizing batteries.