Solar Panels?

[unsubscribed]

So @AZRoadrunner 's musings bring up an interesting question, for which I've not found a simple rule of thumb on the net. And that question is, what is the ratio of solar panel power to battery power that one should have for a usable solar station? (I suppose that "intended load" should be the actual metric - but since the battery bank has to supply more than the intended load, and perhaps with a safety factor of 2, I used battery bank capacity instead of load.)

Based on what I'm finding now with my own little experiment, I'm thinking the ratio is 3 or 4:1, to be really safe.

So here's what I'm currently doing. (Pun intended.) I am powering a mother-in-law sweet with my solar station. With one light on continuously, and this desktop computer, I draw about 170 Watts 24/7. It will be a bit more when I hook up the small refrigerator too.

Let's call it 200 watts/hr. That means that I will use 4.8 kW a day. My battery bank is 4.8 kW.

Just on its face, that doesn't sound good. A day without sun, and I have no more power.

So I wonder if there's a Rule of Thumb about the ratio of one's battery bank to the intended load?

It seems 2:1 at least.

Fortunately, it's been sunny here in AZ. So I'm going to focus now on just getting through a solar day, to get a feel for this problem.

As a baseline, at the equinox, I harvested 3.7 kW.

Naturally, that will increase as summer gets here. (Although maybe not as much as simple math would suggest, since the panels will be at 180 degrees F (or more?) in June, July and August and won't produce as much power.) I have to extrapolate to Dec 21, but I think I could have made 2 kW.

I should be using the shortest day worst case number of Dec 21 for my musings here. But for now, in real life, on the equinox, generating 3.7 kW doesn't fully recharge my battery bank with my 200 W load running continuously, I have to top off the bank at night via my Multiplus. (When electricity rates are lower.)

I haven't been able to wrap my head around a full day's calculations. But let's just focus on a solar day of 10 hours as a subset to see where I stand.

I harvest 3.7 kW and use about 2 kW for my load during the solar day (200 Watts x 10 hours). That leaves me with 1.7 kW to charge my batteries. Assuming that I started the morning fully charged (NOT a valid assumption, as below), that leaves me 1.4 kW short in the evening.

But in reality, I will never been fully charged in the morning if I were only charging with solar. So I will be even more discharged in the evening. I'll come back to this later.

If I had another 330 W panel, then (all things being equal) I could harvest 5.5 kW a day.

That should give me about 1.8 kW extra, which would fully charge my bank by the end of a 100% sunny day. (Again, assuming that I started with a fully charged battery in the morning.)

But this doesn't leave much of a reserve for partly cloudy days.

AND - my Victon MPPT maxes out at 1000 Watts. So if make more than 1000 Watts at peak day, then all things aren't equal.

(Since the best I've seen so far is 680 W from my two panels, I should never be much over 1000 watts. But this means that I am limited to three panels now. Unless I buy another MPPT.)

So all this means that, in the best case of a fully charged battery in the morning, I would break even if I had 1 kW of solar for my 4.8 kW battery bank.

That's a best case, with no reserve. And all this is with a crummy 200 W load.

If I keep my load at 200 W, it seems that I should have 2 kW of solar panels for rainy days. (Which means two MPPT's.)

Now, the reality is that, from the end of the solar day until the beginning the next morning, I will have used about 2 kW powering my load. (200 watts x 10 hours.) If I had started with a full charge at the end of the solar day (which will not happen, because I'll be losing power as the sun goes down) then already my battery bank will be down to 50% by morning.

In fact, here's an interesting aspect to this 'equation:' Even if I had a bazillion watts of solar, having more than enough solar power does not change this part of the analysis.

While, during the day, the batteries would reach full charge sooner with a bazillion watts of solar, they will begin to discharge at the same rate after the sun goes down as they would if I only had 2 kW of solar. So regardless of how much solar I have, I always lose the same power to my load overnight.

This tells me that there is a point of diminishing returns here. That you reach a point where adding more solar doesn't change this part of the equation.

And again this brings me back again to the thought that there should be a Rule of Thumb for the ratio of battery bank to load. Because, fundamentally, if your bank capacity is too small, you'll run out of power before the night is over.

Anyway, having lost 2 kW overnight means that I need to harvest an extra 2 kW in solar to fully charge my battery by the end of the day. (And I think "to the end of the day" is a clue here to optimizing how much solar power one needs.)

So, since I currently harvest 3.7 kW with 660 watts of panels, I would need yet another 330 W panel minimum (for a total minimal solar array of 1.3 kW) to fully charge my batteries by the end of the day.

Double 1.3 kW for reserve, is 2.6 kW. (And three MPPT's.)

So that's a solar power to battery powre ratio of 0.5.

And all this for a measly 200 watt load.

If my load doubles (think running a freezer and refrigerator, a window AC, a hot plate to cook food) I think that the math is NOT going to be linear (due to that overnight usage) and I would need maybe 2x the solar power.

I'll have to do that math for that case and see what I get.

But I'm too tired now from writing this all out.

Hopefully someone smarter than I am has the answer already.

 

[Ols]

Just look for the panel specs even on so called 12V or 24V panels.
Voc
Vmp
Imp
Isc
Forget about calling panels 12V or 24V if you are using a MPPT scc.
12v or 24v is referring to battery voltage catergories for PWM scc.

Frobisher, I am very green when it comes to solar just purchase a solar generator that's a 24v system. Now looking into buying panels. Trying to understand all this is Greek to me for now. So how would I know a panel is 24v to match up with my 24v generator. Just starting to get into solar, for now going with a portable generator and a couple of panels.

 

[unsubscribed]

@AZRoadrunner Well, the Victon MPPT will limit your power if too much. With no harm done. So you can add the third panel and that will buy you some extra power in the morning and late afternoon. But not more peak power.

 

[iamrich]

So how would I know a panel is 24v to match up with my 24v generator.

Just make sure the panel voltage is greater than 24v and you will be good. (more than better as long as the MPPT can handle it). You can also get there by using panels in series. Or just ask this forum

 

[unsubscribed]

I agree with @Forbisher that referencing panels by "12v" or "24v" is not very helpful.

I suppose that if you're running a 24 volt battery system, as I am, then by calling a panel a '12 v panel,' it tells me that either 1) I cannot use that panel by itself to charge my 24 battery bank or 2) I will need at least two of them to do the charging. (Since the panel has to put out more voltage than my battery in order to charge the battery.)

The better way is to simply look at the Voc and Vmp of the panel that you're interested in. In my case, using a Victon brand MPPT, Victon says the panel must produce 5 volts more than my battery voltage to start the charging process. In my case, that means that I need a panel that can put out more than 24 + 5, which means that Voc must be greater than 29 volts. (And, really, since a 24 volt batter will nominal be producing 27.2 v (13.6 x 2), then I need Voc to be 32 volts. )

 

[Forbisher]

Frobisher, I am very green when it comes to solar just purchase a solar generator that's a 24v system. Now looking into buying panels. Trying to understand all this is Greek to me for now. So how would I know a panel is 24v to match up with my 24v generator. Just starting to get into solar, for now going with a portable generator and a couple of panels.

Yeah you are so green that you don't think it is important to mention the brand and model of the solar generator you bought. Lolz

The solar generator has a max volts input and either has a PWM scc or MPPT.
You gotta read the specs on what you have.
I never heard of a 24V system solar generator.

 

[iamrich]

I never heard of a 24V system solar generator.

Bluetti AC200P is 24v, no? At $2k, probably not what we are talking about though.

​

 

[Forbisher]

Bluetti AC200P is 24v, no? At $2k, probably not what we are talking about though.

​

Not 24V
An AC200 has a ~52V battery.
AC200P is about $1700 now.

 

[Ols]

Bluetti AC200P is 24v, no? At $2k, probably not what we are talking about though.

​

Lol with a, it's a Point Zero Titan with one battery for now, in the future a second one.

 

[Ols]

Yeah you are so green that you don't think it is important to mention the brand and model of the solar generator you bought. Lolz

The solar generator has a max volts input and either has a PWM scc or MPPT.
You gotta read the specs on what you have.
I never heard of a 24V system solar generator.

It's a Point Zero Titan with one battery, in the future a second one

 

[Forbisher]

Lol with a, it's a Point Zero Titan with one battery for now, in the future a second one.

Are you getting the Titan in 2022 or 2023? Haha
Plenty of time to study up.

Read their website as they will tell you how many watts of panels, max input voltage, amps, etc. .

I think it has 2 MPPT scc?
Did you know that?

You can use any solar panel with the Titan if you connect correctly.

 

[Forbisher]

It's a Point Zero Titan with one battery, in the future a second one

2 minutes on Google for Titan specs.
Just because the Titan has a 24V battery it doesn't mean you have to use "24V" panels.

You could use 10 100 watt 12V panels arranged 5S2P

Obviously using 3 320w panels would be easier and cheaper if the Voc x 3 is comfortably under the 145V max input when you allow for low temps.

Solar MPPT Charge Controller:
Dual MPPT controllers
Input Volts: 35V to 145VDC
Maximum input Amps: 30 amps per input port
Maximum charging amps: 80A (40A per port)
Charge rate: up to 2000W (using both ports, and with two or more batteries)

 

[Ols]

2 minutes on Google for Titan specs.
Just because the Titan has a 24V battery it doesn't mean you have to use "24V" panels.

You could use 10 100 watt 12V panels arranged 5S2P

Obviously using 3 320w panels would be easier and cheaper if the Voc x 3 is comfortably under the 145V max input when you allow for low temps.

Solar MPPT Charge Controller:
Dual MPPT controllers
Input Volts: 35V to 145VDC
Maximum input Amps: 30 amps per input port
Maximum charging amps: 80A (40A per port)
Charge rate: up to 2000W (using both ports, and with two or more batteries)

Have watched other you tubers and they called it 24v system. So when looking at solar panels need to look at the voc and have above 35v and below 145v for the mppt. If I would buy three 300w panels with a voc of 40.1v ×3= 120.3v I would be in the range.

 

[Forbisher]

Have watched other you tubers and they called it 24v system.

So when looking at solar panels need to look at the voc and have above 35v and below 145v for the mppt.

If I would buy three 300w panels with a voc of 40.1v ×3= 120.3v I would be in the range.

Yeah that would work.
Three 300w panels in Series.

So what if you tubers called it a 24V system as that only means the battery is 24V.
Panels can be 12V or 24V or whatever works with that 35Vmp to 145Voc.

Bluetti AC200 has a 52V battery so can you only use 52V solar panels???

 

[unsubscribed]

@Ols But watch out that the MPPT can take the voltage. If you're going to with a system that produces 120 Voc, then the Victron 150/35 might be your only game in town.

 

[Forbisher]

@Ols But watch out that the MPPT can take the voltage. If you're going to with a system that produces 120 Voc, then the Victron 150/35 might be your only game in town.

He is using a big $$$$ Titan "solar generator" that has 2 MPPT SCC's built in.

Titan Solar MPPT Charge Controller:
Dual MPPT controllers
Input Volts: 35V to 145VDC
Maximum input Amps: 30 amps per input port
Maximum charging amps: 80A (40A per port)
Charge rate: up to 2000W (using both ports, and with two or more batteries)

 

[Forbisher]

Lol with a, it's a Point Zero Titan with one battery for now, in the future a second one.

Where and how are you using your Titan powering what?
Where are you installing the panels.
When did they say they will ship?

 

[unsubscribed]

Okay, I figured out the math.

My gut instinct that a ratio of 4:1 was right, although it's a function of load, not of battery bank capacity.

Basically, you do this:

24 hours x L (where L is your average one hour load, in watts) - 6 hours x P (where P is your actual (not theoretical) peak solar power, in watts) = 0.

This equation says that, during the day, you have to put in to your batteries the power that the load took out in the past 24 hours. Do this and your SOC should stay at whatever it was when you started. So if your battery is fully charged by the end of the day, you should end up with a fully charged battery by the end of the next day.

Solve for P and you get 24L=6P => 6P=24L => P=24/6 L => P=4L.

So, as a minumum Rule of Thumb, to establish a lower boundary, you need to generate a peak solar power that's 4 times your average one hour load to keep your battery from discharging over a 24 hour period.

(And so the ratio of 4 comes from the number of hours of good daylight divided by 24 hours in a day. Which is what my intuition was telling me.)

And so, for my case of a continuous load of 200 watts day and night, I need 800 watts of peak solar. Which, as I had guessed, means that I need to buy another panel.

This equation assumes no clouds. It got the figure of 6P from my anecdotal observations of my current system during equinox, where my typical peak power yields 6x of total daily harvested power. At my latitude. (Arizona.)

 

[Ols]

Where and how are you using your Titan powering what?
Where are you installing the panels.
When did they say they will ship?

Forbisher, last summer went thru a power outage for a couple of weeks this open my eyes to alternative power sources. A friend let me use his gas generator which was loud and of course could not be brought inside. Like I said this greenhorn is looking into solar. So I am starting out with portable generator and a couple of panels. The panels will be on the ground which I will move around to follow the sun. My plans are to use the Titan as a backup for another power outage to run a refrigerator, TV, and fan not the whole house. When the power is on, tv, a lamp, laptop, will play around with it. Still in the process of putting it all of it together.

Also, want to say "thanks" to all who commented with their input. And to those who had the patience to learn this greenhorn.

 

[AZRoadrunner]

Hopefully someone smarter than I am has the answer already.

I'm not the one with the answers, but maybe a little perspective. I've been a ham radio operator since 1974, worked professionally as a radio communications officer for the federal government, and earned a piece of paper along the way that says I'm a certified electronic technician. Been working in, around, and with electronics for most of my working life (now in my 60s). The two most important things I learned along the way is that 1) the devil is in the details, and 2) humility is my best asset. Don't let this stuff frustrate you -- even those of us who might be considered "professional" miss the mark on a regular basis. Pushing electrons around on wires and trying to control their behavior in a manner that provides us with something useful is a very tricky and sometimes mind-altering experience.

 

[iamrich]

I'm not the one with the answers, but maybe a little perspective. I've been a ham radio operator since 1974, worked professionally as a radio communications officer for the federal government, and earned a piece of paper along the way that says I'm a certified electronic technician. Been working in, around, and with electronics for most of my working life (now in my 60s). The two most important things I learned along the way is that 1) the devil is in the details, and 2) humility is my best asset. Don't let this stuff frustrate you -- even those of us who might be considered "professional" miss the mark on a regular basis. Pushing electrons around on wires and trying to control their behavior in a manner that provides us with something useful is a very tricky and sometimes mind-altering experience.

I would add that in a sub forum specifically titled "beginners corner" I would expect a little patience with any posts. What seems obvious to some, might not be obvious to others.

 

[AZRoadrunner]

I would add that in a sub forum specifically titled "beginners corner" I would expect a little patience with any posts. What seems obvious to some, might not be obvious to others.

Exactly. At least this place isn't run like so many other online forums -- where newcomers are publicly ridiculed and told to "search the forum" or "RTFM". Sometimes you don't know what to search on or the search results generate too much noise. The separate discussion area for beginners is an excellent solution.

 

[Ricktara]

Wow, can’t get near that price here in NH

 

[unsubscribed]

Smokin' Deal of the Day

I should probably post this somewhere else in the forum, to let others know. (If there's a Moderator, feel free to cross post this.) But since we've been talking about buying Solar Panels from SanTan Solar (in the Phoenix, AZ area) in this thread:

I just got back from SanTan Solar. I was planning to buy two REC 350W Twin Peak 2S poly's (REC350TP2S72) for $150 each. (Per their website.)

When I got there, the sales guy asked if I wanted "in the box or out of the box." I asked, "What's the difference?"

Answer: (rough quote) "Out of the box is where an installer buys a bunch in boxes, takes them out of the boxes for a job, doesn't use some and returns them. Out of the box are not covered by REC's 25 year warranty."

But I know that in the box REC's are not covered by REC either if not professionally installed. So no difference to me.

"How much for out of the box?"

"$110"

(Some drama: "But I don't know if I have any left." (Not on their website.) "I can check." Yes, he had some.)

I bought two. (As I've gotten older, I figure that if I'm going to drive an hour or two to buy a solar panel, I might as well by two of them.)

(BTW, I had asked about a discount for cash. (Since I'm saving them the 3% credit card fee.) "Yes." But I don't think I got a discount on these. In retrospect, since no discount, I should have paid for these with a credit card, just in case of a problem.)

They check out okay here with a Voc test.

As a comparison, a few months ago I had driven up to Flagstaff (two hours each way) to buy some REC N-PEAK 330 W mono's. They were also "out of the box." (I think that's because Northern Arizona Wind & Sun buys them by the pallet this way.) I paid $205 for those. They work well. And I know mono is more efficient than poly (by 1% in my case). But even if I harvest only 330 W out of my new 350 W poly's, that's at half of what I paid for my N-PEAK's.

And considering that I was paying about $90 from Amazon after tax for 100 W panels (Hey, Amazon, Arizona doesn't charge sales tax on solar panels. So what's up with you charging me tax?), I am now officially kicking myself thoroughly.

Also, SanTan also sells large gauge wire. (Although not yet posted on their web site.) They don't crimp connectors on for you. But a customer there said that NAPA Auto Parts will.

 

[AZRoadrunner]

Thanks for the report @pmikep -- Santan Solar is on my short list of places to visit in the near future. Google Maps indicates it's exactly a 2 hr drive from down here by Tucson. For now, I'm proceeding with the Eco-Worthy 195w panels, but waiting for the second one to come in (one of them was smashed to smithereens by Fedex). If I had known about Santan there's no way I would have had panels shipped in!