Just when I think I got it...

[monicaj]

I lose it. I feel like I almost grasp it, then poof it's all gone.

I'm not trying to do anything huge here. I have a friend who needs some power. I have 2 of these solar panels. I don't know if I need to wire them in series or parallel. All I know is that I'll need an mppt controller if he's going to use a 12v battery bank.

He's not going to be running much. He hopes he can run a washing machine, and small freezer or fridge. Everything else will be minor... 12v lights, 12v fans, etc.

He's a tad more clueless than me and he says he wants to have 2 systems in case one goes down. So that would mean each solar panel having its own charge controller and both controllers feeding into the same battery bank. Do you guys see any benefit for him to do it this way?

I'm going to try to talk him into just hooking these 2 panels together for one system, and then later if he picks up panels that are different than these, then he can make another array. Am I right? What can make the system fail would be the charge controller right, so couldn't he just have a spare controller instead of having 2 sets?

I like the idea of having a monitor showing what the panels are producing and where the batteries are, etc. Don't charge controllers that monitor these cost a bit more, right?, so maybe better to just buy a separate gadget to monitor things?

I'm so lost. Will you please suggest to me which charge controller and inverter he should get, and how many amps he needs in his battery bank? He's probably going to go with golf cart batteries... my solar panels.jpg

[SunEagle]

I would say that 2 of those 280w panels will require an MPPT CC rated at or above 45amps (2 x 280w = 560w / 12v = 47amps) similar to the Morningstar TS-MPPT 45A

With 45 charging amps the battery system should be rated between 360Ah to 540Ah. So if he installs a 500Ah 12v system it should safely provide about 1200watt hours daily (~ 20% DOD) and that 560 watts of pv should be able to recharge the battery usage.

Now the question is how many watt hours will his loads (lights, fans, fridge, washer, etc.) use a day. I can tell you that you can forget the washer and any large fridge. Also that wattage of the lights and fans need to be low if you plan on running them for long.

For an inverter you need to go with a pure sine wave type around 600 to 800watt. Going larger will end up draining your batteries faster.

[Amy@altE]

He needs to step back and do a loads list. Assuming he already has what he wants to power, I suggest he gets a Kill-a-watt meter and measures the usage over a few days. Then go to an off-grid calculator to see what size system he needs. He will soon find that he cannot run what he wants off only 2 solar panels.

But to answer the original question, since you are already planning on using an MPPT charge controller, you can wire the panels either way, series or parallel, although series would be better, because you will have higher voltage and lower current travelling on the wire from the panels to the charge controller, resulting in needing smaller wire and achieving lower voltage drop.

[SunEagle]

He needs to step back and do a loads list. Assuming he already has what he wants to power, I suggest he gets a Kill-a-watt meter and measures the usage over a few days. Then go to an off-grid calculator to see what size system he needs. He will soon find that he cannot run what he wants off only 2 solar panels.

But to answer the original question, since you are already planning on using an MPPT charge controller, you can wire the panels either way, series or parallel, although series would be better, because you will have higher voltage and lower current travelling on the wire from the panels to the charge controller, resulting in needing smaller wire and achieving lower voltage drop.

1+. I agree with the series wiring. The OP just needs to pay attention and does not exceed the max DC input voltage for that CC.

[monicaj]

Thanks for your responses. I used an excel spreadsheet calculator to try to determine his kwh (kwh right?) and I'm going to see if this post will let me upload it... not sure if you can open it if you don't have excel. Wouldn't let me upload.

The calculator said he would use 1130 kwh a day. I know that the calculator is only as good as the figures entered in... for instance, you can insert a number for the quantity of devices you'll be using, the volts of it, the watts of it, how many hours a day you'll be using it, and the number of days you'll have it on.

The spreadsheet had a list of appliances and had the figures already in there, and I know that not all freezers are the same wattage, they aren't all the same. But for example, the spreadsheet had a 10 cu. ft. deep freezer, 117 volts, 125 watts, 7 days a week. We inserted a 3 for how many hours a day we thought the compressor might run, total. BUT WE WERE UNSURE, were we supposed to enter 24 hours, or just how much time the compressor might use? -. then the formula on the spreadseet said that the deep freezer would need 375 kwh a day.

10 cu.ft. Deep Freezer

1

117

125

3

7

375.0

After adding all our info, the spreadsheet would transfer our total kwh to the next page where it would calculate batteries needed, how many days of storage etc. But it was still confusing to me, and didn't help with the cc or the inverter.

He's going to be very frugal on his energy usage (he's basically camping right now, so even having lights for an hour at night and a 12v fan for 3 hours will be a step up). But who knows how accurate the spreadsheet is... is he screwed if he wants to have a freezer and washer run off this system? He hasn't really bought any appliances yet so we didn't test the wattages.

I appreciate your suggestions on the charge controller and inverter. I'm going to get those.

[SunEagle]

Using the voltage or watts that are listed on an appliance could be misleading to what it actually uses in a day or week. I would suggest getting a low cost device called Kill A Watt. It allows you to measure the watts used by an appliance over a period of time. If you collect data for 48 hours you can get a pretty close approximation of how many watt hours that appliance will use in a 24 hour period. The same goes for lights, fans or cloths washer that plug into a receptacle.

Just remember that some days there will be no sun so you can't recharge the batteries. If the sun isn't out for two days your battery system starts to be critically drained which shortens its life. All good off grid systems include a generator that can be run to supply power for your appliances when the batteries are low as well as allow you to recharge the batteries.

Final thought would be how the battery system is built. If it is going to be a 12volt system then I would suggest getting low voltage batteries 2, 4 or 6v rated at the Ah needed and wire them in series to create a 12volt system. Most golf cart batteries are 6volt but at most only 232Ah. To get the 500Ah needed you have to wire batteries in parallel which increases the chance of not charging or discharging them equally. Trojan makes high Ah rated batteries that cost more but last longer then most other deep cycle or semi deep cycle (golf cart) batteries.

[monicaj]

Thanks so much SunEagle for your direction in this. You've given me the exact information I needed.

[Amy@altE]

Since the appliances aren't purchased yet, and you can't test them with a Kill-a-watt meter, go to the EnergyStar web site to see the energy consumption of the models considered. If they aren't EnergyStar rated, I recommend picking one that is. Although the appliance will likely cost more, it will save money by requiring a smaller solar system. It'll tell you kWh a year, divide by 365 to get daily use. It should be more accurate than guessing, but obviously not as good as measuring. https://www.energystar.gov/productfinder/

[monicaj]

Thanks Amy.

[VTSE]

after you v determined the wattage of the appliances you wanna install then you should now determine the inverter size and controller.

[monicaj]

I would say that 2 of those 280w panels will require an MPPT CC rated at or above 45amps (2 x 280w = 560w / 12v = 47amps) similar to the Morningstar TS-MPPT 45A

With 45 charging amps the battery system should be rated between 360Ah to 540Ah. So if he installs a 500Ah 12v system it should safely provide about 1200watt hours daily (~ 20% DOD) and that 560 watts of pv should be able to recharge the battery usage.

Now the question is how many watt hours will his loads (lights, fans, fridge, washer, etc.) use a day. I can tell you that you can forget the washer and any large fridge. Also that wattage of the lights and fans need to be low if you plan on running them for long.

For an inverter you need to go with a pure sine wave type around 600 to 800watt. Going larger will end up draining your batteries faster.

Wow! I forgot that I put a post to this forum almost a year ago. I only found out after landing on a different post and deciding register, and found out I was already registered.

SunEagle, I read through my old post and again, felt much appreciation for your detailed answers. It was actually one of your comments on a different post that prompted me to register and ask a question. Here's the post I was looking at since it relates to my question. https://www.solarpaneltalk.com/forum...ight-direction And further down I'll paste your response so you can see what my new question refers to.

I haven't looked at solar stuff in a while (so now I have to get familiar again), and my situation changed from this old post, but I'm hoping that by commenting here, it'll bump this post to the top of the forum list.

My new situation is that I kept both of those panels instead of giving them to a friend. So now I have a total of 4 panels (all the same specs) that I want build a system with. And instead of using golf cart batteries, I just want to use some bigger deep cycle batteries.


So here's what I gleaned so far, please tell me if I'm on track: It's better to wire the panels in series to get lower voltage drop and use smaller wires. I can figure out what size controller I need by using the formula 4 x 280w = 1120. Then 1120 / 12v = 93.3 amps, so I would choose an MPPT CC that is over 93.3 amps. SunEagle said, "The OP just needs to pay attention and does not exceed the max DC input voltage for that CC."

So first quesrtion: To figure out max DC input voltage, I'm looking at the Voc number? So the Voc # on my panels is 44.8. Then 44.8v x 4 = 179.2, and I need to make sure the CC is rated to handle that much voltage coming in, plus a little more?

Second question: Figuring out size of battery bank and inverter... is it a simple matter of doubling the figures since my solar panels are doubled? Like, for the inverter, go from around 700watt to around 1400watt? and battery bank Ah doubled as well? (I have full doubt that it will be that simple but had to ask.)

So then I finally come to SunEagles response to the other post that is about the same questions I been asking... I'm pasting his response, and then below that, my question is in regards to using a 12v battery bank vs a 24v.

----------------

"... So for a 1000 watt ( 4x 250watt) solar panel 24volt system you should get an MPPT charge controller rated 45amps. 4 x 6volt batteries rated about 800 to 1000AH. A 24volt inverter rated no more than 2000watts max. You will wire the panels in series to the charger controller (make sure that CC is rated for the total Voc of the 4 panels plus a little more). The wire between the panels and charge controller need to be rated about 1.25% greater than the Imp rating of one of those panels, and sized not to exceed a 3% voltage drop. The wire between the charge controller and batteries (depending on the length) should be rated 50Amps and have a 50amp fuse.The wire between the batteries and inverter needs to be rated about 100amps with a 100 amp fuse. This system should get you about 5000 to 6000 watt hours a day if you don't discharge your batteries more than 25%. I do not know if that is enough or too much since you haven't determined your actually daily watt hour needs.

You can go to a 48volt system which requires 4 more 6 volt batteries. This will reduce the size of your wires and fuses due to the higher voltage as well as your Charge controller amp rating. The area you have to watch out for is that you will still need enough amp output for that CC to keep that battery system happy with a C/8 to C/12 charge rate where C = Battery system AH."

------------------

I'm not yet focusing on fuses and wiring between batteries (and when I do, I think the above info will get me through - Thanks again, SunEagle!).

As far as 12v battery bank vs 24v... I need to see if I can get a general understanding of the differences/benefits. Is it that a CC that is 24v will more efficiently get power to the bank? Faster? From the last sentence in the above paragraphs... Higher voltage means reduction in wire and fuse size? Whew. This is some hard stuff to grasp.

The last question: Most of the things I'll be using are going to be 12v... so if my battery bank is 24v, do I need something in between the battery and my 12v stuff to make the volts going to my stuff 12v?

Notes: I know that the normal way a system is created is by first looking at the load, figuring out what you want to run. But I only have these panels and I really don't have funds to add more. So I'll learn to use a killometer and check surges, wattage and amps, etc, before I try to add it to the system. I lived for a year on 80 watts of solar power, and really couldn't do much (still loved it though, lived near a huge forest), so when I get back off grid, 1,000 watts will seem like a luxury.

Thanks in advance to all you people on here for freely giving your time and knowledge to help. It's very much appreciated.

[SunEagle]

Welcome back monicaj.

The problem with using over 1000 watts of panels to charge a 12volt battery system is you need to use a charge controller that is bigger than 80 amps which they usually do not make. So going to a 24volt battery system is a much better choice.

The maximum DC input voltage I referred to is listed on the Charge Controller specification documentation. You are correct it is determine by adding up all of the panel Voc ratings that you wire in series. So if you had 4 of those 280watt panels shown in your first post with a Voc = 44.8v you would get a value of 179.2v which easily exceeds a quality 80Amp CC DC input rating. You could wire those 4 panels as 2 pairs (2 wired in series) wired in parallel which would lower you DC input voltage to about 90volts but you would still probably exceed the maximum wattage for a 12volt battery system.

So if you want to use all 4 of those 280 watt panels I would suggest getting at least a 60 amp CC and build a 24volt battery system. You would get about 47 charging amps out of the 1120 watts which would be enough for a 470Ah battery system.

If you understand that part we can move forward with wire and fuse sizing.

[monicaj]

Welcome back monicaj.

The problem with using over 1000 watts of panels to charge a 12volt battery system is you need to use a charge controller that is bigger than 80 amps which they usually do not make. So going to a 24volt battery system is a much better choice.

The maximum DC input voltage I referred to is listed on the Charge Controller specification documentation. You are correct it is determine by adding up all of the panel Voc ratings that you wire in series. So if you had 4 of those 280watt panels shown in your first post with a Voc = 44.8v you would get a value of 179.2v which easily exceeds a quality 80Amp CC DC input rating. You could wire those 4 panels as 2 pairs (2 wired in series) wired in parallel which would lower you DC input voltage to about 90volts but you would still probably exceed the maximum wattage for a 12volt battery system.

So if you want to use all 4 of those 280 watt panels I would suggest getting at least a 60 amp CC and build a 24volt battery system. You would get about 47 charging amps out of the 1120 watts which would be enough for a 470Ah battery system.

If you understand that part we can move forward with wire and fuse sizing.

I love the way you break it down in simpler terms, so I really think I do have a grasp on what you said.

At first I wondered why the 470 was still the Ah of the battery bank when I was adding 2 more panels, but then realized if I were using 12v bank, I would end up using a bank with higher Ah.

I'm ready for the wiring and fuse sizing.

[monicaj]

"The wire between the panels and charge controller need to be rated about 1.25% greater than the Imp rating of one of those panels, and sized not to exceed a 3% voltage drop."

Sorry for the way I'm about to mangle terms...

So, let me see if I understand the first part of that (Imp). I have 4 panels, all have 79.5A as the Imp rating, and since they'll be connected in series, that Imp figure doesn't change. If I split them and connect 2 in parallel, and then connect those together in series, am I right that the Imp rating of the system would be 79.5 x 2? And the Voc of the system would be 44.8 x 2?

I'm going to connect them all in series and make it a 24v system as recommended but wanted to ask that only to see if I'm understanding how things work.

Imp "of the system", and Voc "of the system"... is that the right term to use?

[jflorey2]

The last question: Most of the things I'll be using are going to be 12v... so if my battery bank is 24v, do I need something in between the battery and my 12v stuff to make the volts going to my stuff 12v?

Yes, you need a DC/DC converter. They are readily available on the net. Search "24V to 12V converter." I would recommend getting an isolated one.