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morganplus8
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# Posted: 1 Nov 2012 02:46pm
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I want to concentrate on certain aspects of my system. The goal is to buy a system that can be expanded upon, over time. For this reason, I won't give you the load requirements as they will be limited to drawing power down by 50% max at any given time and then allowed to re-charge back up again to 100% before being used again. I will listen to music, run a couple of LED bulbs and recharge a phone or two.
I want to start off with a 100 watt mono solar panel.
2 - 6V Golf Cart batteries connected in series to produce 12V
1 - 20 Amp MPPT controller
2 - invertors for 120V ( I own up to 1500 watts now)
4 - Hours of direct sunlight per day
1 - 1800 watt generator for charging and daily use
With this in mind, I would like to be able to double the batteries and 100 watt panel down the road. I have read so much on this site that now I'm a bit confused. Will a 100 watt panel receiving 4 hours of sun per day charge the two batteries up in one day? Will there be some additional charge available? I know that doubling the panel and batteries won't double my available watt/hours without some loss but it will be close. I plan on having two invertors available, one for short-term high power needs and one for running/daily needs. I purchased a Kill-a-watt meter to see what my small fridge (135w/20 mins/hour) will draw against my 50% capacity system, should I decide I need a fridge for a few hours.
Thanks in advance for any help you may offer.
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GomerPile
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# Posted: 1 Nov 2012 03:42pm
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A lot of things affect how much power you can harvest from your panels. One of the best investments I made was a good system monitor. I bought one from Xantrex, but others make them too. The monitor will track amp hours in vs amp hours out so you have a good idea how well you are harvesting sunlight.
Another member here recommended an inverter from Morningstar (http://www.morningstarcorp.com/en/home). I have been looking at them for a data logged charge controller for my new house. You might check their web site. Their sunsaver MPPT is basically a black box that you hook panel, battery, and a load...looks very simple.
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MtnDon
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# Posted: 1 Nov 2012 03:53pm
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One problem with adding to it down the road is the new batteries will act more or less like the old ones in their ability to hold charge. Generally it's not the best to add batteries to a set that has been in use a year or more.
Doubling the battery bank and the PV modules will double the capacity the way I see it. Any losses are already there in charging and using the stored chemical energy in the first set of batteries. So it should follow that doubling the batteries and the PV would give twice the capacity. At least that's the way I see it.
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VTweekender
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# Posted: 1 Nov 2012 04:38pm
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As far as the recharge time question....1 day will not be enough to recharge if your battery bank is 200 amp hours or so...(just estimating the capacity of your batteries)...if you drain 50% you will need 100 amps back in to top off the batteries again.... your 100watt panel will get you about 5 amps per hour of good sunlight...so in 4 hours you will gain about 20 amps per day...
As far as system capacity....with the 100watt panel system you suggest....you will have your LED lights , small portable music device and charging a cell covered OK.....forget the fridge on it..even if you double to 200 watt panels...
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CabinBuilder
Admin
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# Posted: 1 Nov 2012 05:05pm - Edited by: CabinBuilder
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How do you plan to charge batteries using generator?
I think you also need add a battery charger to your list, AC 120V -> DC 13-15V. (Unless your generator has 13-15V output specifically for that purpose. The gen's 12V output, if present, will not work for charging batteries.)
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groingo
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# Posted: 1 Nov 2012 06:12pm - Edited by: groingo
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Figure your power requirement then double it is a pretty good rule of thumb and tripple the cost.
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MtnDon
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# Posted: 1 Nov 2012 08:12pm - Edited by: MtnDon
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Quoting: morganplus8 Will a 100 watt panel receiving 4 hours of sun per day charge the two batteries up in one day?
ooops, made a boo-boo; corrected reply coming soon.
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MtnDon
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# Posted: 1 Nov 2012 08:39pm
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I re-ran the numbers and with the given parameters...
4 hours good sun
2 x 220 amp-hour (typical golf cart) battery
50% draw down
Lowest temperature of the batteries 30 F
I come up with needing 5 of the 100 watt PV modules
(100 watts each = 18 volt Vmp and 5.5 amps Imp)
480 watts is what the calc comes up with
If those 5 x 100 were in parallel they would overwhelm a 20 amp controller.
If you are getting an MPPT controller anyways there would be an efficiency advantage to get PV modules with a Vmp in the mid 30 volts range. usually used with 24 volt systems, an MPPT controller that could handle the higher voltage would be able to wring out amperage from the lower values from early morning and late day sunshine. Just a thought.
I'd give thought to increasing the number of batteries as well. Double that to 4 and you'd have one day of reserve in case of cloudy days.
And cabinbuilder is right on about needing a charger. To put a charge into a 12 volt battery you need to be able to produce at least 14.4 volts to charge in a reasonable time.
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MJW
Member
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# Posted: 2 Nov 2012 12:28am
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MtnDon,
If you were designing a solar set up (panels, batteries, charger, inverter, the entire set up) for yourself with a need for 4kw a day, what would it include?
This would be if you were in a southern MO/northern AR location.
Thanks for the insight.
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Dillio187
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# Posted: 2 Nov 2012 03:31am - Edited by: Dillio187
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my quick stab at it at 2am (thanks for waking me up dog), to replenish 4kwh per day, while leaving 2 days extra for cloudy days.
12kwh would take a 3000 watt array 4 hours to replenish (theoretically).
In the real world with system inefficiencies, plan on about 77% efficiency from the panels, so now you're talking a 4000 watt array. (3080 watts derated)
Now if we start talking battery banks and inverter efficiency, that can muddy the water even more, anticipated loads, maximum wattage of those loads, and overall system voltage come into play here.
to the OP: your 2 6V GC batteries will be around 220ah in a 12V config. Assuming you take these down to 50%, you will want to fully charge them the next day. you will need MUCH more panel than you are planning...
I'm going back to bed, I'm sure Don will post more and I'll follow up in the morning, hopefully more well rested 
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morganplus8
Member
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# Posted: 2 Nov 2012 12:22pm
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Wow, plenty of useful information here!
I'll work my way down the list of responses and see what's left to think about.
GomerPile,
I like the idea of a Xantrex as a system monitor, didn't think of that so thanks. I will add monitoring to my system, great idea.
Mtndon,
I didn't mention the battery issue but you are right, I need to buy all the batteries at the same time and not mix old with new. I was aware of this from my readings on this board but failed to mention that I will deal with that issue, thanks.
VTweekender,
Fare enough, even though most of the time I will only draw 20% during my visit, I do plan on going there for 6 - 8 hours once a week (50% consumption), and 2 visits per week (20% consumption) and if I consume 50% it will take longer than my next visit to replinish those batteries. I would say that 4 hours represents high noon sun and that those panels will see perhaps 12 hours of sun in the summer. Like so many say, "be conservative with your numbers" so I chose 4 hrs. Thanks for looking into the "numbers of the system" that's a big help for me. Would you please look at my picture here and say what you think of me installing panels on my gazebo? I could fit 2 - 100W panels up there. Thanks.
CabinBuilder,
I will be using a charger, should have mentioned that! The gens a 1800 watt unit and should bring those batteries back quickly enough. It will be used if I want my fridge running longer than a few hours some day. Thanks
groingo,
I used 4hrs as my sunlight while there is probably 12 hours worth at various angles and I included that MPPT unit to help with other factors. Thanks
MtnDon!
Hey, if you can error in my favour, cool! Oh yeah, 5 panels aren't going to fit in my system, I can't get that many to target the sun without undue changes. I would top out at 200 watts of panels and make use of my generator once a week when I might hit the 50% draw level. Of the 3 days visiting, I might use 20% on two days and more on the third, weekend day. I have no problem buying a bigger MPPT unit but I will be only using 200 watts max on this project, for that reason, I will stay with 12V too. You are convincing me that 4 batteries and 200 watts in panels to start with makes great sense. The charger is a given, but the move to double the capacity was the problem. So far, things are pointing to a bigger system right away. Please look at my gazebo and see if you like the idea of placing the panels up there, batteries close by, and running 120V to the building via an invertor. Thanks!
Dillio187,
Thanks, you are right, I think I'll be going with 200 watts for panels, possibly more, say, 270 watts which will be over-kill for all but once a week when more people will be there. I won't be taking this system down to 50% often, just the radio playing most of the time without lighting. But, if I decide to run that fridge at 135w x .33% = 42W/h, I will need 4 batteries to get me through the day at that consumption level. I will report back when I use my new Kill-A-Watt unit on that fridge. Whatever else happens, the beer and streaks have to be cold at the end of a day of fishing!!!
Thanks to everyone for helping me out here.
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MtnDon
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# Posted: 2 Nov 2012 12:57pm
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To morganplus8 and anyone else
Keep in mind that if the batteries are not fully recharged at least every other day their life will be shortened. My rule of thumb is that if the PV system is not capable of bringing the batteries through bulk and absorb every normal day, the PV source is too small. Also if you do have to use a generator regularly to bring the batteries to full charge then you are better to pick your day and start it with the generator charging even before the PV kicks in. That is a better use of fuel than finishing the charge with a generator.
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MtnDon
Member
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# Posted: 2 Nov 2012 01:09pm
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to MJW
Here are some rough numbers. If the daily consumption was limited to around 3300 watt-hr then 4 - L16 (Trojan 6 volt 370 amp hour) could work with the 50% depth of discharge. Draw more than 3300 watt-hr and you'd be wise to use 8 - L16, or a battery with higher capacity.
That would give no cloudy day cushion. For a two day cushion at 3300 watt-hr use 8 - L16; 3 days, 12 - L16.
If using 4 - L16 drawn down to 50%, IF there was 4 hours of good sun about 1750 watts of panels should be able to bring the batteries back to float each day. That would be adjusted for more batteries with a 2 or 3 day cushion, depending on how fast you'd want to recover from the cloudy days. IE, if cloudy days are frequent then you'd likely want to increase the panel numbers significantly so you could recover in a day or two. If cloudy days are infrequent you could likely stand to take longer to rebuild the charge. What you don't want is to have the batteries remain in some state of discharge for days, weeks at a time.
As for hardware like charge controllers that would be figured out once the system batteries and PV panels were known. I'd recommend a 24 volt system for sure; 48 volts could be a good option too as that would reduce the number of parallel strings if you get into the range where you'd be having two or more strings in parallel. Trojan makes a 2 volt L-16 that suits some systems very well.
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MtnDon
Member
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# Posted: 2 Nov 2012 01:18pm
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MJW that was also figuring the battery temp might fall as low as 40 F. If warmer then the available capacity increases, colder, it decreases. And keep in mind that, IMO, batteries do not belong in living spaces. Boxed, sealed and vented to the outside, maybe. Think worst case.... batteries get depleted, they freeze and crack and leak.....
Full charge won't freeze until -92 F
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MJW
Member
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# Posted: 2 Nov 2012 01:42pm
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Thanks for the help, MtnDon.
Trying to figure out this solar thing is giving me fits. I calculated that we would need 3kw+ per day so I wanted to go with 4kw to make sure we had enough and I also want to build a system that will allow for expansion if for any reason we ever need more.
We are currently looking for property in the southern MO/northern AR area and are pretty sure this will be the area we buy in. Of course in that area there will be occasions where the temps will be below the 40 F you mention. I am planning a separate shack to house the batteries and other solar equipment.
It sure would be nice if someone offered a "package" based on your location and power needs. haha
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MtnDon
Member
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# Posted: 2 Nov 2012 02:27pm
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12 of the 2 volt Trojan L16 would give a 24 volt system 2 days of autonomy with a 40% discharge depth at a daily 4000 watt. Something to think about
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morganplus8
Member
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# Posted: 2 Nov 2012 11:11pm
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MtnDon,
Good point, there would be one day per week where the generator would have to assist the panels in order to recharge the batteries within 24 hours. The rest of the time there wouldn't be a problem. I have to confirm what the actual draw of the fridge is to see how many hours of operation it will receive.
I won't go above 270 watts worth of panels (2 times 135 watt panels) with this setup, 4 - 6V batteries and a charger that will be used a few times per year. Thanks for your help!
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TomChum
Member
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# Posted: 2 Nov 2012 11:33pm - Edited by: TomChum
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I have 4 6v batteries, 2 85w panels, 12volts, 250Ah. Have 5 hours sunlight. In the summertime its not enough to run a fridge, and I have a very efficient fridge. Its close though. I just run some LED lights and charge phones, and a laptop computer. I intend to add a 135w panel for 85+85+135= 265watts. Adding the 135 mainly for the fridge.
Incidentally, my Kill-a-Watt will not operate with my Magnum Energy MM-1012 pure sinewave inverter. Kill-a-Watt tech says it works with some inverters. He said there's no way to make their device compatible with every inverter.
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MtnDon
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# Posted: 2 Nov 2012 11:41pm
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I don't know anyone who successfully runs an electric refrigerator, other than a SunDanzer in a totally off grid setup. I haven't looked at the standard fridges for a few years to see if and/or how much they might have improved over the years. We went propane as my calculations back then showed I'd have to double the system I planned on if we were to not have to worry about cloudy days when we were not there. Hope the fridges have improved.
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TomChum
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# Posted: 3 Nov 2012 12:04am - Edited by: TomChum
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Hi Don, I have a Sundanzer DCR50. Mfr claims it uses about 114 watt-hrs per day. (or 100 watts for 1.14 hours a day). I'm sure it does, and then some. Seems like it uses about 400w-h per day. I would not leave it running unattended, a few cloudy days in a row or batteries will be drained. It is 1.8 cu ft and has no freezer. There is a freezer version that uses 2.5x the power. (DCF50)
It was a very expensive little fridge, and the thick insulation takes up a lot of space too. Its about the size of "a block of four coolers" but has the capacity of one cooler. If I could do it again I'd spend the 700$ instead on more panels and just run a cheap 120v fridge off my inverter. $700 would have bought 400 watts of panels. Kind of a brain fart purchase that I'm not too proud of. Propane wouldn't risk batteries, would have a known runtime too.
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MtnDon
Member
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# Posted: 3 Nov 2012 12:12am
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Quoting: TomChum I would not leave it running unattended,
See, that was what turned me to a propane fridge. I understand the concern about running out of power and food spoiling, but I like the convenience of leaving a fridge with things we use from week to week, weekend to weekend. Yes, we use some propane doing that but it's only about a pound a day. The same thing applies to an electric fridge; I'd want sufficient power storage to not have to worry about running out.
Or have an automatic start propane fueled backup generator feeding off a 200 gallon tank (or bigger) to replenish the batteries when they hit a preset low voltage level.
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MJW
Member
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# Posted: 3 Nov 2012 12:13am
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A fridge and small freezer will be using the lions share of the 4kw a day we will need. We will be going propane on the stove and hot water but I really want to stay with electric on the fridge and freezer.
The best numbers I have been able to find so far on a fridge run around 1200 watts a day for a smaller, apt size unit. Ouch.
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morganplus8
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# Posted: 4 Nov 2012 03:42pm
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I have been doing some research and I plan on buying 4 batteries:
Costco 6V 225 AH Deep Cycle Golf Cart batteries ($90/)
Giving me 450 AH at 12V roughly 450 X 12 = 5400 Watts/AH equivilent
(The rating of 450 amp-hours in a battery bank means that the battery can supply 450 amperes for one hour, so to speak. In reality, these ratings are normalized to an eight hour rate, which means that this battery can supply about 56 amperes for eight hours. Actual run time at 450 amperes would be somewhat less than an hour.)
I hope i have this right? This means that my load of 45 watts/hr will be roughly (45W X 14 hrs) = 630 Watts/day of Consumption or about 630w/5400w = 12% draw?
By using 200W of panels with 4 hrs of direct sun, I will be supplying 800W during the high point of the day to replenish the draw of 600W. I realize that not all of the 4 hrs of sun will charge the batteries as they aren't drawn down enough but it is close. Eight of the 14 hours the panels will keep up with consumption here.
I know that there is resistance and time issues with this setup, but if my numbers are correct it looks like I should be able to run my fridge for 14 hours and top up the batteries well within 24 hours of consumption using my original 200W panel system.
Can someone please set me straight on these numbers? I have brain fog now. Thanks
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offgrid ME
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# Posted: 4 Nov 2012 06:47pm
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First you should never use more than 50% of the stored energy in your battery if you want it to last more than a year. You have about 450amp hours at 12 volts x 50% which is 2700wh of power. When using an inverter and a battery bank the overall efficiency from panel to outlet is about 52%. This being said your 200w of panels will produce around 400wh of usable power per day. A modern energy star fridge uses about 1kwh per day. You will be defficit charging the batteries and will kill them in short order. You would need about 500w of panel just to run the fridge
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morganplus8
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# Posted: 4 Nov 2012 07:58pm
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Offgrid ME,
I have never read a post where the efficiency from the panel to the outlet is only 52%, can you explain this for me?
The panels will see 10 hours of sun with 4 hours of strong rays, how does this equate to only 400 watts? I have come to expect 800 watts plus a % of less efficient watts for the balance of the sunlight.
The fridge has been spec'd at 600 watts total for the 14 hours of the day using a Kill-A-Watt. It is very small, not a full size fridge.
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offgrid Me
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# Posted: 4 Nov 2012 08:14pm
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It is generally accepted that a battery based solar PV system usually runs around 52% end to end overall efficiency. The panels usually produce around 77% of there name plate rating, the charge controller runs around 92%, the inverter around 95% and the batteries which can be very inefficient in the final stages of charging(absorbe stage) less than 80%. All of this equipment also uses power just to run. The inverter itself can use upwards of 30w per hour times 24hrs is 720wh per day. I have been living off grid for a while now and can tell you you never generate as much as you expect, and your loads are always greater than you planned. How long did you have the kill a watt hooked to the fridge? One day is not enough to get a good average. Try it for a week and see what you come up with. The fridge will run longer when you plug it back in to bring the inside temp back down to t stat setting
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morganplus8
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# Posted: 5 Nov 2012 08:52am
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Well that's not good news offgrid Me!!
I was hoping the panels would be far more efficient and the fact that they continue to take in more sunlight long after the sun moves of the high point of the day. This coupled with a MPPT Controller should give me the minimum 800 watts that I expected. The invertor will always be turned off when we aren't there which is most of the week. The problem here is the load, in this case, the fridge, I ran that test for 24 hours and I admit it was under better conditions than it will operate in at the lake!
I have only enough room for 2 panels, they can be 135W units to compensate for losses at that end. With the invertor off most of the time, I might still hit my numbers. I'll run the fridge test for a week and look at the numbers. The fridge number was from a warm start but the temperature of the room was ideal for an easy showing (about room temp).
This is a tiny fridge that will be used to store meat/booze while we boat, it costs $ 3.00 for a bag of ice, hmmmmnnnnn???? Hard to justify anything but a very small lighting/radio solar system isn't it? Thanks
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TomChum
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# Posted: 5 Nov 2012 09:50am - Edited by: TomChum
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I use ice and a fridge. My fridge sits out in the shed, and we don't open it much. The cooler sits on the cabin porch. When it runs out of ice after about 1.5 days then the important stuff moves to the fridge. I thought about burying a "5-day cooler" (big ugly things!) in the ground outside the cabin and put a wood cover over it, like a storm shelter.
You might consider downsizing your fridge. After buying a $700 12v fridge I had the thought of buying a "normal" small fridge, and gluing styrofoam insulation to all sides of it. You can then glue panelling to the styrofoam then it might look like a wood fixture. Of course you want to start with one thats efficient. Also wide temp swings cause a fridge to run more. For example when it's 40 deg outside some fridges can get confused, not knowing which way its cycle is supposed to go. Most fridges are designed for efficiency with the interior always cooler than outside, and also ~30deg difference. I don't know anything about fridges ability to handle temp swings or inverted temps - but was told that simply putting a fridge outside does NOT mean the thermostat never turns it on (told by a refer representative who seemed to know something about it - www.truckfridge.com)
You might confirm your kill-a-watt works with your inverter, before heading to the cabin expecting to gather data. My Kill-a-watt doesn't work at all with my inverter.
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morganplus8
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# Posted: 5 Nov 2012 10:47am
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TomChum,
I was thinking of burying a few things too! I got this idea that I could buy a 500 gallon stainless steel tank and put it under the cabin with inside access for storage. Then I could place a bunch of stuff in there and not worry about them. A cooler would be good as well as things that take up space in the cabin itself.
This fridge is so small it should be able to run for a few hours while we are boating. I brought the fridge home to do the Kill-A-Watt testing thats why I claim it is under more favorable conditions. I can't find a smaller fridge and I might place it in a styrofoam container too.
I'll run the test and get a meter for my batteries and see if I can get something to work in reality, sometimes "theory" can be off when we don't address all of the variables!
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TomChum
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# Posted: 5 Nov 2012 11:21am - Edited by: TomChum
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Yes a cellar, with trapdoor hidden by a rug, I wish Id'a dug a cellar too. But the rug has to be one that nobody would lift to steal. Or if it moves when they look behind the couch.
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