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OliveSheep
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# Posted: 1 Dec 2016 11:18am
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Facebook of all places... Stumbled across this posting today (see image). It broke down the sizing aspect of a solar setup pretty good. Along with the follow up comment.
My question, is this information, a pretty accurate way to size a system?
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bldginsp
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# Posted: 1 Dec 2016 01:10pm
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Yes, that's the basic start, but remember-
-it's bad for deep cel batteries to continually drain them close to empty, so you should get a lot more battery capacity so that in an average day you don't run them down more than a third
-the calculations are based on what you imagine you will use, but you will probably use more much of the time.
But yes the first baseline for designing a pv system is determining usage, but that determination needs to be realistic and the batteries need to be able to handle the reality without being strained.
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MtnDon
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# Posted: 1 Dec 2016 02:39pm - Edited by: MtnDon
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The above mention of 50% maximum discharge is for lead acid batteries. Some, like LiFePo4, are very different, less of a worry, but more $.
The last sentence about solar panel array size is wrong... the # of watts of panels has many variables across the country. Some place have better sun, fewer clouds and some places (south) have more sun hours in winter.
Keep in mind that lead acid batteries lose capacity as the battery gets colder and that a fully charged lead acid battery does not freeze until -90 F but a 40% charged lead acid battery will freeze at 16 F.
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Atlincabin
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# Posted: 1 Dec 2016 06:06pm
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I agree, this is the basics. For reality, I would assume perhaps 20% more usage than you think (depends on how well you can really estimate), and then another 20% for parasitic losses, inverter use etc. As was said by others, the most critical aspect is having enough battery capacity so that you don't discharge them too far (I try to keep mine above 75%). Solar panels are relatively cheap these days, so its worth adding a couple extra above what you think is the minimum. Also, consider whether you are using the system constantly or just on weekends (which would allow the system to recover while you are gone). And how many cloudy days in a row are likely in your location that you have to get through. Having a gasoline generator can help with long strings of cloudy days.
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DaveBell
Moderator
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# Posted: 1 Dec 2016 06:34pm
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Here are some calculations I use.
Planned Consumption in Watts
60 watt light bulb x 5 hours = 300 watt hours
1800 watt MW oven x 15 minutes = 450 watt hours
Total = 750 watt hours
Convert watt hours to amp hours
Total 750 / 12 Vdc system = 62.5 amp hours
Calculate for total battery capacity needed (limit 20% draw down)
62.5 amp hours x 5 = 312.5 amp hours battery capacity
Calculate recharge time to find solar array size
(750 watt hours used / 500 watt solar panel array) x 2.5 = 3.75 hours to charge
Calculate Charge Rate (Amps)
Total 20-hour amp hour rating x 0.13 = charger size/charge rate setting (Amps)
Find sun angle in summer vs winter to optimize array position
Use online sun angle calculator
Canada more extreme sun angle change, adjust array 2-4 times a year?
Florida less sun angle change, adjust array 1-2 times a year.
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OliveSheep
Member
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# Posted: 1 Dec 2016 09:34pm
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Thanks for the great followup guys. Appreciate your thought on this. I'm in early planning stages for things at our cabin. We run generator mainly now, mostly just in the evenings to have some lights (cfl bulbs) to charge up phones and cameras.
I'd like to eventually have a small enough setup that we can have a light or two here and there, or charge a camera / phone, without having to wait until the evening hours when the genny is running. And your comments put me on track. Thank you again.
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Steve_S
Member
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# Posted: 2 Dec 2016 06:47pm
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Here is a site with calculator that takes the guesswork out and makes it simple. Enter your Country, State/Province, nearest city and it does the rest. This seems to only work for Canada & USA. There are many out there but this one works and does the hard work easily.
http://solarelectricityhandbook.com/solar-angle-calculator.html
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LS1
Member
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# Posted: 4 Dec 2016 09:21am
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I came up with a simple excel spreadsheet to help me calculate solar system sizing. It allows you to input in the yellow areas and will give results for battery, array and inverter size.
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Wendigolake
Member
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# Posted: 4 Dec 2016 11:06am
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LS1, thanks for the spreadsheet!
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rachelsdad
Member
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# Posted: 29 Dec 2016 04:00pm
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soooo.
Using that spreadsheet LS1 graciously provided:
I will use 2632 watts per day
2 day back up...weekend use mostly but I'll build in some capacity.
The formula says I need 487 ah at 24v
and a Solar array of 460 watts.
Seems small.
And not quite sure what line 27 means.
happy New Year all!!
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Nate R
Member
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# Posted: 30 Dec 2016 08:27am - Edited by: Nate R
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I've found that this site: http://www.pveducation.org/pvcdrom/calculation-of-solar-insolation
Has some of the best data I could find. You can put in your latitude, panel angle, and get an output of how many sun-hours per day you'll get throughout the year, by day (Hit the show data button)
Been great for calculating how many hours of sun I can expect in winter, summer in between, etc.
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creeky
Member
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# Posted: 30 Dec 2016 11:07pm - Edited by: creeky
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SMA has an awesome solar software online or for download that is free. Or free with registration. "Sunny Design" You can put in everything from panels, voltage, location, battery etc. All in one. Really nice. See it here.
NB: don't forget your hidden loads. A fairly good inverter will use 500 watts a day on idle. A cheap one. More. Esp. if it's oversized. I saw a chinese inverter that was at 50w idle. That's over 1kw a day idle loss. Then factor in your what 80% efficiency charging?
Note: the 2632 watts you need with lead acid is less than one days storage at 467a/24v. 50% dod on lead acid means you get 2 years of battery lifespan.
Lead should be set a 20% dod. Not 50. Maybe as a weekend user you're okay. because you can recharge during the week. Not that many cycles.
If you look at the tables tho. Realistic cycles at 50% dod are like 300.
as a general observation.
Lead is Dead.
Go lithium. The real costs are way cheaper for lifepo4 or li. And if you go diy on the lithium its actually cheaper day 1 over lead.
With lithium for 2 real days at 2632 you'd need 260a/24v. Batteries will now go 5000 cycles.
Note: I've actually recycled chevy volt batteries at 10/kw usable (80% dod). And used them.
So I'm just sharing my experience with 5 years full time solar powered with lead. Plus that's 365/24/7 uptime for 3 years. So it was dialed. And let me tell you. with 1/2 a year now on lithium. Night and day. One system was driving a model T with a wonky left wheel on washboard roads. The new system is like driving a Tesla on the autobahn.
Lead is only cheaper for one day. Lithium is cheaper after that.
After all. The new year is 2017! Cheers!
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groingo
Member
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# Posted: 31 Dec 2016 12:37pm
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The nice thing about solar is that it is flexible and you can ad or subtract power as needed, you can never have too much.
Exposure is vital....full sun is nice but bright clouds are even better....shade, and rain and snow on panels are bad.
Reduce energy requirement before going solar....find out how much juice you actually need and when you do that then tripple it and you will have a good realistic idea how much solar you will need.
Look outside the box, try things and don' t believe everything you read....there is no substitute for hands on experience while mistakes can actually be good.
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LS1
Member
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# Posted: 1 Jan 2017 02:28pm
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You need to change the charging voltage for a 24v system, the spreadsheet was set up for a 12v battery and panel. Line 27 was a note about lines 29-33 and a way to check the max wh given a certain size battery.
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