I need help and plenty of it. Most not related to this forum. I am adding solar to a camper and bought three used 315 watt panels. Panels are rated at 37.2 volts each and 8.48 amps. Now when I bought them I checked voltage and all three read about 46 volts. Now I think I know why as I live in the frozen north and it was around 35 degrees when checked. My research showed that to be correct based on temp? It also gets at least 55 degrees colder here. Yes 20 below and sometimes worse. Based on these temps I am going with lead acid batteries. So now for the questions. Series or parallel? Why? What size mppt and brand? Why? Inventor charger? Why? Here is what I will be running. A few lights, USB ports and a water pump all 12 volts. The main reason for the solar is to run a 5,000 btu window A.C, a microwave in short bursts and a dorm fridge/freezer. I am also not opposed to 24 volts if that is the answer. I also have a 2000 watt and a 1000 watt gas portable generators when needed. Okay folks open to any and all ideas.
Need Advice
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MisterSandals
Participation Medalist
You need to check the Voc on the panel label, its probably closer to the 46 volts you are measuring without a load. I think this Voc is normally determined at 25C temp so it will be higher in colder weather.
You REALLY need to get the number of watt hours that you will use in a day figured out. Running AC and a microwave off of batteries is a tall order, it will require a substantial battery bank not only for the overall power but also for the discharge current.
Once you have your energy usage figured out, your battery requirements can be determined. Then its pretty easy figuring out your SCC requirements, especially since you jumped the gun and got solar panels.
Come back with your energy audit numbers and someone can help you from there.
First thank you so much for the reply!!! Labels on the panel say 37.2 volts and 8.48 amps. So I know I didn't explain it well but my concern was getting an MPPT that would handle the volts if I hook up in series and it is 10 below zero. I am guessing due to the angel of the sun this won't be a concern but again looking for advice or experience with cold temp voltage. I find the energy audit an extremely interesting discussion. The reason I feel this is interesting is I think it is impossible to do accurately until you take real world figures. So let's take the AC for a minute. So I do have some insulation it is unknowable exactly what the R value is. Also from the many many YouTube videos I have watched a 5,000 BTU window AC uses about 450 watts when running. The big unknown is how often it will actually run. 70 degree day with higher humidity might have much longer run times than 70 with low humidity. I probably didn't explain this exactly but I just see may to many variables to accurately predicate the watt hours. I also didn't explain this correct but the plan would be to run off solar as much as possible but use one of my generators as needed. You are so correct about battery capacity and once I have some real life numbers I would then add batteries if needed. Another idea I have as have others is shade cloths so the rig doesn't get as hot. Also I think I understand this correctly. Let's say for an example the 5000 BTU AC runs for 5 hours straight at 450 watts. If the panels produced 450 watts for those same 5 hours it would be a push? 600 watts for those same 5 hours and I can run other items and not lose any capacity. Would love to hear your thought and anyone else please educate me.
MisterSandals
Participation Medalist
Thats pretty much correct, if running at 100% efficiency (and i know you mentioned “produced 450 watts”).
For an AC unit, getting the watts when it is cooling and when it is idling, and what percent of the time it is cooling will be a guess, but good for a ballpark watt hour number.
Or get a Kill-a-watt meter and get real data.
Sounds like you will produce what you can and use the generator as needed.
Once again you are so correct. Real life with a complete system will be the test. Kill-A-Watt meter over many different temps, humidity and sun or partly cloudy days will give me the answer. Hopefully someone on here has already had this experience. I have always said my best ideas were copied from someone else. So what does everyone think on a brand and size of MPPT? I can always add batteries but I need to get the MPPT correct the first time. Series or parallel? Maybe I will need to add a couple of portable solar panels? Again I can add them but only if I get the right MPPT.
MisterSandals
Participation Medalist
Well yes I want to hear from many but really!!!! appreciate your responses. So now I am confused. I guess I don't understand the terms "charge watt hour or amp hour"? Isn't an MPPT sized to fit the solar panels regardless of what I actually use? Let's say I am in Arizona with great sun shine and tilting panels and am hooked up in series. Let's also so it is 25c temperature all day long. In theory I would be producing 111.6 volts and 8.48 amps. If my MPPT was a Victron 75/15 wouldn't I ruin it with 111.6 volts going to it? Again I welcome and appreciate your response as I am new to solar. I have had boats for thirty years with dual battery switches so used to battery capacity and monitoring but 0 experience with solar.
MisterSandals
Participation Medalist
My point was to try to figure out how much solar you need/want, BEFORE building your system. But I have no problem helping you building a system without regard to what you expect it to do. Upgrading or changing components is expensive and I find that getting the system sized correctly, the first time, is not only easier but BY FAR less expensive.
MisterSandals
Participation Medalist
So working with the 3x 315w panels, and since you choose to keep the Voc numbers secret for some reason, we have to use you measured voltage (all three read about 46 volts) as the max voltage.
It sounds like you prefer to have them in series but that too you are keeping secret (are you a poker player?).
So 3x 315w panels in series will 138v at 8.48 (because your Isc is a well kept secret too).
A Victron 150/<anything> will work nicely.
If you run them parallel, 46v at 27A. A Victron 100/30 will work.
And since you choose not to figure out your energy usage, i will make a swag at a battery bank of lead acid batteries (your prefs based on temp ranges, good info!).
Based on your panels' ability to charge for the 5 hours you mention:
315w x 5 hours = 1545wh.
1545wh/12v = 128ah (operating at 100%)
With lead acid batteries 50% discharge is the accepted safe level. So your array could recharge a 50% discharged battery bank that is 128/.5 = 256ah.
Will it run what you want for as long as you want? I dunno, that's not important apparently.
MisterSandals
Participation Medalist
Solar panel investgation
Hi I’ve been given these panels. See attached pic. If you would be so kind I would like advice on building a system with some or all of them. I live in Australia -31.06, 152.84. I have 8 of them second hand. Yet to test them. I want to power a fridge. I’ve got a freeer I’ve changes the...
diysolarforum.com
Check out post #9 for panel voltage and temperature. This may apply to you.
Thanks again for your help. Just checked label again and the VOC is 46.2. The VMP is that 37.2 that I have been talking about. Sorry about the bad info. The IMP is the 8.48 that I have talked about. The ISC says 9.01A. I have no preference for series or parallel. What ever works best. I also don't have any preference on brands like Victron as I have no experience. I have used Xantrex 2000 watt inverters on the boats. I wish I had a better idea on usage but just don't. I have looked at your swag calculations and I have three 315 watt panels. Would the calculation not start wit 945w times 5 equals 4,725w? I will also look at the other post you referenced. I can't thank you enough for all your help with my new adventure in solar.
I just looked at the other post and the light bulb just went off. There is voltage and now I get VOC. My exact fear, buy a MPPT that I think is correct and temps drop and MPPT gets smoked. Thanks again for all your help. Ever need advice on the best shipwreck scuba diving in the world and I am your guy.Solar panel investgation
Hi I’ve been given these panels. See attached pic. If you would be so kind I would like advice on building a system with some or all of them. I live in Australia -31.06, 152.84. I have 8 of them second hand. Yet to test them. I want to power a fridge. I’ve got a freeer I’ve changes the...
Check out post #9 for panel voltage and temperature. This may apply to you.
MisterSandals
Participation Medalist
MisterSandals
Participation Medalist
Yes it did. Did not know that Vmp was not the correct number but Voc is. Sounds like parallel is the way to go. Also get a MPPT bigger than needed in case I need to add portable panels. I am thinking a 100/30 might work but should go bigger? Agree? Again thank you so much!!!
MisterSandals
Participation Medalist
I'm a "strongly prefer series" guy. I think keeping the amps as low as possible, almost always, is the right decision. I'm familiar enough with the affects of temperature on Voc so use your judgement on that.
Have you seen this?
Thanks again for the response. I am less worried about temps now as I didn't understand I was measuring Voc when I checked the panels. I thought they had jumped from 37.2 (Imp rating) to roughly 46 due to the colder temp but what i didn't know until your help is that I was really checking the Voc. I thought if it jumped from 37.2 to 46 when it is 35 degrees Fahrenheit what the heck voltage will I get at 10 below zero. Do you like series due to wire size? Other advantages? Again thanks for sharing your knowledge and the MPPT calculator link.
