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Old 06-13-2018, 09:22 AM   #1
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Question Solar Panel Theoretical Maximum ?

Hi all.

So I was wondering if anyone has some insights as to what the theoretical (but still practical.. i.e. using off the shelf, widely available equipment) maximum amount of solar energy one could every hope to collect by mounting them on a standard size (~40ft?) bus frame.

I thought it might be an interesting thought experiment.. if any of you pros want to weigh in i'd love to hear your thoughts.
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Old 06-13-2018, 09:45 AM   #2
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Solar system design needs a "balanced" approach.

Sizing components to work properly together.

I could fit at least 7200 watts of panels on my roof but the properly sized battery bank to go with it would be completely impractical.

A great resource for solar info: Forums - Solar Panels - Solar Panels Forum
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Old 06-13-2018, 09:48 AM   #3
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Using my SunPower E20 435 watt panels as a starting point (cause I have the documentation at hand for them)... They are 1312 sq. inches and produce 435 watts. That is about 1/3 watt per sq. inch (round numbers). A 40' coach is something like 40' long x 8' wide which is 46,000 sq. inches. That times 3 = 15,000 watts (15kW).

Of course, you can't cover every square inch very easily.... So, a more realistic calculation would be fitting 11 of these 41" wide (81" tall) panels which would equate to 4785 watts. Of course, that isn't very realistic either but this is all theoretical, right?!?!
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Old 06-13-2018, 11:56 AM   #4
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I think that by exploring the upper limits of what solar panels can do we can universally help anyone who wants to build their own system.

I always try to start with the ideal and then walk my way back to what my budget allows. In any event maybe someone will be inspired to try and create whatever ideal system we can think up here
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Old 06-13-2018, 12:16 PM   #5
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As I played with system design for my bus I found the limiting factor for me was the cost and weight of the battery bank.

I had hoped to follow JD's example and not have a generator. Unfortunately the battery requirement exceeds my budget by a significant amount.
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Old 06-13-2018, 03:05 PM   #6
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I have a 40' bus. There is 26' of clear space between the two roof hatches, between which I made a central 12"-wide walkway. On either side of this walkway are hinged four support frames, each about 5'6" long, that hold each of the eight Sharp 255W grid-tie panels, and they extend out to a few inches from the edge of the roof. Between the last panels and the rear hatch I have 4' of space that will be for the two eventual water heating panels there. So, I have 2040W of PV occupying 22' of roof, but with a 12"-wide walkway down the center, or occupying about 154 sq.ft. measured in a horizontal plane.

Could I have fitted more panels? Maybe, but A) I don't need more power (during the summer I'll be able to produce way more power than I need), and B) I wouldn't have the walkway that is important for cleaning the panels safely (I put two quick-connect water outlets on it so I can easily and safely wash the panels from up on the roof), and C) I want space for the two water heating panels that will each be about 20 sq.ft. and hinged just like the PV panels. This amount of PV will charge my eventual 900 aH of golfcart batteries at about a 13% charge rate, the most that FLA batteries will accept without too much water loss, and even in winter I should still be able to get a useful amount of power into them, hopefully even to Float each day unless it's pissing down with rain.

Sure, you can cover the entire roof from front to back with panels, but why? And besides, you'd need those flexible panels to do so, and we all know how long they last . . .

John
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Old 06-13-2018, 03:18 PM   #7
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Using my SunPower E20 435 watt panels as a starting point (cause I have the documentation at hand for them)... They are 1312 sq. inches and produce 435 watts. That is about 1/3 watt per sq. inch (round numbers). A 40' coach is something like 40' long x 8' wide which is 46,000 sq. inches. That times 3 = 15,000 watts (15kW).

Of course, you can't cover every square inch very easily.... So, a more realistic calculation would be fitting 11 of these 41" wide (81" tall) panels which would equate to 4785 watts. Of course, that isn't very realistic either but this is all theoretical, right?!?!


Those numbers look amazing, i checked out the pdf specs here https://us.sunpower.com/sites/sunpow...-datasheet.pdf for the sunpower e20 435 and it is indeed beefy. I only wonder on the price point and availability..

I suppose in the interest of theoretical maximums would there be any possible system that you could fit on your roof to get that extra 30% sun power by tracking the suns zenith across the sky ? Perhaps on a raised roof rack? I'm racking my brain trying to imagine optimizations
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Old 06-15-2018, 11:34 PM   #8
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I suppose in the interest of theoretical maximums would there be any possible system that you could fit on your roof to get that extra 30% sun power by tracking the suns zenith across the sky ? Perhaps on a raised roof rack? I'm racking my brain trying to imagine optimizations
Practically, no, there's no realistic way to have two-axis tracking on a bus or RV roof. However, vehicles can easily do something that houses have difficulty doing - they can orientate themselves to maximize solar harvest by simply parking to face either due east or due west. Then you can do what I've done, and have all the panels hinged so half of them can be raised to face the sun better, and the other half are angled down against the roof (which you can't do anything about). It's still better to do this than to have them all flat on the roof, unless you live on the equator where having them flat is OK. My benefit from having all my panels tiltable will be in the winter when the sun is lower (not that I ever intend to be further north in the winter where the sun is lower still); any gain in solar harvest at that time is well worth it, but during the summer I'll probably have more power than I need.

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Old 06-16-2018, 05:53 AM   #9
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Adding to John's post re. tilting panels...

After several years of full-timing, I have found that my flat mounted panels achieve a maximum output of about 85% (of rated output) during the summer months and a maximum of about 60% during the winter months. I suspect tilting the panels would help the winter output a great deal.
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Old 06-18-2018, 05:09 AM   #10
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Originally Posted by Iceni John View Post
Practically, no, there's no realistic way to have two-axis tracking on a bus or RV roof. However, vehicles can easily do something that houses have difficulty doing - they can orientate themselves to maximize solar harvest by simply parking to face either due east or due west. Then you can do what I've done, and have all the panels hinged so half of them can be raised to face the sun better, and the other half are angled down against the roof (which you can't do anything about). It's still better to do this than to have them all flat on the roof, unless you live on the equator where having them flat is OK. My benefit from having all my panels tiltable will be in the winter when the sun is lower (not that I ever intend to be further north in the winter where the sun is lower still); any gain in solar harvest at that time is well worth it, but during the summer I'll probably have more power than I need.

John

I saw someone else's post on their diy roof rack linear actuator solar panel tilting system.

It appears to be only 1 axis but i think with a few more linear actuators and
a photo sensor it could be fully automated and the other axis can be
controlled by how you park the bus.

Check it out..

http://www.skoolie.net/forums/f11/so...tml#post133025
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Old 08-19-2018, 08:01 AM   #11
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Since this is theoretical I have noticed that there are allot of tesla model s batteries for sale on ebay selling for around 1400 dollars. The spec says 250 Ah and 5.3 kWH 24vdc if I were to buy 4 of these for under 6000 dollars how large would my solar need to be to be able to charge the battery bank assume I only use 50 percent charge overnight or 10 kwh. I have watched a few installs of these on youtube but they never do a follow up on how well the system works or how happy they are with it.
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Old 08-19-2018, 08:27 AM   #12
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Howdy Capt'n!

Firstly, I have no experience with Tesla batterys. I do have experience living with Lithium batteries though.

In my opinion, the battery management system (BMS) is extremely important (critical) for lithium batteries. I don't know what the Tesla batteries require in this regards but be sure to add it to the cost.

The answer to your question is: "It depends!"

On a lot of things, of course. What is the solar insolation where you plan for this to operate, time of year, hours per day, how are the panels mounted (do they track the sun, or fixed, flat, or tilted, etc...), and so forth.

In general, Lithium will accept a lot of power very fast. My 1.7kWh array/controller will pump a maximum of 60 amps @ 24 volts into my 10kWh lithium bank (24VDC). The 60 amps is the maximum for the charge controller that I have (TS-MPPT-60) but matches up with the array (theoretical maximum output of about 64 amps at a charging voltage of 27V. Typically, however; the array is producing more like 1200-1300 watts (during the summer months) and the coach is consuming ~10 amps so about 30-35 amps (@27 volt, charging) is going into the batteries.

None of that answers your question but hopefully gives you some things to research to find the answer for your situation.

Also, you can discharge lithium down to about 15% SOC without damage (in reference to your 50% note) - just fyi if not already known.
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Old 08-19-2018, 09:06 AM   #13
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Hi JD!
Youve probably answered this a dozen times how many panels do you have and what size are they (watts) if you discharge your batteries down to 15% will your panels charge them in one day or over a few days time? With direct sunlight in an area that is almost optimal for you solar. It is a bus and I dont plan on staying in one place or going somewhere thats cold or overcast lol.
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Old 08-19-2018, 09:20 AM   #14
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Hi JD!
Youve probably answered this a dozen times how many panels do you have and what size are they (watts) if you discharge your batteries down to 15% will your panels charge them in one day or over a few days time? With direct sunlight in an area that is almost optimal for you solar. It is a bus and I dont plan on staying in one place or going somewhere thats cold or overcast lol.
Indeed I have - that's why I added that link in my signature, all the details are there.

If it is a "good" solar day, the battery will be fully charged in a single day. "good" = nice sunny day here in the southwest. About the only time my battery get's discharged to that extent is when there are several "poor" solar days in a row.
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Old 08-19-2018, 09:24 AM   #15
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Thank you very much. I have spent the last 25 years on ships and I find overkill to be best especially when your 2000 miles from anything lol.
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Old 08-19-2018, 09:44 AM   #16
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Yep - agreed!

I've tried twice to transition to sailboat living but failed both times. Already working on the plan for attempt number three!!

With buses/RV's we are generally limited by roof space and or storage space (for solar panels). So, putting as much on the roof as possible is about the best you can do. From there, it is plan B (generator or shore power).
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Old 08-24-2018, 12:51 PM   #17
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I have been trying to design my electrical system and learning about the math involved. I have been searching on-line for the straight forward math and this is what I have come up with so far from piecing together what I have found.
Iím using my roof airconditioner in this example. What if I run my airconditioner from solar? It takes 20Amps to start and runs about 10A. The documentation recommends a 3.5KW gen. Lets use 20A x 120v = 2400W. If I use a 3KW inverter, 3000/12v = 250Ah, and if I run the airconditioner 10 hour a day Iíll need (10Hx250Ah) 2,500Ah battery bank. I could use 20 of those 6v 235A/Hr batteries to get 2.35kA/Hr for $3,200. And for charging. I read this somewhere:Ē As a general rule of thumb, a 100W solar panel can generate about 5A/hr at peak power, thatís about 25Ah per day. ď I take the daily need divided by the rule of thumb for a 100W panel, 2,500/25 = 100 solar panels. If I get 500W solar panels, I donít know if the rule of thumb translates, but if it did, that would be 125Ah per day, so 2500/125 = 20 solar panels, that is more reasonable, but can I get a 500w panel? I did find a 350W panel for $350.00. for the rule of thumb 350/100=3.5, so 3.5x25=87.5Ah a day from each panel, 2500/87.5=28.6 panels that measure 5.3íx3.6í each. 28 panels at $350 each = $9,800. If I place them in two rows on the top of my bus thatís 10.6í wide and 50.4í long. Iíll need a bigger bus for this set up.
Iíll look at a more efficient battery, the lithium iron phosphate battery can be discharged 100%. I can get a 12v 100Ah battery for $950 each. So thatís 2500/100=25 batteries for $23,000 Yikes! I could get a 12v 300Ah battery for $3,488 ea, 2500/300=8.3 8 of those would be $27,904.
I think I could get away with 10 golfcart 6V batteries and 10 of the 350W panels. The panels would cover 18í of the roof. 10 batteries for $1,600, 10 panels for $3,500 and the 2.5kW inverter for $800. Thatís $5,900 for this system. 10 6v 235Ah batteries will give me 117.5Ah x 5 = 587.5Ah. The inverter 2500W/12V = 208.3Ah, so 587.5Ah/208.3Ah = 2.82 hours that I can run the airconditioner. Add a charge controller and the wiring into the cost. My airconditioner is old, and the newer replacement draws 11Amps to start, so more efficient appliances should be considered. Also consider charge rate and losses, and the battery array could be set up for maximum efficiency, I havenít looked into that yet.

ďSince this is theoretical I have noticed that there are allot of tesla model s batteries for sale on ebay selling for around 1400 dollars. The spec says 250 Ah and 5.3 kWH 24vdc if I were to buy 4 of these for under 6000 dollars how large would my solar need to be to be able to charge the battery bank assume I only use 50 percent charge overnight or 10 kwh. I have watched a few installs of these on youtube but they never do a follow up on how well the system works or how happy they are with it.Ē
That would be 1000Ah if you have them set up in parallel for a 24v system. 10kW/24v=416Ah
Take the daily need divided by the rule of thumb for a 100W panel, 10,000/25 = 400 solar panels. The 350W panels would be 10,000/87.5= 114 panels needed to charge 10KWh in 5 hours a day. Wow!
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Old 08-24-2018, 01:05 PM   #18
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Lots of math there!!

A few items to add:

1. LRA - Locked Rotor Amps. That is the power that an A/C unit draws when starting. A soft-start device can be a big help (I have no experience with them though).

2. Read this article: https://www.solarpaneltalk.com/forum...-size-tutorial

3. Even the thought of running A/C from battery requires, IMHO, lithium batteries. The size and weight of anything else would be horrendous.

4. I have 435 watt panels. I don't watch the industry very close but I don't think they get much 'bigger' (output) than that. Probably wrong, again...

5. FLA/SLA batteries should not be discharged below 50%. Lithium is generally 15% (though I have heard 10% a few times).
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