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5 Solar Facts You Can't Ignore


rbertalotto

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Roy,

Very good general info about solar. Also a very interesting magazine. Thanks for sharing.

 

It is definitely good to remember that when panels are rated at 100watts, that is max output under laboratory conditions, simulating the sun perpendicular to the panel at the equator and no haze in the sky.

 

One area I am not in agreement with. That you should have 300 watts of solar for every 100 amp hours (AH) of battery. That would be nice, but generally for most of us in the US who boondock or dry camp there just isn't enough roof space for that many solar panels. Partly because many of our battery setups have 440AH of battery. That is four 6V batteries. Trying to fit 1200 watts of solar isn't practical.

 

The rule of thumb I use and have seen mention is 100AH of battery, have at least 100 watts of solar.

 

My system is 400AH of lithuim and 640watts of solar. About 1.5 watts of solar for 1 AH of battery.

 

A lot of how soon I get to full, depends on how much I used since I was last charged to full.

 

Note, I am using the term "full charge" or "charged to full" and not "charged to 100% full". If you are using lead acid (including AGM) going from 90% full to 100% full takes several additional hours of charging at a low charge rate. Generally 5amps or less. So when someone says I am fully charged by noon I would think that is charged to about 90-95% full and not 100%.

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I'm like Al, I DO NOT have 300 solar watts for every 100 battery amp hours, that would be nice if I had rooftop mounting space lol. However, with a mere 470 Watts of fixed (no tilt) rooftop solar, a temperature compensated MPPT Solar Charge Controller, and 450 battery Amp Hours, I've never had a problem reaching full or near charge by afternoon, subject of course to sunlight and how much I depleted my batteries overnight. It gets back to any solar is good solar and the more the better, all subject to your space and budget constraints. I just don't hear of even the "big boys" who may have as much as 1000 watts of solar having 3000 battery amp hours WOW you could run a welder with that lol

 

John T Still in Bee Cave Texas but back to Florida soon

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Actually.. I tend to agree with the 3:1 rule.. but would amend it to read for daily usage rather than actual battery bank size. If you two were to stop and think about it.. you're both actually doing the same if you consider it in terms of what you're actually using and replacing each day.

 

John.. you use what daily? Around 75-100ah's on a "heavy" day? And you have "by the book" 470watts. So you're 4.7+:1.

 

Al... you use.. I would guess.. 100-150ah's on average.. if that? Even with 200 daily you're at 3.2+:1.

 

I'm a little lighter on average so probably 5.4+:1.

 

On average I believe each of us is generally topped off well by noonish.... SO... sounds to me like we are all just proving the authors point. :lol:

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Yarome, your estimate of my 12 VDC based energy use sounds about right. A few LED lights and occasional fresh water pump and vent fans in our 29 footer isn't much at all HOWEVER my small extra dorm fridge requires around 35 Amp Hours per day BUT IF ITS COLD that furnace fan is probably my biggest energy hog. I'm talkin dry camping of course when we may fire up the genset for the microwave or the wife's hair dryer, but when its running my PD 9280 can crank out 80 amps into the batteries theoretically subject to their charge state. Anywho, my 470 solar watts seems to keep up fine subject to sunlight.

 

John T

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my 470 solar watts seems to keep up fine subject to sunlight.

 

Yeah.. you're positioned really well based on your daily usage... and you've got the right gear to back it up. Even on lower production days you still have enough "reserve" capacity to still keep your bank happy.

 

3:1 of daily is really the minimum to maintain sustainability, IMO. Not that it's a sin to crank over the genset once a week or so.. but still.. it's sure nice not to "have" to, right. ;)

 

If you were a 200ah/day guy then you might be a little "light". Battery bank capacity really isn't a good measure when trying to determine solar needs though. Heck! Based on bank size... I'm about 780watts shy of what I "should" have.

 

My reality... I run 540watts against a 440ah bank... 1.2:1. Typical daily usage is around 40-50ah... so 11+:1 of daily.. "by the book". (We all know that's not how life really works) Still though.. on a "bad" day it's not often I'm not topped off by the end of the production day. On a "good/normal" day.. after the bank is topped off I still produce enough FREE energy to run my electric camp washer and spinner, make a batch of ice for the evening and/or charge a device or two without having to cut into my batteries.

 

I would say I'm doing pretty good for being 780watts "light". :lol:

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1110 watts and 490AH on our toy hauler 99% boondocking without an onboard genny. With the use of LED's and a catalytic heater we were totally solar as long as the sun was shining and only minimal genny run time from a little Yamaha EF1000is during an entire week of cloudy weather. I'd support the 3:1 theory if I was full timing to eliminate genny run time.

 

Planning for 1250+ watts on the new build along with wiring it up for grid inter-tie for when it's docked in the backyard.

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1110 watts and 490AH on our toy hauler 99% boondocking

 

Another great example. I would "math that out" as 1110w@80% for 888 watts (reasonably expected production). 490ah@50% SOC for 245ah daily usable. That puts you right about 3.6:1 of your daily.. or better since you probably don't discharge to 50%SOC daily. Works.. don't it! ;)

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I too agree with the 1:1 rule. Have 720 watts with 660 ah. The extra watts are to help make up for lost sunlight due to less than ideal conditions. Love our setup and have no NO problems or come up short of power. Also firm believer in the newer battery technology as higher charge rates can be used and shorter charging times req'd. We have 6 AGM batteries. NO cleanup or maintenance req'd.

 

rocmoc n AZ

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1200W X 80% = 960W (Reasonable production)

800AH 50% SOC = 400AH (Available for consumption) (2.4:1)

800AH 75% SOC - 200AH (What I prefer to work with, and target for overnight usage.) (4.8:1)

As we target overnight usage to average 70-75% range of SOC after the coffee is perked in the AM. It makes me feel I made the right move adding the 5th panel for added insurance. (X's 5 240W 48V Panasonic High Efficiency panels) Though 960W would have yielded 3.84:1 at 75% SOC, and 1.92:1 at 50% SOC. So, I would have been probably have survived OK with 4 panels too.

And yeah, we can always run the generator a bit more as needed. Depending on where we are geographically, and the time of the year, we're usually at 100% SOC between a very broad 11:00AM - 1:00PM, dependent upon how much below the 70-75% SOC target we may drop overnight. (Not often, but the DW will run a fan for several hours in hot weather. As one example of where we consume more then the norm. Suspect the Samsung refrigerator is working a bit harder in that kind of weather too.)

And sure, 1200W's does help us during cloudy days, and increases are odds of more power in shaded conditions from trees too.

Fun thread, and good article!

Best to all,

Smitty

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Actually.. I tend to agree with the 3:1 rule.. but would amend it to read for daily usage rather than actual battery bank size. If you two were to stop and think about it.. you're both actually doing the same if you consider it in terms of what you're actually using and replacing each day.

 

John.. you use what daily? Around 75-100ah's on a "heavy" day? And you have "by the book" 470watts. So you're 4.7+:1.

 

Al... you use.. I would guess.. 100-150ah's on average.. if that? Even with 200 daily you're at 3.2+:1.

 

I'm a little lighter on average so probably 5.4+:1.

 

On average I believe each of us is generally topped off well by noonish.... SO... sounds to me like we are all just proving the authors point. :lol:

Except, the author stated "300watts of solar for 100AH of battery capacity". He did not state having solar panels which would replace 3 times the AH of daily usage.

 

In theory my 650 watts of solar will provide 54 amps of 12V electricity, I could replace my 150AH usage in just 3 hours. In practical use it is more like 5-6 hours.

 

I do like your thoughts about having 2-4 times as many watts of solar as your expected AH of usage. That does sound like a good rule of thumb.

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My rule of thumb for any but the smallest, budget systems, is to put as much solar on the roof as will fit. Size your battery bank for at least two days of use without recharge. Kinda different than what others do.....

 

The logic is that solar is relatively cheap unless you are building a budget system.

 

Our current rig has 1220 watts and 1200 AH of storage in AGMs. Next setup will have a 42'-45' rig roof totally covered with panels. No rooftop AC units, vents or skylights. All venting will be either low profile under the panels, or side vented. Skylights will be side transom windows, instead. AC will be minisplit and basement air combination. The minisplits will be wired to run off the battery bank. Battery bank will be at least 16 KW of some sort of Lithium chemistry.

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My rule of thumb for any but the smallest, budget systems, is to put as much solar on the roof as will fit. Size your battery bank for at least two days of use without recharge. Kinda different than what others do.....

 

The logic is that solar is relatively cheap unless you are building a budget system.

 

Our current rig has 1220 watts and 1200 AH of storage in AGMs. Next setup will have a 42'-45' rig roof totally covered with panels. No rooftop AC units, vents or skylights. All venting will be either low profile under the panels, or side vented. Skylights will be side transom windows, instead. AC will be minisplit and basement air combination. The minisplits will be wired to run off the battery bank. Battery bank will be at least 16 KW of some sort of Lithium chemistry.

I look forward to that, Jack!! You are a great example of pushing the envelope of what CAN be done and then you do it.

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Roy,

Great article and thread guys. I am looking at our next step as we are winding down with home hospice and then taking care of estate issues, then moving, possibly to B.C. as we loved that area as well as Colorado. I just found that after four years living in Canada you get dual citizenship, and their excellent socialized medicine program from the third month in. We improved our five acres and have a lovely new home all decked out, a steel workshop on a slab and a finished out portable building as well as new culverts and gravel road. We will be looking at a rural full/part off grid setup with grid available for emergencies. But the advice in that excellent magazine and article applies even more so for a full house. Up in Oregon we encountered some all off grid 12 volt powered homes up in the mountains around the Sutherlin CoOp where we stayed several times.

 

Jack,

as said before, like me in computers, you go and do it then pass along the results. Sometimes neutral but rarely bad with your excellent research and grasp of the technologies you adopt regardless of past exposure or not. As is the case with the others here in this thread. Thanks for sharing guys. ;)

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Except, the author stated "300watts of solar for 100AH of battery capacity".

I do like your thoughts about having 2-4 times as many watts of solar as your expected AH of usage. That does sound like a good rule of thumb.

 

Which was why I added, "but would amend it to read for daily usage rather than actual battery bank size". I agree with the 3:1 ratio when calculating solar requirements, but disagree that it should be calculated on battery bank size. I should have been more clear.

 

So even several folks all packing the same size of battery bank may in actuality have very different solar sustainability requirements.

 

I agree with Jack in that utilizing all available real estate is never a bad idea.. more is always better.. but with the caveat of focusing more on "quality real estate" and a well designed array/s of hi-efficiency/quality panels over simply maximum coverage at the least expensive price per watt.

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I have 850 watts of solar panels and 800 ah of Li batteries. I have only had this for several months, and in the northern part of the country, but find that I come up short on replacing my daily consumption. My roof is as covered with panels as I can get up there, I am going to see how it goes through the next summer, and if the trend continues will look to do some mods to get panels over the rear AC and vent.

Having a 35' class C does limit available roof space

Maybe I could mount some on the hood :)

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but find that I come up short on replacing my daily consumption.

 

What is your average daily consumption? Which solar controller are you using? What is your solar array configuration? All in parallel or do you run series/parallel? Are you using a meter to measure daily production? If so.. what is your daily average coming out to? Type/brand/model of your panels?

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I have 850 watts of solar panels and 800 ah of Li batteries. I have only had this for several months, and in the northern part of the country, but find that I come up short on replacing my daily consumption. My roof is as covered with panels as I can get up there, I am going to see how it goes through the next summer, and if the trend continues will look to do some mods to get panels over the rear AC and vent.

Having a 35' class C does limit available roof space

Maybe I could mount some on the hood :)

Ditto on what Yarome asked.

 

In addition I would add:

-- You either use a huge amount of electricity every day. (400-600AH a day?)

-- Or there is something else happening, other than the amount of solar panels you have on your roof.

 

Are you unable to replace your usage on a clear sunny day? Or is it on days with a hazy sky?

 

On a sunny day with clear sky:

-- How many amps & voltage does the Solar Controller control panel show going to the batteries?

-- How many amps and voltage does the charger control panel show going to the batteries?

 

Does your BMS have a video display where you can see the individual cell voltage and temperature?

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With limited roof space, I'm planning on using a couple solar panels as window shades. Mounted on hinges they will fold flat for travel and out once set up. They will serve three purposes...solar energy, shade to help keep coach cool and allow windows to be left open when raining.

 

I'm surprised no one else has gone down this road.

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I'm surprised no one else has gone down this road.

 

Unless they were mounted at roof level (which might negate them being very effective as window shades), rather than directly over a window, it might be very difficult to always be able to orient your rig so that they would be effective and the production "window" would be very limited with a "wall" directly horizontal to the panels. If you're in a CG you might also have your neighbors rig casting shadows that might affect their ability to produce.

 

I once saw a setup that I thought was somewhat slick. The guy built a framework that allowed for 2 layers of panels. The lower rack slid out on rails from underneath the top rack of panels when parked. He ran into some problems when the wind picked up and was planning on a redesign, but I don't know if he ever did. Kind of a nifty idea though.

 

To me.. the easiest way to add additional production capability would be to just setup a remote portable array that can be positioned and plug n played as needed.

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I once saw a setup that I thought was somewhat slick. The guy built a framework that allowed for 2 layers of panels. The lower rack slid out on rails from underneath the top rack of panels when parked.

To me.. the easiest way to add additional production capability would be to just setup a remote portable array that can be positioned and plug n played as needed.

I've seen that on a couple of rigs. It should work pretty well if properly designed. The key is stabilizing the "moving" panels when driving.

 

I also like the portable array. The key is storing it, though.

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That was exactly my thought.

X3! I use 400-600 a day, but only when I heat water and run the refer on electric.

 

Assuming the pv system is functioning correctly, might need to focus on more efficient appliances. My new to us rig has an LCD TV that is very thirsty. Simple and inexpensive fix to save 40-80ah a day.

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I'm surprised no one else has gone down this road.

I have seen a few people go this route with smaller RV's. There have been a few pictures of these set ups shared on the forum in the past. It is a good way to go when roof space is at a premium. You just have to make sure you securely mount those panels.

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I can easily get my usage to < 200 ah per day, but when I am in hog mode it is about 500 - 550 ah. I have several very optional accessories which are big time power users.

And like I said, I have only had the system for the last couple of months. Shorter days, lower angle of sun. I can, but did not, tilt my panels.

My system is working properly. Victron MPPT 150/70 controller.

I am still learning how to live with the setup

 

One of the things I really like about the Li batteries is how fast the fill up on the generator. Very impressive

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