How much solar do I need?

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akrvbob

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First, before you do anything else, go up on your roof and measure to see how much room you have. You should only buy what will fit on your roof. Then see what different panels have for different sizes, maybe you can fit more small ones, or, maybe you can only fit one big one.

Of course you can always buy a portable suitcase system or flexible panels and not mount them on the roof, that's another discussion.

Many people suggest you add up all your electrical use and get enough to meet those needs. But solar has gotten so cheap I say start with your wallet and buy all you can afford to spend right now. After all, if you can't  afford it, why bother finding out what your needs are? 

Also remember that you are buying for the worst day of the year--a storm that lasts for a week in the winter when the days are already short and the sun never rises very high. If you buy JUST enough for the summer, you WILL run out in the winter. Plus, you are buying for latitude, northern cities get less sun that southern cities.
 
Here are my recommendations. 
 
Minimal Power User = 100 watts. This person mainly recharge his phone, laptop, camera and a few other devices. Buy the 100 watt kit and adjust your use to it. If it's all you need, then you are done.
 
Average Vandweller = 200 watts. The majority of vandwellers need and can be satisfied with 200 watts. It meets all the basic needs plus, 12 volt compressor fridge, fans or even TV.I have 190 watts on my trailer and I've never been unhappy with  it. If you have the money, buy a 200 watt kit. If it's not enough, buy another 100 watt kit. Yes, you have two systems, but I consider that a blessing since you have redundancy; if one system fails you still have power. I have three distinct systems and I like it! Or, get the 200 watt kit and get the upgraded controller so you can add another one to it later when you can afford it. But controllers are so cheap now I think just buying a new cheap one is better.
 
Power user = 400 watts. If you have the money, start out with a 300 or 400 watt. You'll never have to worry about power even in a storm and nearly all of us can find a way to use extra electricity. I use my extra power to run a microwave and watch Satellite TV. [img=21x21]file:///C:/Users/Bob/AppData/Local/Temp/msohtmlclip1/01/clip_image001.gif[/img]
 
Why increments of 100 watts? Because you can get 100 watt Renogy panels cheap with free shipping. If you jump up to the big panels like 200 or bigger, they are cheaper per watt, but the cost of shipping is so high it will end up being much more. 
 
But, if you are near where you can get big panels and they fit your van better, get them instead.
 
Be aware that higher watt panels are usually higher voltage and require a MPPT controller. 
 
Starting with your wallet is just my personal bias, lots of people will disagree, hopefully they will speak up and explain there thinking.
Bob
 

rvpopeye

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I like your thinking on this Bob.

I have a 75 watt panel , BUT , I bought it back in 2001 . It cost me $600 at West Marine!
And that WAS all I could afford. It was installed on a 29' class A so there was plenty of roof left to cover.

I did get a 25 amp MPPT controller so I can still get some more cheap panels to go on this 23' class C without needing an upgrade there.

I'm gonna throw out a cool idea I saw someone doing though. Panels mounted with tilting mounts on the side of the rig , some over the windows like awnings and some others on the walls. All removable to go on the ground etc when shade parking.
 

Spaceman Spiff

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Being an engineer, I always recommend starting with a needs list (not likes or wants).  Especially if you have a medical condition that requires power for equipment (CPAP, nebulizer, refrigeration for medications, etc.).  Determine how much power your needs require.  Now you can go up on the roof and see how much solar you can fit (and measure you wallet to see how much you can afford).  If you can't meet your needs, you need to get creative or find a different solution.  And remember, you must build margin into your system for that inevitable week of rain  :(

Other than where you start from, I think you are giving great advice.

Personal experience: I have 200W of solar and 208 AH of battery.  My big loads are a 12 volt, 2.1 ft³ refrigerator and a Maxxfan.  I am seldom below 90% battery capacity in the AM.  Additionally, I am usually up to 100% by noon on an overcast day.  My system hardly notices my charging a cell phone, laptop, digital camera or running LED lights (all on 12V, I don't have an inverter).

-- Spiff
 

Trekking

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Are you folks using a 6V or 12V batteries? What kind of 12V fridges are you usingusing?
Thank you!
 

Spaceman Spiff

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Trekking said:
Are you folks using a 6V or 12V batteries? What kind of 12V fridges are you usingusing?
Thank you!

I use two 6 volt golf cart batteries in series (208 amphours).

I have an old (1982) Norcold 2.1 ft³ front loading refrigerator (12V/110V).

-- Spiff
 

akrvbob

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Since this is a sticky we need to stay on topic. Off-topic posts will be deleted. Start a new thread for new topics.
Bob
 

gcal

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Thinking of getting some solar when we get back near the family this winter. Love boondocking, but don't like to be without our electronic goodies. Right now, we're low on gas, so we have to leave the site to fill up about halfway thru our intendind stay. Well go whole hog, the 400 watt setup if we can get it on the roof. We may start now out thinking that we only want a couple lights and our devices charged, but will ease into more.
 

mockturtle

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200W works for me.  It runs my 4.8 cu ft compressor refrigerator, lights, water pump and 12V fans and charges all my electronics.  I use a very small [200W] inverter which grinds my coffee and runs my pencil sharpener when I need it [I do crossword puzzles].  When I don't bother setting up my catalytic heater, I run the furnace in the morning. It's important to get a high-quality controller.
 

Seraphim

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My way of thinking was to have enough panels, theoretically, to quickly replace my overnight usage; I figured an average of 3 hours usable sunlight. Then I bought enough solar to theoretically replace my overnight usage in one hour, because theory is never reality and there are overcast and rainy days. *grin*. Coupled with sufficient battery storage, another topic, it's been way more than sufficient.
 

IGBT

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I have been running various items during the daylight hours to make use of excess solar capacity.   We bought a little rice cooker that uses 200 watts of 120V AC.   It makes two cups of cooked rice in about 27 minutes, purely off the sun.   I also was able to heat the water in our 6 gallon propane/AC water heater up to 140 degrees F using purely the sun.   Our 9 cu/ft 24V DC fridge runs on the sun and I experimented with changing the duty cycle of the compressor such that it cools down a bit more during the day hours than the night hours.

I guess I am coming around to the idea that maybe you can find uses for excess solar even if your battery bank is technically too small for the amount of panels you have.   I am sure the amount of propane I save cooking rice, heating water, and running a compressor fridge is fairly small, but the solar would be otherwise going to waste.
 

BigT

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I just added wider crossbars to the roof of my Transit Connect, so all the "facts & figures" I put together (& posted about in other threads) over the last several months are out-the-window and I have to start all over again, figuring out what I can fit on the roof and what CC/battery to get. Ugh.
I'd like to run a minimum of 200 Watts. maybe even 300 Watts. That should be more than enough for keeping my CPAP, LED lights and electronics happy.
 

ALuckyDuck

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So I mainly want to use solar to power one, or two Fantastic Vents while at work (to keep the dog cool). This is my only energy need. Please let me know if I've fudged these calculations, and if the equipment seems appropriate...

I've read that at their max speed settings, Fantastic Vents draw 3 amps. So I suppose two fans running 8 hours per day will draw 48 amps per day? With a 12V set up running two fans 8 hours per day, I calculate my energy needs to be 576 watt hrs.

Since deep cycle batteries should not be discharged beyond 50%, I'll determine my need to be twice of 576 watt hrs, or 1152 watt hrs.

I divide 1152 watt hrs by 12 (volts) to determine the minimum capacity of Amp hrs my battery can be, which I've calculated to be 96 Amp hrs.

So would one 12V 125 Ahr AGM battery be sufficient for my needs, assuming the only devices I run are twin fans and I limit their use to 8hrs per day?

The above calculation was based on another blog's description of estimating battery size needs. Intuitively, I'd assume my needs would be met with a 125Ahr battery because that would give me an effective 62.5 Ahr battery, which is greater than the 48 amps per day the vans would draw.

So assuming my calculations are correct (please tell me if they are not) and the 125Ahr battery is sufficiently sized, my main concern is whether or not (pun intended) a single 100 watt solar panel would be sufficient to charge that battery? I plan to minimize cable length, use a PWM charge controller, and will not be using an inverter.

I am not sure if I can manage with Renogy's single 100 watt panel/PWM kit, or if I should spring for a 200 watt/MPPT configuration. Given my modest energy needs, and desire to keep a minimum budget, do you all think a 100 watt PV array/PWM charger/125Ah 12 V battery would cover my 48amp per day needs?
 

akrvbob

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If everything goes perfectly, it'll work. In my experience, things rarely go perfectly and a plan with no wiggle room soon leads to disappointment.

If you just can't afford more than 100 watts and that battery then go ahead and give it a try. The worst that can happen is that you won't get the battery 100% charged every day and it will slowly die. It should easily last a year and at the end of the year you'll know if it works or not. If the battery dies, then buy a second 100 watt panel and a pair of golf carts. Or just buy a new battery every year or two.
Bob
 

ALuckyDuck

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Thanks for the input  :idea:

I will probably go for a 200 watt set up, since I would plan on upgrading eventually and I can afford it now. I've noticed there aren't many people complaining that they overestimated their power needs. 

Not sure if this is the right thread, but would the MPPT charger be worth it, as opposed to PWM, with a 200 wt array?

From what I've read, they make a huge difference, but less important for small set ups.
 

SoulRaven

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I am also curious about the MPPT advantage. 

After all my big plans last year I just went with a single battery [font=Arial, Helvetica, sans-serif]DEKA DC24 Group 24 12v 625 MCA Marine RV Deep Cycle. Battery mounts under the sofa. I have average electric usage, TV with a Dish receiver and little else.  [/font]

[font=Arial, Helvetica, sans-serif]I would like a 200 Watt system and am unsure of the benefits of an MPPT. I am going to power a small inverter for the tv and dish. I have a generator and don't plan on parking by anyone so won't bother them but still would like the solar. Will it work with just the one Battery?[/font]
 

SternWake

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For best battery life, AGMs really want higher charging rates.

100 watts Ideally is not going to replace 50AH a Day, and things are never Ideal.

if the solar is your only charging sourde and you run those fans at high speed for 8 hours each day, it is not going to be a happy battery, and you will need supplimental charging sources.

200 watts and you should be good except for periods of bad weather, But during those you will not be needing the fans to run on high all day long.

Fantastic fans move a lot of Air, so If your vehicle is small, 2 of them running on High, one intake, one exhaust, could be overkill.

Better to have too much power, than too little. Needs....evolve.

200 watts and a 125Ah AGM should make for a pretty happy AGM, but still it will want the occassional higher amp recharge, About every 7 to 10 cycles you want to give it 30 amps or so from 50% charged to ~80%.

The Alternator can do this if it is well wired to the house battery. If you intended 125AH battery is the Vmax tanks from Amazon, then you do not want to exceed about 40 amps, but if it's the top end 360$ Lifeline GPL-31XT, it will love 40 amps, or 100 amps if you can muster it in the morning, then let the solar finish the job.

AGMs are great batteries, when they get fully charged according to their individual demands. Their price tends to make some people think this instills them with magical qualities, but this is not true. Their demands are a bit more petulant than flooded batteries in the same usage.

There are lots of PWM vs MPPT threads all over the net. I don't know whether this sticky is the place to argue it, but it is Bob's forum
 

jimindenver

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Honestly with pets involved I would take any amount of solar and battery that you think you need and double it. What to explore MPPT, IF you have room for one larger panel in place of the two smaller ones, it might be worth it. Why? Because the larger panels are the same size up to the 270w range and that one panel will likely be less money than the two 100w panels and be good for up to 20amps of charging. It also depends on what you can get your hands on.
 

Canine

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Put all the panels that will fit on the roof.Solar power is a progressive addiction.The more you have,the more you like it and the more you want.
 

MikeRuth

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I can't say for sure, but my MaxAir Fan draws 5 amps on the highest speed setting.
I'm seriously doubting the Fantastic Fan draws as little as 3 amps on it's max setting.

Just a thought since I have measured mine.

MIke R
 

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