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How much solar do I need?
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.
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frater secessus (08-06-2017)
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.
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BobBski (12-07-2015)
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?
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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.
2015 GMC Savannah 2500 van, 480 watts of Solar Panels--and a wonderful furry best friend named Cody. I'm out to change the world!
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frater secessus (08-06-2017), ALuckyDuck (09-17-2015)
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.
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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 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.  

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?
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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
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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.
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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.
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Tele (12-31-2015)
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
Box Stock 2007 Ford E-150 All Build Pics HERE
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ALuckyDuck (09-18-2015)

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