Help with new system design / my brain hurts

sabud · November 2011

Hi,

My ultimate goal is to go off grid. I am starting my solar adventure by taking my bedroom off grid to get my feet wet so to speak. I bought a Kill-a-watt device and if all my electronics and lights are on in my room, I use 323 watts. I bought 2 100 watt panels off craigslist as it was a deal vs having panels shipped. I intend to add and grow this system but I need help with getting my room off grid. I do not yet have all the components and need advice on what to buy.

I have great southern exposure, no trees at all. How many batteries can I charge with only 2 100 watt panels? I am ok with buying more but if these 2 will work, than that is ideal for now. Thanks.

Cariboocoot · November 2011

Re: Help with new system design / my brain hurts

Welcome to he forum.

I'll leave the "why are you doing this?" for someone else and just answer the questions. :roll:

What you can reasonably expect 200 Watts of panel to support:
200 * 77% efficiency = 154 / 12 Volts charging = 12.8 Amps peak potential current.
That would be a 128 Amp hour 12 Volt battery. Enough to supply no more than 768 Watt hours, not including conversion loss. If you factor the total system efficiency in you get about 400 Watt hours AC 'out the door' when all is said and done. Enough to run your 323 Watt load for a little over an hour.

Disappointing, isn't it?
Your actual efficiency will be different; that's based on "typical" performance numbers. For instance you might get more than 4 hours equivalent good sun. Or you might be at higher elevation. Or perhaps you can use more load during the day once the batteries are full.

Growing a system is one of the most difficult things there is about solar. Systems do not grow well. As the power levels rise there is a need to do things like increase system Voltage and alter array wiring/charge controllers and battery banks. Most of the time this necessitates tossing everything you have and getting new stuff. Even the panels can be a problem as manufacturers frequently change specifications, meaning the ones you bought yesterday are not compatible with the ones you plan to buy tomorrow.

It's no wonder your brain hurts. So does mine.

icarus · November 2011

Re: Help with new system design / my brain hurts

Just because others will say it, and it is important to note. Going off the grid is rarely a good idea economically. What you need to know is that battery based off grid PV comes at about twice the price per KW,,at half the efficiency, leading to power that is closer to 4 times the price for the off grid power,, nearly ~$1.00 wh.

Additionally, battery based systems need the batteries replaced every 5-15 years at ~ 1/2 the total system cost, (only on part of the added cost) They also require good habits and a good service regimen. Killing a set of batteries comes at considerable cost when they are abused.

The grid is your best friend if you have it available. Before you buy anything (else) condier reading up on the the relative advantages, costs, tax credits, rebates etc that can apply to grid tie system, and try to understand why grid tie makes much better sense in nearly every incarnation.

If the issue is that your grid is unreliable, it is often cheaper and more reliable to use a generator for occasional power outages rather than a battery based system.

All advice that is not asked for,,,

Welcome to the forum,

Tony

sabud · November 2011

Re: Help with new system design / my brain hurts

Thanks for both comments. I am really tinkering and learning for an eventual off-grid homestead. I live in PA and usually have consistent power so that is not why I am after solar energy. I understand the costs and that is not my main concern when compared to my goals of eventually living completely off-grid. I am fascinated by alternative energy sources. My main problem right now is the terminology and calculations. I have been reading and mainly watching youtube for help. When it comes down to actually doing this stuff I am finding it quite complicated. So many pieces to fit together... getting there. Slowly...

waynefromnscanada · November 2011

Re: Help with new system design / my brain hurts

icarus wrote: »

Just because others will say it, and it is important to note. Going off the grid is rarely a good idea economically. What you need to know is that battery based off grid PV comes at about twice the price per KW,,at half the efficiency, leading to power that is closer to 4 times the price for the off grid power,, nearly ~$1.00 wh. Tony

WHAT!!!???? Tony, are you kidding me? I was told that all I had to do was spend a couple hundred dollars, stick a panel on the roof and all my energy problems would be over! Better still, buy a bunch of cells off the net, glue them to a piece of plywood, cover that with a sheet of plastic and it would do the same thing for under $30.00 ? Tony, Please tell me you're pulling my leg!

And NO "sabud", this is NOT aimed at you, I'm just having a bit of fun with Tony:p:p, but you'd be surprised at how many people actually do think that way.

icarus · November 2011

Re: Help with new system design / my brain hurts

Yea,, I read about it on the internets tubing!

I even know of some solar cells that produce at night! All our energy woes are over!

T

Ps in all seriousness,, to Sabud, As you design your "off grid homestead" realize that your cheapest energy dollar is the KWY you don't consume. In other words,, focus like a laser on conservation, given the expense of your power per kwh, every hour your don't need to generate makes your system that much less expensive to build.

Realize that in an off grid system will look like this,, take the name plate rating of the PV, divide that number by TWO to account for all cumulaltive system loses, then multiply that number by 4, to account for the average number of hours of good sun one can reasonably count on, per day on average over the course of the year. So a 1000 watt system might look like this. 1000/2=500*4=2000.

Also realize that all design considerations come from the loads. These loads can and will grow with time,, they always do when you are off grid. The biggest single mistake that people who are trying to build an off grid system is, they at the same time, over estimate the amount of sun they can actually harvest and put into a battery, and they underestimate the loads they actually use.

Start with the load, add in a good amount of head room to grow into.

Tony

waynefromnscanada · November 2011

Re: Help with new system design / my brain hurts

icarus wrote: »

take the name plate rating of the PV, divide that number by four to account for all cumulative system loses, then multiply that number by 4, to account for the average number of hours of good sun one can reasonably count on, per day on average over the course of the year. So a 1000 watt system might look like this. 1000/2=500*4=2000. Tony

Just wondering if I'm reading something wrong in the above statement, or if I need my glasses changed again - - - Asking to avoid possibly confusing Sabud.
First part says to take the name plate rating of the panels and divide by "four" (to account for all losses) then multiply by 4 (the average expected sun-hours per day.
The example then lists a 1000 watt system as such:
1000 watts/2 (should that be /4 instead of /2) ?
If so, that would make it 1000/4=250*4 = 1000. - - - Or is it the first part that has a typo?
If I read your post correctly, I suspect a late night typo error. But at 2:30 in the morning and unable to sleep - - who am I to talk about errors?

BB. · November 2011

Re: Help with new system design / my brain hurts

The "English" is wrong, the equation is correct (I think):

1,000 watt panel * 50% system efficiency * 4 hours of sun = 2,000 Watt*Hours per typical sunny day

-Bill

waynefromnscanada · November 2011

Re: Help with new system design / my brain hurts

BB. wrote: »

The "English" is wrong, the equation is correct (I think):
1,000 watt panel * 50% system efficiency * 4 hours of sun = 2,000 Watt*Hours per typical sunny day

-Bill

Thanks Bill - - - - - - -

icarus · November 2011

Re: Help with new system design / my brain hurts

Sorry folk, brain faster than fingers. (not to mention,, as I have always said,,,,never trust my math,, as Bill says,, the basic equation is "right".

NAME PLATE RATING/2*4=WH/DAY

1000/2=500*4=2000 WH/DAY

One could simply say rating times two, but and it would be correct, but it wouldn't explain why. also, there would be no room to adjust either the time factor up or down, or the efficiency.

Respectfully self flagellating,

Tony

PS I probably have written that equation and notion on these boards a couple of thousand times, one would think I could get it correct. Thanks guys,

T

mike95490 · November 2011

Re: Help with new system design / my brain hurts

Generally, I would say, 100a of batteries for 100W of panel would be a good starting point

BB. · November 2011

Re: Help with new system design / my brain hurts

I would assume Mike is talking about a 12 volt battery bank (24 and 48 volt would be 2x or 4x the panel wattage).

-Bill

mike95490 · November 2011

Re: Help with new system design / my brain hurts

BB. wrote: »

I would assume Mike is talking about a 12 volt battery bank (24 and 48 volt would be 2x or 4x the panel wattage).

-Bill

yep, I was assuming the OP was planning a small 12V system. Good for learning with, and using it as a learning springboard to bigger & greater things.

simagic · November 2011

Re: Help with new system design / my brain hurts

mike90045 wrote: »

Generally, I would say, 100a of batteries for 100W of panel would be a good starting point

???100 AH battery for 100W of panel???.. So Mike is suggesting he get a 200Ah battery for his (2) 100 W panels??? Is that right???. Whaddabout the whole 5% (minimum)-13% ratio thing for charging amount vs battery amount?????????? If his 2 panels are putting out (about) 5 amps each (probably less with derating), so we're looking at 10amps (best case scenario) and lets say we use 10% for the charging ratio. (good easy figure). Well then, 10 amps (produced) is 10 percent of what? 10 is 10 percent of 100. Therefore wouldn't a 100ah battery be best for him or am I wrong. A 200 ah battery would put him in the 5% range (the very minimum) only if his panel were getting great sun and producing the nameplate ratings on the panels.......so if ya figure that not every day gives ya great sun, wouldn't a 100-120 Ah battery be best to keep the battery healthy???.....Sure you can use 5%, but if he hasn't yet purchased a battery, wouldn't ya shoot over 5%, more like 10% if you could ???

sabud · November 2011

Re: Help with new system design / my brain hurts

my goal is to take my bed room off grid. I will buy as many panels as I need. That is not the issue. I only have 2 panels now because I found a good deal on cragslist. I was hoping it would be enough to power it but based on the little amount they can charge, it is likely not possible. I just need help with my goal. All my electronics in my room plus lights is around 323 watts if all running at the same time. I want enough battery power to last until the next days sun can recharge. I will add alternate battery charger for no sun / little sun days...

Thanks.

niel · November 2011

Re: Help with new system design / my brain hurts

sabud,
your loads do determine what you need as far as batteries and the pvs to charge them. there is some wiggle room, but the 323w at most would be the determining factor of how large of an inverter to purchase. the wh per day will determine what you will need for batteries and that determines what to get in pvs. like i said there is a bit of wiggle room, but one will find it varies if the wiggles go too far or not to make a system work or viable. now that needed said and you have the means of determining the wh with that killawatt meter.

mike95490 · November 2011

Re: Help with new system design / my brain hurts

mike90045 wrote: »

Generally, I would say, 100a of batteries for 100W of panel would be a good starting point

I came up with this, based on my experience with a small portable system, using a 64w panel and 80ah battery.

so the math works out - 100W panel, @ 15V that's 6.6A
de-rated to 80W, it's now 5.3A into a 100AH battery, that's just above the 5% lower limit. That works for me, unless I've made a gross math maistake?

Anyone can easily kill a system by pulling more load that it's capable of. I didn't claim this to be idiot proof. You can't pull more out, than you have hours of sunlight to recharge with. (for very long)

But this is all bassakwards, you calculate the LOADS first, to get the battery size, and then you work up the wintertime sunhours to size the array to fit the battery.

Cariboocoot · November 2011

Re: Help with new system design / my brain hurts

Well Mike you misspelled "mistake" but ...

Using the "standard formula":
100 Watts @ 77% efficiency / 12 Volt minimal Voltage = 6.4 Amps which is 5% of 128 Amp hours @ 12 Volts which somebody mentioned before.

The main problem with using the 5% minimum is that it assumes no loads will be active during charging, which is usually not the case. When you figure in net charge current you have to subtract what the loads are taking out of the battery from what the charge controller is putting in.

That's why I usually recommend trying for 10%; it gives you something to work with.

niel · November 2011

Re: Help with new system design / my brain hurts

good point coot as i often forget to mention that loads do take away from the charging amps. i guess i said it so many times in the past that i figured it sunk, but that all got buried in the wayback machine.:D:p

Help with new system design / my brain hurts

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