# Choosing the number of solar panels?



## a_dahl (Jan 29, 2014)

So my question is? If I want to power my cabin requiring 3kwh a day using 250 watt solar panels I am getting the answer that I should get about 12 panels. That equals 3000 watts total. Saying I get 4 hours of sun a day does that mean with those panels I am producing 12 kwh a day? If that is the right way of calculating solar panels does that mean I will have the 3kw to run my cabin plus charge the batteries to run it when the sun goes down into the next day? They say to divide your panel wattage by your needed wattage to determine how many you need and When I do that I get 12 250 watt panels totaling 3000 watts so wouldn't that mean with those 12 panels I am producing 3000 watts an hour or for that whole day? I am a newbie to solar and I am confusing myself. I know there is a lot of other factor determining how much power it will produce but this is just question of general nature. I am sorry if it sounds confusing.


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## wy_white_wolf (Oct 14, 2004)

For off-grid first thing is to calculate in for system losses:

3000WH * 1.25 = 3750WH

Then you need to divide by the lowest month of solar insolation. I know you say 4 but most likely that is the yearly average and not the lowest. Very few places in the US have a low of 4hours, but I'll use that just incase you do:

3750WH / 4H = 940W This is how big the whole array has to be.

Then divide by panel watts rounding any remaining up.

940W / 250W = 4 panels

For grid tied, Calculate losses:

3000wh * 1.1 = 3300WH

Account for solar insolation. Here you can use the yearly average:

3300WH / 4h = 825W

Divide by panel size rounding up.

825W / 250W = 4 panels

I know here both calculations here came up with 4 panels but in reality the low and average insolation will not be the same. If you give a location I can addjust the numbers to match.

WWW


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## a_dahl (Jan 29, 2014)

Thank you for the answer it helped clear up the confusion I was having! My cabin is going to be located in either the interior of alaska just southeast of Denali or around the wrangell forest. There is a couple spots I am looking at in those areas. My goal is to use a system of solar panels and a generator to provide power for a couple lights and a refridgerator. That's my starting goal. I will expand the system from there. The generator will be powered by gas and eventually a second by a wood gasifier. This is all a work in progress so any suggestions would be great.


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## wy_white_wolf (Oct 14, 2004)

For Denali you have lows of Nov. 0.33, Dec. 0.07, Jan. 0.17, and Feb 0.76. Wrangell - Nov. 0.46, Dec. 0.16, Jan. 0.29, and Feb. 0.79.

Solar can't help then and you will have to depend on the gennies all winter. Trying to use isolation numbers of under 1 result in charge rates above what your battery bank can handle.

WWW


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## Gray Wolf (Jan 25, 2013)

You might think about going a bit short on panels and use the money to buy more batteries instead. We live in eastern WA and aside from the short days, we have many cloudy days in winter where a hundred panels wouldn't help. Winter = generator. 

You can charge a large battery bank at a higher rate than a small one so, since your generator will be burning gas anyway, you might as well use more of the power you are making. We charge our bank of sixteen 6-volt batteries with 50 amps from either a 5,000 or a 3,500 watt generator. Both of which are overkill for that charge rate. I'd like to double the charge rate to save gas and but the bank can't take it. 

We have eight 235 watt panels and have more power than we need most of the year but winters suck.


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## a_dahl (Jan 29, 2014)

All good points. As far as winter being bad for power production I would be able to use the cold Alaskan winters to my advantage. My main thing for needing power is just to have a fridge to help keep some food fresh during the short summers. During winter I can use the weather to keep food fresh. Other than a fridge my main need for power is for just a couple lights and minimal use of small dc pumps to help circulate water. I have no problem using a genny to help charge the battery bank for that. Are any of you familiar with wood gasifiers? They are pretty neat and actually work to run gasoline engines. Poorly built ones will gum up a engine fast though with soot etc. I am in contact with a guy who has made a couple you tube videos on there design and construction.


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## a_dahl (Jan 29, 2014)

Gray wolf what kind of household items do you power with your panels?


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## a_dahl (Jan 29, 2014)

WWW insolation numbers confuse me as far as like kwh/m2/day. For my locations that number falls between 1 to 4. It seems like it's saying I could produce anywhere from 1 to 4 kwh a day per square meter of panel. I should probably by a good book on solar power. It just seems like there is so many factors to consider and so many ways to try and calculate how to design a system. I was told on a different forum to consult a specialist but I like to learn and do stuff myself. If I needed the system sooner than maybe I would but for now I still have time to learn how to do it myself.


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## Gray Wolf (Jan 25, 2013)

a_dahl said:


> Gray wolf what kind of household items do you power with your panels?


Our house has a web site since we are trying to sell it and move to a much larger town to be closer to the specialized medical care my wife needs. Feel free to visit the web site to see how we do it.

offgrid150.simpl.com

We use propane for cooking and hot water, and backup heat a few times a year when we will be gone for a few days in the winter. We heat with wood.

One refrigerator is propane and the big one upstairs is electric. 

We have satellite dishes for internet and tv. Tv is the biggest single power user in the house.

All lighting is zoned to death so we don't waste power. For instance, I'm in the upstairs great room and count 17 light fixtures on 8 circuits. The master bedroom/bath has 13 fixtures etc. etc.

We have a cell phone amplifier system that uses power. Driveway alarm uses power. 8 smoke detectors and lots of GFI indicator lights all use a bit of power. 


Have a backup generator or two - they break when the snow is deepest.

Have a good way to store generator fuel or else plan on going to town every week or so in winter. Enjoy the summer sun up there!

Watch it if you go with a tankless hot water heater. Cold will come down the exhaust pipe and freeze the coils if the room the water heater is in is not kept a bit warm. We have one and it works fine once we replaced a solid door with a louvered one to get some heat in there.

Have a cheap inverter as a spare. (See generator.)

Have a battery charger in addition to the one built into the inverter. (See generator) Our system is 24 volts which is about as high as you can go with cheap automotive chargers. I loaned our backup charger to a neighbor while his inverter was in the shop since there wasn't one around for him to buy in town. 

Spares should always be considered if you are your own power company.

It's tempting to go with a 12 volt system but the system and wire losses are higher.

Go to a place like Backwoods for good information and pretty good prices. 

Shipping is a killer on a few panels. Find a friend and buy a pallet of them if you can. Inspect them before the driver leaves in any case.

Shipping inverters is pricy. They are way heavy! See if you can pick it up somewhere. I bought ours from a dealer in California and picked it up myself at the Magnum factory in Everett WA.

Good luck on your cabin. Solar works!


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## Forcast (Apr 15, 2014)

Just thinking... in the new mailer from harbor freight they have a 45 watt solar panel kit #68751 for $189.99 I would like to power 2sump pumps in the gold fish pond, my house water heater, and heat lamps in the chicken house. Would this system work for me? I don't have a clue about what's what with watts.


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## TnAndy (Sep 15, 2005)

Forcast said:


> Just thinking... in the new mailer from harbor freight they have a 45 watt solar panel kit #68751 for $189.99 I would like to power 2sump pumps in the gold fish pond, *my house water heater*, and* heat lamps* in the chicken house. Would this system work for me? I don't have a clue about what's what with watts.


They will work.

Unfortunately, you'll need to buy several hundred of them.....and a whole bunch of batteries.


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## Forcast (Apr 15, 2014)

No such thing as cheap plug and play? Years ago I had a company come give me a price it was like $50 thousand if I can't afford my electric bill now that ant gonna work for sure. So solar is only for the wealthy still.
.


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## 12vman (Feb 17, 2004)

"*So solar is only for the wealthy still."*

No it ain't. I'm not rich but I ain't paid a power bill for 20 yrs. I don't have much but I live within my means. Just gotta learn to live with whatcha got.. (And be a little creative)


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## PorkChopsMmm (Aug 16, 2010)

Ditto what everyone is saying but I would also recommend getting more panels than you need. We have 16 6V batteries, which does us well, and we have 12 240 watt panels. That size array is a little overkill for us but it allows us to do nearly anything a person needs to do with electricity at our house on any day that it is sunny. That is a good feeling.

Also, FYI, most charge controllers are picky about how you string panels together. My Outback MX60 MPPT controller is maxed out at 12 panels, 4 sets of 3 panels in a series.


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## giles117 (Jun 22, 2012)

A 50k solar rig sounds like someone needing power for a McMansion. 50k, I could power a city block...

My solar rig is costing me $2200. That's with batteries and wiring. More than enough juice to run my operation...


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## Jim-mi (May 15, 2002)

giles please be careful with the wild claims you make.
At $2200 Obviously the batteries you are using are bottom of the line and wont last long.
One system I installed the battery bank alone was worth $14,000 . . .
And you can spend even more than $14k for long life high quality batteries

So before making wild claims please compare apples to apples.........


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## giles117 (Jun 22, 2012)

Wild claim? My check book supports this WILD claim. I only share facts. PERIOD. A claim is something that has to be proven.

The batteries I use are not top of the line, (Crown) but they have lasted 2 years so far and top off nicely. I chose the J185 series as my primary form. Still not a wild claim. This system provides ALL the power I need. Nothing WILD about that... and I was frugal enough to get the batteries at a serious discount. Deals always abound for those who don't live on "buy new always lane." 

All I need is about 200Ah of power each day. I didn't move to the country to act like I was in the city. I moved to the country to shed the city life out of me...

So be careful how you judge a situation. One size does NOT fit all...

Finally, I didn't make ANY comparisons. I merely shared what I DID... So follow your own advice. Thanks...


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## okiemom (May 12, 2002)

for a "city normal" house I would agree $30,000 to 60k is reasonable. that "might" run ac. If solar is the goal the electricity reduction is mandatory. I could do with out a lot but my ac is not one of them. currently the freezers and fridge would be a buster.


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## Gray Wolf (Jan 25, 2013)

$2,200 will get you a good inverter, disconnect, combiner box, some wire, and a charge controller for a small system.

Panels, mounts and batteries are still missing from the picture.

A good set of batteries, properly managed, should last 8-10 years. Cheap ones not so much.

We moved off-grid to the country to live responsibly, reliably, and comfortably. A $2,200 system won't do that. We prefer to have tv, internet and cell phone coverage here, not in town. We perfer lights whenever we want and a refrigerator.


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## giles117 (Jun 22, 2012)

Gray Wolf said:


> $2,200 will get you a good inverter, disconnect, combiner box, some wire, and a charge controller for a small system.
> 
> Panels, mounts and batteries are still missing from the picture.
> 
> ...


You see, everyone's situation is different. $2200 fit the bill for my situation. I don't watch TV. I haven't used a refrigerator since 2007 when I lived in Philadelphia. I figured out what a root cellar was for.

Matter of fact, when I lived in St Louis, I managed to get my public utility electric bill down to $35/month. That's running 4 computers, internet, AND a recording studio.

How you manage your power matters. I just fell into it because my income dropped from 12k/mth to $600/mth in 2008.

Also keep in mind, I have been involved in electronics and electricity for over 37 years. What most people buy, I build. Example, your combiner box. What a waste of money for me to buy one when I can build one for pennies on the dollar. 

Trojan batteries. I don't buy new batteries. I buy year old batteries. Between US BATTERY and Trojans, I never pay more than $100 for a decent quality 6 or 12v deep cycle battery.

Again, homesteading to me is about frugality. Not throwing money at a problem, but using your brain to figure out cost effective methods to solve problems. Real computer programmers know about this. It's called compute cycle management.

My sharing what I did was not to make a standard, but to merely share how much I spent to solve MY electrical needs problem. 

It is foolish to think every homesteader needs an electrician. Go buy an NEC code book. Study it. Take the test. Do the rest. To me, real homesteading by a money rich person is like a preacher buying a church building and having zero carpentry skills. What happens when the money runs out...

Btw, we have lights whenever we want. Cell phones stay charged, blah blah blah... and we live comfortably... 

Too each his own. I have not knocked anyone's way of doing things, don't criticize mine...


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## 12vman (Feb 17, 2004)

Quote, giles117..
" Also keep in mind, I have been involved in electronics and electricity for over 37 years. What most people buy, I build. Example, your combiner box. What a waste of money for me to buy one when I can build one for pennies on the dollar."

Kudos, my friend. I too have been blessed with an electronics background. I've built all of my light fixtures throughout, being nothing on the market fits "my" bill. Built my combiner with 1/2" copper pipe and a propane torch..

Cutting corners with quality has always been the challenge to me. So far I've done rather well. I do OK on 512 watts of panel..


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## giles117 (Jun 22, 2012)

12vman said:


> Cutting corners with quality has always been the challenge to me. So far I've done rather well. I do OK on 512 watts of panel..


Honestly, I could do with JUST 500-600w of panels, BUT I want to run my business with my rig and the gear uses (max) 800watts continuous and I want to run it exclusively off power generated by the panels, not pulling from the batts.


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