# Have I finally got this?



## TxGypsy (Nov 23, 2006)

I am a person that most things come easily to me. I have struggled and labored over comprehending solar power!! I think I may have finally grasped most of it. I'd appreciate it if y'all could look through this and see if I'm correct.

The first thing on my system is also the biggest energy hog. I have a small window A/C unit that is currently cooling the same very small approx sq ft in a poorly insulated RV, in the desert sun, as what I will have at my new place under shade trees. The space there will be well insulated as well. I have tried every way I can think of and I just can't get away from needing A/C in Texas. A swamp cooler won't be an option in the humid eastern part of the state that I'm moving to.

The A/C uses 515 watts....4.9 amps...115 volts. Vitamix blender....120 volt...1380 watts...11.5 amps. 12 volt pedestal fan that uses 25 watts. Laptop...40 watts. 12 volt fluorescent lights...2.2 amps...34 watts. Very small, efficient chest freezer converted to a refrigerator with an external thermostat...uses 700 watts per day.

A/C- 515 watts used for 12 hours a day= 6180 watts per day
Fan-25 watts used for 24 hours a day= 600 watts per day
Computer- 40 watts used for 5 hours a day= 200 watts per day
lights- 34 watts used for(all lights combined) 5 hours a day=170 watts per day
Blender-23 watts per minute used for 5 minutes= 115 watts per day
Refrigerator-700 watts per day

So my bare basic gotta have it power needs are 7965 watts.

If I figure sunshine on my panels for 10 hours a day that means that I need at least 800 watts of panels(8000 watts of power needed divided by the numbers of hours of sunlight received daily). I'll get a bigger system than 800 watts...I just want to make sure my math and methods are correct.

If I have 4 - 240 watt, 24 volt, 8 amp solar modules, I will need a charge controller that can handle 40 amps(8X4=32).

Since I am using 24 volt panels I will need a 24V to 12V converter to run lights and the fan. Also, since I am using 24 volt panels I will need an mppt charge controller(I figured the system using 10- 12 volt panels and the money is the same as using the 24 volt panels and the mppt charge controller). I'm also looking at using a cobra 2500 watt inverter. I decided not to go with the horrifically more expensive true sine wave inverter because I don't use a lot of electronic gadgets.

Pant. Pant.

I've done lots of house wiring and wired up a 100 watt panel in my camper van a couple of months ago, so I think I can handle the wiring. I didn't list all the connectors, wire and breakers that I'll need because I think I've got that part.

So, if y'all would be so kind as to look over my figuring and tell me if I have left out any major system components...other than the battery bank...., or tell me if I've figured incorrectly, I'd appreciate it. Gosh I hope I have finally figured some of this stuff out!


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## lonestarbugout (Jan 25, 2013)

Hey Tex,

First off your basic assumption of 10 hours solar is way off. Best you can do is from 10 am to 4 pm.
There are alternatives tooling that will help. The big one is earth tubes. Do some research on that one.

What part of east tx? I'm in the northeast not too far from Omaha.


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## TxGypsy (Nov 23, 2006)

Maybe the sun shines more around my place. :shrug: I'd think that 10am - 4pm would be the hours you could expect in the winter time. Have you tried earth tubes in east Texas? I would steer clear of them due to the mold and mildew problems we have in this area. 

I'm guessing I'll be about 2 hours south from where you live.


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## TnAndy (Sep 15, 2005)

TxMex said:


> *So my bare basic gotta have it power needs are 7965 watts.*



7965 *watt/hours* ( 'x' number of watts over so many hours )

1st, as LSB said.....you can't figure 10 hours. 

You can't even figure 6 hours of FULL power....800w of panels at 9am will be producing about 200 watts if you're lucky......same thing by around 3pm. You "might" peak around 700w at noon....little more on cold days, little less on hot ones.

For example: Yesterday was a pretty good day here....except a little warm, about as good as it gets. 20 of my 50 panels are on are Enphase micro inverters, and give me a daily report of their production. 

5100 panel watts. (20 x 255w)

--9am they were at 2538w
--12noon they were at 4341w
--3pm they were at 2800w
--5pm they were at 1,000w

Total for the day: 26.3kw/hrs.....26,300 watt/hrs.....a little over 5 times the panel rating in watt hours.

For a battery based system, you could figure 15-20% less production.


To produce the 8kw/hrs you need, you'll want at LEAST 1500watts of panels, and 2,000 would be better to allow for cloudy/rainy weather, with the appropriate sized battery bank.


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## TxGypsy (Nov 23, 2006)

Hmmm, that does make it danged difficult to figure the size of a system doesn't it. At least I will be getting the most solar gain as far as time of day and time of year for running the A/C.


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## SolarGary (Sep 8, 2005)

Hi,
PVWatts is a good tool to use to figure out what a given size system will actually produce in your area -- it gives output by month, which might be helpful in your case since the AC is probably not as much of a load in the winter when you also get less solar:

http://rredc.nrel.gov/solar/calculators/pvwatts/version1/

I've used PVWatts on my system, and comes quite close to its actual output.

The AC does seem to dominate your load. Any chance of finding a more efficient AC unit? Or, providing some type of shading? More reflective exterior colors? Less solar gain through windows using external sun screens or shades or interior reflective shades? Orienting the RV for less solar gain?

Gary


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## Gray Wolf (Jan 25, 2013)

Crank in some system loss too. How did you get the usage numbere?

No cell phone charger, ipad charger, cordless tool recharger, etc? Aside from actually charging, the design of most of them creates heat which is wasted electricty. Toaster, microwave? Small and phantom loads add up. 

Do you watch much tv? We live off grid and tv is our biggest draw other than our big refrig. The tv alone is a bit more than 200 watts and then you add in DVD player or sat dish and sound system. It adds up.


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## wy_white_wolf (Oct 14, 2004)

You will also need to run your numbers for different times of the year. I don't expect you'll be running the AC unit in Decemeber or January so your seasonal load will be different. 

PVwatts will be good for figuring out what size array you need. Also calculate in 20% to 25% loss using MPPT controllers or 50% loss using an PWM controller. I can't see doing a system that size with a PWM controller. The extra panels needed to cover the loss would cost more than the MPPT charge controller.

You also should not drain the battery bank any more than 20% in one day. 8KWH daily load will require a 40KHW battery bank. That's: 

1666AH at 24V
833AH at 48V

WWW


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## Jim-mi (May 15, 2002)

. . ."Have I got this" . . .NO.

Two different animals....Watts . . . and . . .Watt hours.

5 hours a day direct sunlight on the PV panels........
Not "daylight"---your 10 hours.

Best that you be sitting down when you see the price of a battery bank......


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## Bellyman (Jul 6, 2013)

You've gotten some good advice above. Yup, watts and watt hours are two different animals. That AC unit is killing you. 

You've got a lot more to think about before you get to this but 24 volt panels don't necessarily equate to a 24 volt battery bank.


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## Gray Wolf (Jan 25, 2013)

MPPT is the only way to go. We have "24 volt" panels wired in pairs to push up to 60 volts to the charge controller. 

Volts, amps, watts, AC, DC..... Much to sort out before you buy anything. 

Also, now's the time to start figuring out your battery bank. You could have acres of panels that would just be wasted if your battery bank can not accept the charge rate. The system needs to be somewhat balanced. 

And, living off grid eventually will fail due to cloudy days, unless you have a humongus battery bank. Get a generator that will run your charger at the max your battery bank can accept.

I'd also think about scrapping your 12 volt lights and fan. The cost and loss of the voltage converter may not be worth it. It sounds like your inverter is going to be on 24/7 anyway and they are more efficient at higher output. Sine wave inverters have less operating loss.

Why do you need a 2,500 watt inverter?


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## TxGypsy (Nov 23, 2006)

Well.....I now know the answer to why everyone isn't using solar power.

Thank you for the advice. I am honestly beginning to think that this is such a huge complicated pain in the backside that it simply is not worth the effort. All the hours and hours of research and studying seems to have been for nothing.

Yes, the A/C is the big energy hog, but in the area I'm in it isn't a matter of shading, better insulation or increased air flow. It is Humid! Sweat starts pouring the moment you step outside. The air is absolutely sticky and when we hit over 100 it is plumb miserable.


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## lonestarbugout (Jan 25, 2013)

There's a lot of good information here that will help a lot of people. Other options for reducing your heat load are high mass building such as rammed earth, earth berming at least part of the building and earth tubes. In high summer earth tubes will stay dry. When properly installed mould is not as much a problem as conventional forced air duct work.

As far as a conventional window unit goes suggest running it only during peak solar from around 11 to 3. If you have a lot of thermal mass this will hold you over during sleep periods when you need it most. Over size your panels to handle the air unit during peak load. Shut it off during cloudy periods.

Living off grid solar requires changing habits and expectations to a new paradigm. Conservation and conservative energy behavior are requirements.


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## TxGypsy (Nov 23, 2006)

Why do you need a 2,500 watt inverter?

Because if I were to get something smaller my ex would never give me a moments peace. Lousy reason to size something huh.


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## TnAndy (Sep 15, 2005)

lonestarbugout said:


> Other options for reducing your heat load are high mass building such as rammed earth, earth berming at least part of the building and earth tubes........



Yep.....MASS is where it's at, and not just in Texas. 

I built our house with 10" exterior insulated wood walls, concrete floors and a huge fireplace for MASS. You can walk in the house and it's 10 degrees cooler than outside on a hot day just from cooling all that mass at night ( whole house fan ). We have AC, but rarely need it. And the reverse is true in winter.


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## Gray Wolf (Jan 25, 2013)

Could you turn off the air and fan when you are out?

Off grid can work. Here is our web site to see a bit more. I installed all of the system myself. We are selling in order to move to Texas.

offgrid150.simpl.com


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## Jim-mi (May 15, 2002)

Don't give up on renewable energy..............

If you can find a good designer / installer in your area he/she can head you in the right direction.

One thing you are learning --and I hope others who are reading this---you just can not go to a wallyfarts and get---**on sale today $39.95**--- the equipment for a good system...........................


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## Gray Wolf (Jan 25, 2013)

We live where it gets 100 and -15 with no air or central heat.

We have over 60 tons of stone in the fireplaces. Floors are tile over concrete over insulation. We got mass. We open windows at night and keep closed during the hot afternoons. All walls are insulated, interior and exterior. Mass is good in winter too.

Can you add a few tons somewhere? Add more insulation?

Maybe look into adding mass in burried stone fields that you can force air through to cool in summer and heat in winter. We don't have that but I've seen it and it works to take the peaks off temprature swings. Not simple but everything helps.

Don't give up!


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## Jim-mi (May 15, 2002)

Those buried rock piles that you force air thru are subject to mold . . .NOT good.
Being in a very humid area to begin with . . . . . . . . . . . . .

Gray Wolf is a good example of building a big enough system--off grid-- to live very comfortably with.
--But-- he will also admit that "up front" he put a bunch of bucks into his system.


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## 12vman (Feb 17, 2004)

Would it be possible to have a small area in your home to retreat from the heat? That's how I do it. I only have 512 watts of panel installed right now so I don't have enough power to supply a standard fridge for a day. I use my bedroom for my retreat and operate a small window air conditioner with a 2800 watt generator when the heat gets too bad. A little over a quart of gasoline gives me ~3 hrs. of comfort. A couple of fill-ups gets me through the worst part of the day. I might suffer a little during the night but I survive. Small fans work better during the night. Just a thought..

Don't be discouraged just because you don't understand. No one here will steer you wrong. It's simple once you get a grip on it. Design for December/January and it will work all year long..


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## Gray Wolf (Jan 25, 2013)

Actually, since I did it myself, we have well under $15,000 in the whole deal. 

It's mostly a matter of taking your time and working towards a reasonable goal. I started with two panels and a few batteries, which was good enough for weekends when we came over to work on the house. I added panels and replaced 8 old batteries with 16 new ones when we moved here full time. And replaced our old charge controller with an Outback MPPT one. 

We had the exterior shell of the house built in 2004 and have been finishing it ourselves ever since. It was done enough to sell our other house and retire here about 3 years ago.

Random thoughts follow----- Very random.
Buy and learn to use electrical meters. Get a Kill-A-Watt kind of meter and know what your loads really are. Take your time. Don't buy crap because when stuff breaks you will be out of power until the new parts get there and are installed. Returning to be repaired even under warrantee will take weeks. Inverters are HEAVY so you don't want to be shipping them any more than you have to. Check out Backwoods Solar, and others, to learn all you can. Make your own panel mounts. Don't put them on the roof if you live in snow country (duh). Go with the big US brands for inverter and charge controller. Buy panels from a place that is reccomended, not the cheapest - within driving distance would be nice. Don't write off Chinese panels or Chinese panel components - darn few are 100% US anyway. 24 volts system is good. 48 is better. System voltages these days can go much higher. I'm personally afraid of more than 24 volts DC, but look it up and decide yourself. Dip all but the head of wrenches and screwdrivers in plastic dip so you don't arc across the batteries when you drop a tool while you are working on them, and wear plastic-framed saftey glasses. Wire is cheap so use zoned task lighting rather than waste power, the room I'm in now has 19 light fixtures on 12 circuts. Windows - natural light is free, this room has 14. Get old - it's fun.


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## awhobert7 (Jun 1, 2002)

I have a 1.5 KWH system. most of the panels are mounted on racks that can be turned 180 degrees. To track the sun. Need to turn them 3 times a day.


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