Solar system setup question

captcrunch227 · February 2015

Hey guys, I'm new here. I found the site doing google searches and almost every search or question I had always led me back here. Some very knowledgeable people here and I look forward to learning a ton from this forum. I thought y'all could maybe help me out on this setup.

I'm looking to do a solar setup, I have zero experience with this and this will be my first time dabbling in this arena. I'm looking to have something to provide air conditioning to a room. A small room, 150 or so sq. feet. I have read on here a lot that window a/c units have huge demands to start them up and they aren't the best for energy effeciency.

So my first question is, what would be the best unit/system to use to cool a small room that would be reasonable to power with solar? Is a mini split the best to use for this or is there something better to use? I'm in Texas and would be looking at running it 5 or so hours in the day and a couple of hours at night. It would be handy to be able to run the refrigerator/freezer and lights in the house but a/c is the big priority. 112 degree days can be kinda rough with no power, especially on the little ones.

I do greatly appreciate the help on this. I look forward to the replies and I just hope I can understand what most of y'all are saying! lol

CALLD · February 2015

Hi,

First of all air conditioning is pretty much one of the most power intensive things you could want to do with solar power other than industrial applications or say running a pool-pump etc. I'm guessing you would want to run a 14000btu unit or similar and that would pull about 1.4kw. Multiply that by lets say 8 hours in a 24hr period and you get to 11.2kw/h just for the AC. Then you need to factor in the efficiency of the inverter, batteries, charge controller and wiring. Add another 30% for best case scenario or 50% for worst case scenario. Then look at insolation figures for you area, half them if you want to allow for partly cloudy days. Lets assume in Texas it is 4kw/h per sqm. per day averaged over the year (6 in summer and 2.5 in winter). This is the amount of power a 1kw array would produce on average if mounted facing straight up to the top of the sky. I like to use this method because after tilting the panels to suit your latitude the extra power gained roughly compensates for the losses in efficiency through all the components. So all in all you probably want a 3 to 4kw array, 48V battery bank rated at about 1000AH and a 3 to 5kw pure sine wave inverter. This is big by off-grid standards and is nearly enough to power a moderately sized home if not for the AC. Perhaps the money would be better spent on good insulation to cut the AC-run time? My stance would be to size the Solar power system to run everything else except the AC and to use grid power or a generator to run the AC, but if money were no object and I wanted to go all-out and power everything from solar then by all means - consider a 5-6kw array, 48V 1600AH battery and a 5-10kw inverter:)

BB. · February 2015

Off grid solar power is expensive... Roughly ~$1 to $2+ per kWH -- Or about 10x the cost of utility power. So, if you want off grid power, you really want it to be the most energy efficient you can.

There are now Mini-Split A/C (and heat pump) systems that are quite efficient and have much lower starting surge current--Can make off grid solar power + A/C fairly reasonable for a cabin. And there are a few people here that have run smaller/efficient window units here too.

However, if your aim is to save money and you have utility power available--A Grid Tied solar system (solar panels connected to a GT inverter connected to your main utility power panel) can save you money. However, you have to have a utility billing plan that works with GT solar (not all utilities allow GT solar--And as time goes on, the subsidies that people get for solar power are slowly going away--Politics and economics for utilities).

So, it is coming back to knowing your loads and conservation... Insulation (lots of it in the ceiling), double pane windows, weather stripping, Energy Star Rated appliances, etc. all are better investments for your money than just buying a larger solar power system.

Plus, off grid power systems have a fair amount of maintenance (battery watering/monitoring, batteries typically last ~3-8 years or so, new electronics every ~10+ years, etc.). You have to be pretty dedicated and have a need for off grid solar power.

If you are looking for emergency power (days to a couple of weeks of emergency power)--Many times a simple AC Generator is a good solution. Connect to an existing propane tank or natural gas line--Or you need a place to store fuel. Typically you find the smallest genset you need to power your loads. Larger generators can use a surprising amount of fuel (1/2 a gallon to 1-2 gallons per hour of gasoline for larger units).

Before you purchase anything--Decide why you want off grid/backup power, how much power you will need, look at any conservation you can do--And then make some paper systems. Does not have to be exact--But rough designs and costs before you purchase any equipment. Going out and buying stuff because it is a good price can make it difficult to build a solar power system that will be cost effective. It is like buying an engine for a 1/2 ton pickup before you know if you need a Prius or a semi-truck.

-Bill

AuricTech · February 2015

Depending on the humidity in the part of Texas where you are, an evaporative cooler ("swamp cooler") might be suitable for this application. Of course, if you're someplace like Houston, that's obviously a non-starter....

captcrunch227 · February 2015

This is some good help so far, very much appreciated. I'm looking for a completely off grid scenario though. Something that I would be able to use with no power. So hooking things up to a grid as some of you mentioned wont be an option. So it looks like a/c just wont be an option to power off grid, just too energy consuming. Which is about what I had suspected.

So given that, would it be more reasonable to power a couple of pedestal fans off of solar power? Would that be something that would be feasible to do? I would be running them quite a bit more though, closer to 12-15 hours a day though given that the only way they keep you cool is when they're running. Would this be something that is easily accomplished with a basic solar setup? Or are we looking at a lot of batteries and a lot of panels again?

BB. · February 2015

I am not saying that you cannot run A/C--It is just a question of what it is worth to you.

There was a really nice 120 VAC mini-split -- No longer available -- that would run as low as 300 Watts for lower power A/C dehumidifying. Add a well insulated room/cabin, and running the A/C on low during the day--You can have a very comfortable place.

A mini-split on low will cost you something like $0.30 to $0.60 per hour (all installation/maintenance costs for next 20 years / by amount amount power you use)-- Is that worth it to you or not.

Get a Kill-a-Watt type meter (for 120 VAC, other versions available too) and start measuring your loads. See how much they draw.

Then we just need your (rough) location (amount of sun by season), your average kWH per day (by season), and the rest is just straight forward math (which we can help with).

-Bill

Alaska Man · February 2015

Tall windows that open at the top and a basement for the hot days. If the house is built already and without those and you really want AC, I would suggest a small inverter/generator.

If I lived in Texas I would live in an in-ground type house. Why pay to cool a room/house when the earth is 50° all year round?

captcrunch227 · February 2015

Yeah, in ground houses aren't very popular here. Besides the enormous cost of a custom build home that few can afford, the ground shifts so much here. It can be in 114 one day and a day later a cool front blows in and it be in the 60s. The ground shifts a lot here. Foundation problems are quite common here with all the temperature swings. Just this past week we went from 70 degree weather to in the 20s in a matter of days. With the clay soil it expands and shrinks around 30%. Another reason why basements dont happen here, that and the high water line.

I may look into some more on just running a couple of pedestal/box type fans. A/c just seems way to draw way to much power to actually be feasible. I think fans can be a reasonable alternative that would actually keep us cool.

Mountain Don · February 2015

Texas has a thriving remedial foundation contractor business community.

mmag · February 2015

There is a company that sells solar powered swamp coolers

mike95490 · February 2015

i'd suggest the mini-split AND fans, Both working together, in a well insulated sealed house.

MarkC · February 2015

I have a small, rural cabin style home in South East Texas. Couple of comments - it's often like a "swamp" outside in the summer - no swamp cooler will work. My co-op has a good deal on on-grid which made obtaining a 4000 (nominal!!) watt system affordable for me. I decided on the SMA Sunny boy inverter with the secure power supply with limited off-grid availability (you must vet this thoroughly as it IS limited - but will charge a set of golf cart batteries easily). I decided to go the Prius route of generator. You can then end up with an extremely robust inverter system as part of that package (search for techntrek posts - the best!). The main problem is the high DC voltages - so you would need to understand certain limitations of this off-grid design - mainly the solar charger required for these high voltages are VERY limited - and so many series batteries is another challenge. You will also likely need to be prepared to do some panel re-configuration - but maybe not if you design your on-grid properly.

The good part - I have two small AC units, ceiling fans, regular refrigerator, HD TV, many lights which can all be run easily by the "Prius" system. I have a PTAC AC - the heat pump style with good efficiency and 1 ton of AC that will cool the entire cabin easily and only draws 3.7 amps at 240 v split phase. Even with this unit, I will need to run the Prius gen system during the hottest days to use this AC - obtaining enough battery power is just not an option for me. Off-grid for other users (TV, refrig, etc) e OK for every evening of "cool" comfort (albeit ceiling fans - not AC). BTW, I have not resolved the battery system yet - but my start is 8 Trojan 12 volt deep cycle Golf Cart batteries - I have a golf cart and needed a spare set anyway.

Hope this helps and good luck.

Photowhit · February 2015

I've run an A/c off a minimal solar system, but in general we need more information to help you out, as I recall if you travel I-10 across Texas it's 880 miles, over 950 on US 90. Lots of different climates there, but I'll assume you're in a humid one if you have high ground water.

Is the place theoretical or built? Do you have a budget in mind?

In central Missouri, I built a 10x16 cabin with high ceiling (13'), 6" thick walls, 10" in the floor @8" in roof. I built it in the shade and ran a 5300btu A/C for 4-5 hours a night(on thermostat) a 1000 watt array, PWM charge controller and 4 golf cart batteries. I was hitting the batteries pretty hard and didn't try to run during the day. I later increased the array to @1650watts and ran the A/C during the day from about noon until 5-6pm, this helped in cooling the mass so cooling at night was easier. The batteries lasted 5 years, 4 summers with this setup. I used a 1400watt quasi pure sine wave the first summer and an 1800 watt Prosine after that... Summers in Missouri are not as bad as North Florida which is likely similar to much of Texas. And it normally cools down at night, but something similar might well be done. Here's a pic of the cabin, cropped out of a photo of my shed. The batter box can just be seen to the left panels in sun in front of the cedar shading the cabin.
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I don't really think Off Grid electric is quite as expensive as Bill suggests, but close to $1 a Kwh unless you do the build yourself and find ways to use the energy that would normally be wasted. A/C while hard to do on solar usually makes use of the sun's energy when you have the most sun so might be discounted a bit... I hope that makes sense. Here in Missouri our heat comes with sunshine, while in North Florida we could have 85/85 (heat and Humidity)days with little sun.

Solar system setup question

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