Hypothetical, just for fun "How much Solar Do I Need" question

[JohnnyM]

Hypothetical: Let's say you live in a hot, humid climate like, oh, I don't know, the Texas Gulf Coast.

You have all the sunshine you could possibly want, and in summer you have a good 14 hours of it per day. The only problem is that it's astonishingly hot and humid all night long and you want to use your roof A/C to keep cool and dry while you snooze the night away and to keep the mushrooms from growing in your shorts drawer.

The obvious solution, of course, would be to fill up ol' Betsy with gas and head for northern New Mexico ASAP. But suppose that option wasn't available for some reason. Remember, this is all hypothetical; a sane person would just bail and head for the mountains.

I'm not smart enough to calculate this on my own, so can anyone tell me how much battery it would take to do the job and how much solar that battery group would need to recharge during the day enough to run the A/C all night. Let's assume for the moment that all the other needs are taken care of -- just talking about the requirements for the A/C in this discussion. Also, to narrow the field, let's assume just one single, normal sized A/C unit in a small TT or Class C. 

Whaddya think?

Johnny

 

[lenny flank]

It'd likely take more solar panels than you can fit on the roof, and a quarter-ton of storage batteries.

 

[Wabbit]

Drive by math...

Est 6ah x 12hours = 72ah

So battery pack of 200ah would work if you recharged them everyday.

300w solar = about 80+ah day

So 200ah+ batteries and 300w solar is my hypothetical answer.

These numbers are rough. If I'm way off please post it.

 

[John61CT]

Really for 24*7 you'll need to run a genny almost as many hours as the aircon, and the huge battery bank will just allow you to time-shift a little.

800w panels is a bare minimum, only enough in ideal conditions, and only running it in the middle of the day.

1200-1500 more realistic, still not enough for overnight, may be able to recharge the bank a little while also running aircon.

2kW+ starts to let you reduce genny runtime, but that takes a lot of roof space.

 

[Scout]

Drive by math...

Est 6ah x 12hours = 72ah

So battery pack of 200ah would work if you recharged them everyday.

300w solar = about 80+ah day

So 200ah+ batteries and 300w solar is my hypothetical answer.

These numbers are rough. If I'm way off please post it.

How are you getting 300watts=80ah per day? Are you figuring 3.2 peak hours of sunlight?
300/12=25×3.2=hours
Not saying your wrong or anything i just want to know if this is how you did it. I'm trying to learn all this math.

 

[Wabbit]

300w solar is approx 15ah in. Most say 5 hours of good sun a day. 15 x 5 = 75. On a perfectly good sunny day, you would get more than 5 hours(not 14 hours, but more than 5). So I went 80ah in to land on the safe side of equation. I think it's more than that if you're talking about a day with 14 hours of cloudless sun.

 

[Motrukdriver]

A couple of these with your sleeping area zoned off with several layers of furniture wrap blankets might make it comfortable enough to sleep and not have to spend a fortune on a million solar panels and 16 tons of batteries.  I have no personal experience with them but I did have a cooler/fridge unit a long time ago with a similar cooling device and a couple of times it got cold enough to freeze a can of soda.  I can see an experiment in the future....

https://www.amazon.com/Conditioner-8-Chip-TEC1-12706-Thermoelectric-Cooling/dp/B073RCY438/

Note: This is a small thermoelectric air cooling device that operates on 12VDC.

 

[Wabbit]

576w of 12v. No idea how many amps that is, but it's more than 40 when doing it in my head. Just FYI.

 

[Trebor English]

One way is to install a 5000 BTU per hour air conditioner unit. If that doesn't cool the van enough with commercial power then add insulation untill it does. At that point get a Kill A Watt and measure the electricity used. From that you can calculate solar watts and battery amps. If the roof isn't big enough for that size panels add insulation and repeat. Then calculate the battery capacity needed. If that is more weight than the van can carry, add insulation and repeat.  

You have to start with something. Measure the energy used by a 5000 BTU per hour unit in your van in your high temperature low humidity desert or lower temperature high humidity coastal area.  

Since a 5000 BTU per hour air conditioner uses about 500 watts while the compressor runs you will need neighborhood 1000 watts of solar panel to make that and do a bit of battery charging. If your insulation is good enough your air conditioner compressor will run less than 50% of the time. Six to eight hours of sun and 16 to 18 hours from the battery at much less than 50% like maybe 25% would be 500 watts times 25% times 18 hours for 2250 watt hours per night, 225 amp hours, 450 amp hours at 50% depth of discharge.  

The ball park is 1000 watts solar, 4 golf cart batteries, 1000 watt inverter, 5000 BTU per hour air conditioner. $1000, $400, $100, $150, total $1650. More battery is probably necessary. Lots of insulation plus more insulation is not included. It's a starting point to guess the price not an engineered solution.  

Additional connectors, bits of wire, fuses, etc should get you to two grand with flooded lead acid batteries. AGM will get you to lithium iron, LiFePo, prices, another $200 to $400 plus more if bigger battery is needed.  

Woops, I forgot a solar charge controller. Add $200 to $300. Also add $500 to $1000 for a backup generator. You must recharge the batteties, no excuses, ever. Now it's $3200. Oh, to use the generator you need a battery charger, a big one.
So, your ball park is $3200.  

The only way to do this is to measure the BTUs you need to pump by installing a 5000 BTU per hour unit and measuring the energy used. Without that it is all a guess.

Search for posts by BradKW for information from someone who has done this in Key West, a coastal area.  Jimindenver posts are from a non coastal user.

 

[Van-Tramp]

300w wont ANYWHERE near do it. I have 300 and even on the best of days I might get 250 watts of peak incoming power for a few minutes here and there. Usually I get about 13-14 amps of incoming power (200 watts +/-) at high noon. At the end of the day that is about 50 amps of power... in the real world. It is some math we can use to start though...

Here is the problem...
1) for every 300 watts of solar panels you will need a 100ah battery to hold that 50ah you can capture each day. You would likely need at least four of these packages (total of 10-12 panels and 4 batteries) and that will only get you 200 amp hours of usable power each day.
2) Not only do you need a place to carry all those panels, you need to carry all those batteries as well.
3) You then need an inverter large enough to run that AC (3000 watts or better just to start the AC)
4) A standard lead acid battery wont deliver that much power in one go. You would need a Lithium battery, about $5000-8000 worth of battery alone, and another $2000-3000 in panels.
5) You need roughly 150 amps of power to start the AC and still close to 100 amps to keep it running... EVERY HOUR. The above solar setup (12 panels, 4 batteries) will allow you to run your AC for only two hours... maybe three. If you want to run the AC for 8 hours, you need even more panels and batteries... and a second truck just to carry them all.

I know of one full-time RV'r that has 1500 watts on top of their Class-A and 400 amp hours of lithium. They *can* run their AC unit off the inverter for short periods, but not even they do. It just isn't practical.

So how about a generator? Sure, totally can be done as long as you have a 3000 watt genny (or 2000 watt and a soft start system for your AC unit). Then you need to think of the gas, which will be about a gallon every few hours. Let's say you use 2 or 3 gallons a day. Are you prepared to spend $6-10 per day in gasoline just for the AC unit? This doesnt even include the cost of the genny itself! And where are you going to store all this extra gasoline? To make it a week, you would need 15-20 gallons of fuel stored. Again, now we are talking about another truck just to haul around the gasoline, genny, and AC unit.

The math is simple: It is possible, but at huge cost, hence the reason no one does it.

 

[Wabbit]

Ignore my math, the 5a I was working with is 110v, not 12v. I hate when my mouth makes a fool of me.

 

[JohnnyM]

Great discussion and answers. Thanks, everybody!

Math and physics are (two of) my weak areas, but I do know that an A/C compressor takes a huge amount of energy to get started, and I strongly suspected that running a roof A/C would be on the outer limits of our portable solar systems' range.

Looks like I was right -- it's doable, but not practical at all. A good-sized generator would get the job done, but that's not what we're looking for. It would be a lot cheaper to just go to a paid campground and plug in.

Or head for the mountains.

Johnny

 

[frater secessus]

I'm not smart enough to calculate this on my own, so can anyone tell me how much battery it would take to do the job and how much solar that battery group would need to recharge during the day enough to run the A/C all night.

This is a frequently asked question and most folks don't get the answer they want.

Here's a gentle intro to solar, including pratical limitations.

 

[Technomad]

It really depends on the air conditioner. There is a huge variety of them, including ones that don’t have a high amp draw on startup and ones designed for solar.

This site should give you an idea:
http://www.geinnovations.net/solar-electricity-cost.html


Sent from my iPhone using Tapatalk

 

[JD GUMBEE]

http://www.hammondac.com/systems/arctic-breeze/
(Link is to a DC powered A/C unit for trucks.)
One like this might be in my future if I was stuck there.
Hottest part of the day, would have the panels dealing with the load.
Do the math...
Had similar one in my big truck with Lithium bank.
Would run for 8 hours while I slept and kept the sleeper comfy...even in NOLA during August.

 

[Wabbit]

^^^45amp draw so that's a lot of solar. 800-900w. Saying that, I totally messed up with stating number facts earlier, might be wrong here too.

 

[John61CT]

20-30AH per hour adds up very fast. Sure with a big LFP bank, a decent alternator setup or large enough genny can replenish a 8- hour run in an hour or two of runtime, but you're talking a very big investment, and very little to do with solar.

 

[jimindenver]

Actually it can be done with solar, I am running one right now. Now that the heat is returning I will likely go back to running it every day.

So I use the most efficient 5000 BTU window shaker that you can buy at a running wattage of 410w once the head pressure is up. I have to tell you that 5000 BTU is pretty limited in a 25 ft trailer and it would be better if I were to close off the bunk area, bedroom area and bath so that it was just cooling the main area. It did a great job when it was in the bedroom window because on a really hot day it would keep that area almost too cold. Now I can run it mid day in the summer off of the 750 watts of solar on my roof. The tilting/tracking 435w on my truck allows me to start it earlier and run it later without draining my batteries. Running it at night would require at least twice as much solar to replace the power needed for batteries, I would realistically say 2 1/2- 3 times as much is a better number.

Now panel is cheap if you buy off of craigslist and get a good deal. You still need someplace to put them and the supporting gear along with the batteries to do this. That's just for a tiny 5000 BTU A/C. You can find a 6000 BTU unit that pulls 500w or so and a 12v 9000 BTU roof A/C that will pull just 600w too. That last one cost nearly 2 grand so add that into your figures. By the time you add up the panel, controllers, batteries and A/C plus a generator to back it all up, it is quite the investment if all you are doing is running your A/C. I do it because it not only saves me and my neighbors from hearing a generator but because I also run a hotplate to cook with, a electric hot water heater, even a 400 watt oil filled heater on clod clear days plus a bunch of smaller items. Even going cheap I have $3500 into my system and I never need the A/C at night.

 

[WalkaboutTed]

That Arctic breeze takes more battery than most people will be able to put in their vehicles. This is from their PDF. Sorry of the copy and paste quality, as you can see, eight group 31 batteries will only give you 16.7 hours of run time at 100%.

"The following chart will give a good indication of the number of batteries needed for different run times.
This chart assumes the use of group 31 AGM batteries weighing 75 lbs. each.
# of Batteries Weight 100% Compressor
run time
60% Compressor run time
4 batteries 276 lbs 8.1 hrs 13.1 hrs
5 batteries 345 lbs 10.3 hrs 16.3 hrs
6 batteries 414 lbs 12.4 hrs 19.5 hrs
7 batteries 483 lbs 14.6 hrs 22.9 hrs
8 batteries 552 lbs 16.7 hrs 26.3 hrs
These run times do not account for any other possible draws on the batteries like interior lights,
radios, CBs, laptops, fridges, etc. If your truck has other power draws, you should factor them into the
number of batteries you may require."

 

[JD GUMBEE]

LOL
Yes, but think about it...
60% is pretty high overnight.
My mistake...the one I had was "inverter" style, which ramped the compressor up and down as needed.
I found the 8 hour run time to work perfectly for sleeping.
Longer than that, I fired the diesel APU.
(Which, BTW, are a lot easier on fuel than gasoline gensets.)
I have often wondered about setting up an alternator to a small diesel engine to see if I could beat my Honda eu2000's efficiency level.
With a 120AMP alternator, driven slower than optimal, you could grab 45amps with a pretty small diesel.
We made them (with gas motors) for the shops.
A cheap hand-truck with a Harbor Freight (I think they were 5HP) and a one-wire GM alternator...and 15 feet of 4AWG with jumper clamps.
Would spin a stone-dead big-truck over after about 10 minutes wide open.
I think our cost on them was about 250 per unit, including the welder time to stitch it together.
The diesel motor weighs more, but on an RV, it would not mean squat. (On a hand truck in the snow, it DID make a difference.)
The mechanics had a choice of the battery bank, the service truck for a standard jump...or the "cart-start." They chose the hand truck every time. It was always a sure thing and never cooked an ECM or starter lead.