Formula needed to estimate power useage

[GotSmart]

I am looking at possible equipment for my set up, and one item is a Air Conditioner. I can get by without one, but if I have to leave my cat inside all day, I do not want him to bake. I need to know if a 2 battery system and 240W of solar panels will work, or if I need to get more.

I am needing the formula to convert a 120 volt unit to a 12 volt unit. So I can estimate power useage and make an informed decision. This is also why I am starting a seperate thread for it. Others will be needing these formulas in the future.

For example, I am looking at this window AC. I can mount it in a back door of my van hidden behind the spare tire. I am not 100% sold on this unit, but using it as a low cost example.

http://www.walmart.com/ip/General-E...r-Conditioner-AET05LQ/20449871#Specifications

Cooling amps: 4.9
Cooling watts: 515
Volts: 115

 

[akrvbob]

Gotsmart. The easiest and shortest answer is NO, 240 watts is nowhere near enough for AC of any size. Solar for AC is just not practical for most if us.
Bob

 

[Unchained]

If you will be in mostly arid regions during the summer, a portable swamp cooler may work for you instead of A/C. Swamp coolers take much less power to run.

 

[GotSmart]

Gotsmart. The easiest and shortest answer is NO, 240 watts is nowhere near enough for AC of any size. Solar for AC is just not practical for most if us.
Bob

OK. I need to find a way to take care of my cat on the hot days. I can always set up a pass through fan system with heavy wire screen, and a couple frozen jugs of water in a pan.

But what about the formula. I will be needing it for figuring out what can work with the system.

Unchained. I just saw your response after I posted. Thanks. I have a swamp type cooler in mind, designed for the cat.

 

[Zil]

formula. Volts times amps equal watts. amps times minutes divided by 60 equals amp hours.

To get 120 volts from a 12 volt battery you need an inverter. The inverter will have a loss during the operation. The 120 volt motor will have starting amps maybe up to double the running amps.

Your example, 4.9 amps x 115 volts = 563.5 watts. 564 watts at 12 volts = 47 amps. Inverter loss of 20% means about 56 amps from battery. Starting the compressor requires more. There are soft start systems that help with this. 4 six volt golf cart batteries are rated about 500 amp hours. You can only use 50% of that. 250 amp hours will run your example maybe about 270 minutes.

Cruisers and sailors at sea do this so it can be done.

 

[James AKA Lynx]

I put mine in my window. 5000 BTU's. In direct sun during the summer it does not cool to well. You will need a Generator.

OR ---

Get a 12 volt fridge and make a cat door in it. Never heard of this but the amp draw would be much less. about 6 amps at 12 volts continues.

 

[blars]

Zil's calculation neglected that lead-acid are rated at a 20-hour rate, so when using more than 5% of the capacity per hour you get much less. So you might only get 3 hours from that setup, and still have to fully charge it back to get most of that the next day. Technomadia is one of the few RVs that sized a battery bank to run their air conditioner, 500Ah of LiFePO4 planning on upgrading to 1000Ah. (For this application, you would need about 3 times as much Ah of lead-acid.) Covering your roof with solar panels might be enough in a modest climate, but not in a hot or humid one.

 

[GotSmart]

Thank you all for the feedback. I will build a cat cave with a couple computer fans to circulate air. I will have fans set up in windows and I am seriously thinking about a roof vent. Vent size 12v fans are not that expensive. Not that much power draw either.

By asking the right questions, from the right people, I can come up with a working solution. I still might set up the AC for when I have access to the grid.

 

[steamjam1]

To get a rough idea of what your power needs would be, look at the power rating decal of whatever you want to plug in, and look at the watts and volts rating.

Take the watts and divide it by volts and that will give you amps.
So if you have a 110V 150watt appliance, then 150/110 = 1.37 amps.

To get a rough idea of what that means if you use an inverter and a 12v battery.. then multiply your amps by 10.
So 1.37 x 10 = 13.7amps.

To estimate how long that'll go on your 12v system.. Take your battery's amp hour rating and divide it by amps. So if you have a 80ah battery, 80/13.7 = 5.84 hours.

There is an inefficiency in the inverter that will suck an extra 10-20% just to run it, so if you want more accurate numbers you'll haveto factor those in.

But for "round about" figures that will give you a decently good idea of what your looking at.. for me its always worked out well.

 

[James AKA Lynx]

you cannot get 80 amp hrs out of a 80 amp hr battery. More like 30 amp hrs.

 

[blars]

On a new 80 Ah lead-acid, you can draw 4 amps for 10 hours, for 40 Ah usable. After a few such cycles, the usable power goes down. Drawing more than 4 amps at any time will reduce the usable power that cycle, you may only be able to draw 8 amps for 2 hours. LiFePO4 have the advantages of 80% usable (rather than 50%) and being rated at a 2-hour rate rather than 20-hour.

 

[Donedirtcheap]

[/quote]I would not leave the cat in a van or RV that requires a/c. If you leave your pet in a house with or without a/c its probably not a problem.

If its a house with no a/c the temps may be high but not kill the pet

If its a house with the a/c and it fails, you will be back home in a time frame that will not let it get too hot. Like coming home from work or from the store

If you leave the cat in the van with an a/c unit and it fails or out the power goes out or it trips a breaker the cat could die

if you rig up a fan system and any component fails, your cat could die

If you know that the temperature on the day you are leaving the cat is in no way going to put the cat in danger, then go ahead and leave the cat.

I guess the easiest way to understand it is like a safety net. Is there a back up plan or safety net. If there isnt, don't do it.

If the cat is in a house (large box, slow heat up) and the system fails there is a long time frame before it is in danger

If the cat is in a van (small box, quick heat up) and the system fails there is a short time frame before its in danger.

The cat is still at more risk in the van because its safety is dependent on you or someone else remembering to check on it.

Thats the problem that everyone is up against in vans and RV's. Cooling is a big problem. High temps can kill quickly and living things can only get so hot for a short time where as low temps are much more tolerable and take much longer to kill.