A Reply to another members questions...
A)How many watts to run a roof ac 1500?
It depends on the size and age of your AC Unit. The old rule of thumb was 1500watts for a 15k BTU AC unit (12.5amps). I also would take a gamble that a 10K BTU Ac Unit would only use 1000watts (8.3amps).
B)how many watts to run a small refrig( 2.5 cf) 750?
That is a hard one to answer too. It really depends on how well the manufacturer has designed the fridge and what components they have used. I would imagine you could expect 150 to 300 watts on a mini fridge. A good tool to have is a "Kill-A-Watt" Meter. You can find them at Harbor Freight as well.
(
http://www.killawattplus.com)
You plug this meter into an outlet, then plug your device into it and it will tell you how many amps/watts the device is currently pulling.
C) how long will two or three trolling motor batteries last, running a 2500 watt inverter, for the above. 1) alone
2) with 45 watt harbor freight solar panel.
Math can get tricky with this question. Most Trolling batteries will have an Amp/Hour rating on them. And some people say you really shouldn't deplete the battery past 50%, however I run mine down to 20%. There is also another issue that complicates things, Peukerts Law. A bit hard to explain, but to put it simply the more amps you pull, the less capacity your batteries will hold.
http://en.wikipedia.org/wiki/Peukert%27s_lawFor example, I found out last summer that I could run my AC on high with 4 golf cart batteries (500ah) for about 1 hour before the inverter shut down when the batteries depleted. With 8 golf cart batteries (1000ah), I was able to run it full blast for a little over three hours. As you can see doubling the batteries gave me 3 times as much run time on the AC because of Peukerts Rule.
With 8 batteries, I can run the AC through the night if I'm easy on the thermostat. The next issue is getting the batteries charged back up the next day.
The HF Panels are rated at 45 Watt, about 2.5amps in a perfect world. 2.5amps is normally considered a trickle charge. If your battery is 100ah and is completely dead, it will take 40 hours of sunshine to recharge not including any loss due to poor efficiencies. (100ah/2.5a=40hr)
D) can you supplement the above from the chassis alternator?
Alternators have one minor flaw that isn't talked about much, their continuous rating. A 250amp alternator might only be able to handle 250 amps for 10 to 15mins, after that it will most likely overheat and fail. In a regular vehicle situation, this isn't an issue as after 10 to 15 mins your battery is almost charged and the regulator will start to taper back. But if your charging a large amount of batteries that can take the full 250amps for large amounts of time, you will kill your alternator. I read last year that there are a few companies making sort of an amp regulator for alternators, this spring I will look into this for my configuration.
Also an appliance such as a 15k BTU AC unit that requires 12.5amps at 120volts AC will require 125amps at 12v, not including the loss in your inverter. It will require very large wires going from your alternator, inverter and batteries. I used 1/0 welding cable on mine and even they get warm when running the AC.
A few more things to consider, Motors and compressors require 3 times their rated wattage for startup. My 1500/3000 watt inverter would not start my 15k BTU AC, but my 3000/6000 watt inverter does the job. Inverters now days are sold as Modified Sine Wave inverters and Pure Sine wave inverters. Pure Sine Wave inverters cost more, but give out a much cleaner power output. Some devices such as battery chargers, motors and even some microwaves will not run with a Modified Sign Wave.
When trying to figure out Watts, amps and voltage, just remember PIE.
P=I/E
P=Watts
I=Amps
E=Volts
Helpful Links:
The 12volt Side of Life (
http://www.ccis.com/home/mnemeth/12volt/12volt.htm)
Battery Information (
http://www.batterystuff.com/tutorial_battery.html)
Inverter Information (
http://www.windsun.com/Inverters/Inverter_selection.htm)
Some Solar information:
Solar panels are usually one of the three.
Monocrystalline
Multicrystalline (Polycrystalline)
Amorphous (Thin Film)
In general regarding efficiency and price is directly related, Monocrystalline is the best, multicrystalline is second best, and amorphous is on the bottom. Most of the cheaper panels you see on the market today are amorphous, thin film usually painted on glass. They will get the job done, but usually require more space per watts than any other panel. The majority of panels imported from China are also thin film.
Monocrystalline cells are cut directly from a sylicon nugget, Multicrystalline cells are peices of the nugget, up close the cells look like glass partical board.
Space might not seem like an issue at first, but get on the roof with your tape measure and make sure nothing is in the way. I picked up two US Solar panels for my RV, and they are rather large in size, I might be able to fit three more on my roof but that would be pushing it. They are 64 watts a panel, and a total of 5 would be 320 watts. However I've found monocrystalline panels that are much smaller and 96.2 watts. Due to their small size I can easily fit 9 on my RV, 865.8 watts. Depending on your needs, you can see size can be important depending on your available roof space.
