This math doesn't work.....
I have 2 reverse cycle A/C's on my h40.5. Probably pretty similar to what you have on your 42. My two units draw about 28A of 120V power when running, and a HUGE slug of power when starting. My 5KW genset runs them well. However, your plan of 1000AH of batteries and a 3KW inverter isn't going to work very well. Here is why..... First the inverter probably won't handle the startup surge that the A/C units demand. But assuming that somehow they actually start and run, to supply 28A of 120V power is 3360 watts. A bit over your 3Kw inverter. But let's assume that the 3Kw inverter will handle the startup and running load. The next issue is to determine how long it will run. If you are outputting 3000W of 120V power, you must put in more than 3000W of 12V power due to the conversion inefficiency. Probably 10-15% more than 3000W, or about 3400W of 12V power. If you divide 3400W by 12V you get a current draw of 283A. Your 1000AH bank is rated at a 20 hour rate. This means that you can draw 50A for 20 hours out of that bank before it is completely discharged. (50A times 20 hours equals 1000AH) The AH rating of the batteries drops significantly when the current draw is more than the 20 hour rate. In your case the current draw is 283A or nearly 6 times greater than the 20 hour rate. At a current draw of 283A, the amp-hour rating on your battery bank could be as low as 500-600 AH. You would be extremely lucky to be able to run your A/C for 2 hours. Now to recharge. If your 200W of solar panels are lucky enough to have constant and intense sunlight and you achieve the maximum power output of 200W, you will be able to charge your batteries at 200/12 or 16A. To replace the 1000AH this will require 1000/16 or 62 hours of sunlight. Actually at least 5% more than that due to charging inefficiency. Probably more like 66 hours. All of this ignores the rest of the 12V loads in your boat- refrigeration, lights, etc. The bottom line is that if you want A/C on the hook, you gotta have a genset. Nothing else will do the job. The reason your old salt offered the idea is probably because the Ark didn't have A/C and he wasn't directly familiar with the requirements. Sorry.
