Wow!
I didn't expect such a response.
Stan, modify this calculation to figure out your power usage:
First, estimate the ac watt-hours to be used each day:
Microwave, coffee: 1500 Watts for 10 min (1/6 hour) = 250 W-hr
TV: 60 Watts for 2 hours = 120 W-hr
Reading Light: 14 Watts for 4 hours = 56 W-hr
Microwave, dinner 1500 Watts for 20 min (1/3 hour) = 500 W-hr
Charger for cellphone, elec toothbrush, etc = 20 W-hr
Total for day: 946 W-hr
Divide by the voltage (120) to get ac Amp-hrs & round up: 8 ac Amp-hr
Next, estimate the DC Amp-hrs required:
Because the Voltage is one-tenth (12 V vs 120 V) it takes 10 times the dc current to give the same power. Further, not all of the dc power becomes ac. An efficiency of 80% is close, probably a little low. The inverter will draw an Amp or so even if there is no ac load.
AC Amp-hr: 8 ac Amp-hr
Multiply by 10: 80 dc Amp-hr
Divide by efficiency, 0.80: 100 dc Amp-hr
Add 24 Amp-hr to keep the inverter lit: 124 dc Amp-hr
Most batteries should not be discharged below 50%, so to last one full day, a 248 Amp-hr battery is required. This is a big one, a size 8D. West Marine recommends 300 Amp-hr for a 35' boat, so this is no surprise. 6 Volt golf cart batteries, in series, will provide 370 or more Amp-hrs at 12 Vdc.
This leaves the problem of charging. A 50 Amp alternator will take 150 minutes to restore the battery, and only if it has a special regulator. Most of us don't run the engine that much, but as I do a lot of overnights with a return to the dock, what I really need is a sizeable charger run by shore power.
Jeff, I knew I could count on you to have already done it. I assume that you are using 75 or 90 Amp-hr batteries, so three of them drained fits my numbers. You are right on about the remote switch -- a remote panel is one of my requirements. Modified Sine Wave inverters are very cheap, and if they do the job they are fine. I would prefer to have a pure sine wave inverter, since I do not really know what fussy loads I might have for it in the future. I chose 3000W because that is what we can get from shorepower, and I have an 1800 watt microwave, 2000 watt stove, and so on.
Docsnow, I know many folks who use the golf cart batteries. They are flooded batteries and must be properly vented to the outside, but they are a great value. If you are going to use two batteries, it is better to have them in series than parallel. I use AGM batteries, which have $ome di$advantage$.
Barleydave, you have also already done it. Xantrex now owns all the inverter companies, but their stuff has a good rep. Except with Yooper.....
Yooper, I know what you mean about non-repairable stuff. The fact is that the Chinese make a lot of it. But there are good manufacturers in China, I dealt with a number of them when I was over there, actually in Thailand - I have never been to the Chinese mainland.
I have been talking to a company that makes UPS systems for industry and I believe that their quality is well above the harbor freight level. Their design is advanced, microprocessor driven and efficient. Service is the problem, and if I do bring in a bunch of these I will need to set up a repair depot here. That means getting schematics and procedures as well as access to parts. I am not sure that I want to do all that, I am after all retired, but I would need to do it for my own unit in any event.
The inverter store sells the AIMS 3000W unit for $800, and a pair of 6 V 245 Amp-hr golf cart batteries will run around $400, plus motor freight. You can likely buy them locally. There are cables and fuses and battery boxes, call it $1400 total. I was hoping to do it for $900 total, maybe that's unrealistic.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida
