Adding 12-volt service

Hi Larry. I'm rebuilding an old Shasta for full-time use, to be solar powered with no need for a generator, ever (I also spent a couple of years helping to install residential solar systems in electric, hot water, and air heat). I've done a lot of research for this project. I currently have two residential panels of 205 watts each hooked up to four 6v golf cart batteries, and can run just about anything I have for tools and appliances off the batteries, with and without inverters.

Your math was right on, you just used the wrong units. It's 2.352 kilowatt-hours. The only formula you need to know is Watts = Amps x Volts. The unit you want to use to include time is amp-hours, or watt-hours (watts are a constant, and the units are multiplied, there's no such thing as "watts per day"). Amp-hours is more useful because that's what the batteries are rated in, but watt-hours is just amp-hours x voltage, so either will work.

Your cooler will use 96 AH per day (4a x 24 hrs), or about 1152 wh (1.152 kwh).

Your coffee maker will use 10 AH per day AC (5A x 2 hrs), or 1200 WH (1.2 kwh). But since it takes about 10 amps DC to make one amp AC through an inverter, it will use 100 AH of battery capacity (1200wh / 12v). It will be drawing 50a DC from your battery (5a AC x 10)! Together they will use 196 amp-hours of battery capacity per day. Assuming no charging for math's sake, and to keep above 50% discharge, that means you would need a battery bank of 392 ah just to run the two appliances for 24 hours. That's with no lights, water pump, radio, TV or anything else. That's almost the size of my four 6v golf cart batteries, which hold 460 ah. If it rains/snows/clouds the whole trip, there's very little solar power to be had...

To replace those 196 ah every day through just solar charging (in order to keep up with usage for as long as your trip lasts (and assuming no rain or clouds) you would need: 196ah / 6 hrs = 32.7 amps out of your solar charger (@ 6 hrs of sun max on a clear sunny summer day). At the proper 14.8v charging voltage (for "wet" batteries, if you really want to charge them fully), that would be 483 watts of solar you'd need on the roof (32.7a x 14.8v, not counting inefficiencies, so let's round up to 500 watts). That's a lot of roof acreage.

Even if you let it get down around 50% before you start charging, you will still need to replace all 196 ah every following day to keep it there. You could supplement the solar with a generator, but that is one of the most expensive ways to charge, as well as being irritating to you and anyone nearby.

A 100 amp-hour battery discharged to 50% would run your coffee maker for an hour, not two (50ah capacity / 50a DC draw = 1 hr).

IMHO, I'd toss the thermoelectric cooler. They're terribly inefficient, and only get you about 30 degrees below ambient temperature. In Texas in the summer, you might keep drinks sorta cool-ish, but it wouldn't be able to keep meat or dairy fresh for long. In an air-conditioned truck cab with an alternator powering them they work OK, but they're not much good in a warm camper running off batteries. By the time it hits 75 degrees in the camper your cooler is getting close to too warm for spoil-prone foods.

For contrast, I just bought an Edgestar 3.2 cf, 120v compressor-driven fridge. It's Energy Star rated, and uses about 80 watts max, or .67 amps AC (80w / 120v = .67a). Multiplied by 10 for the inverter and it's 6.7a DC out of the battery bank, compared to the 4a your cooler takes. But I've read that most properly-operating fridges have a 50% duty cycle, so that is 12 hrs x 6.7a = 80.4 ah per day. I'm hoping that the energy star-ness will drop that cycle below 50%, but I haven't tested it yet. If it runs too often I might increase the insulation, or even build a new super-insulated box to put the guts in. Anyway that's less power than your cooler, and it's a "real" fridge, and has a zero degree, separate freezer section (for ice cream!).

I'd probably also stick to a percolator on the stove (if I drank coffee). Heating elements take more juice than anything.

It sounds pretty discouraging, huh.

Thanks for the information, folks, I appreciate hearing from people who have first-hand experience with travel-trailer boondocking!

I'm happy to hear that my calculations were correct, so now I can work on revising my "needs" and my solutions with more confidence --- I'm seeing it as more of a challenge than a discouragement, Cow. I'll study your data carefully, thanks again for spending the time on such a detailed reply!

Thanks too for the tip on the 3.2 cubic foot EdgeStar fridge --- I think we'd have trouble shoe-horning that one into our Compact as a replacement for our existing 2.5 cubic foot Sears model (made in the Pleistocene era), but I'll keep it mind as a replacement, possibly installed in a different location. Please let us know how it performs!

I haven't decided yet whether to use propane (our Compact has a propane lamp and heater, but the stove was removed by a PO and the propane lines need re-furbing or replacing IMO), but I'm kinda curious about the propane refrigerator you're using, Turbodaddy, please give me details: size, brand, model, etc.; that might be a reason to re-do the lines and buy tanks.

Truth be told, we would just as soon camp where there's 120V hookups available, but sometimes it can't be that way. My wife and I are old tent-campers and know how to do without DC power, and will probaby use an ice-chest and a propane-powered coffee maker for the near future, but will add DC and solar after I've done some more ruminating.

I love 12v and solar and I have mobile solar panels for boondocking on extended trips with an acceptable inverter. My suggestion is to be as conservative as you can with battery power, using it only for necessities. I consider necessities to be minimal lighting, charging my electronics, and TV when available, radio if not. But I also carry a hand crank radio for emergencies. I do usually power my outside string lights with my 12v. My advice would be to replace the Mr. Coffee with an old fashioned drip pot you can use with a propane stove OR what I do is use a French press. I just boil my water over propane stove and pour it into the French press. I love it. I've had a couple and Mr. Coffee actually makes a nice glass one that has become my favorite. I also like my whistling tea kettle to heat the water. It was my favorite part of every morning. And the kettle looks right at home on my camper stove.

I have a 12v/110 hybrid refrigerator which I usually carry in my vehicle. The power draw is minimal but adds up over time. An efficient icebox or cooler is more practical for boondocking.

Truth is, off grid camping is great and technology has brought us a long way but the conveniences are not equal to plugging into shore power. Sometimes it is nice to be reminded that we don't really NEED all of the conveniences we've come to expect in life.