posted 2 years ago
I'm a bit late to the party, but I have to say that Chris is spot on with his suggestion to go to 48V.
This problem is what is known on other solar forums as being "stuck in a 12V box". The mentality that you somehow must stick with 12V. It becomes almost as silly as some religious restrictions.
Once you break free of the 12V restrictions, many different solar options are available, and I can say that for my own 48V system, it can produce 15kWh per day without even paying attention. It's all about selecting the right components for the right scale of doing things. You wouldn't try to power your 4X4 pickup truck with a lawn-mower engine, would you? You wouldn't try to put a 300hp V8 engine on your kid's go-cart, would you? The same sense of scale applies to solar installations.
I have to say though that even 15kWh is a lot, and I myself find myself consuming less than 4kWh per day, and that's with lights, TV, computer, and the frig going 24/7. For you, with two frigs, I'd say with a bit of conservation, you can live about the same lifestyle with just 5-6kWh.
On solar, 5-6kWh is easy, and maybe you don't even need a 48V system to do that.
The basic concept to start with is the sunhour, and what it is for your area. A sunhour is NOT the number of daylight hours, but the number of hours of FULL output you can expect. For most of the continental US I'd say maybe you'd get 2.5 sh in winter, and 5.0 sh in summer. Maybe in the far North, 1.5/3.5sh. You can go online and find the sh for your geographical location.
So, saying you need 5.0kWh per day, and you get 2.5sh in December, what you need is 5000Wh/2.5sh = 2000W of panels. Don't pay retail for panels. Shipping is baked into the high price. I've never had to pay 1$/W for locally purchased panels. Shop on Craigslist, with local cash and carry pickup. You can find large grid-tie panels for 3-4W/$. Last summer I bought 260W REC panels for 65$ each. That works out to be 4W/$. So, enough solar for ~520$.
A MPPT charge controller acts as a transformer, converting raw high-voltage solar down to battery charging voltage, making extra amps out of the extra volts. I'm running my solar arrays with four panels in series for 120VDC, which gets transformed down to the 26-28V needed to charge the batteries. The high voltage solar allows me to position my panels ~130' away from the system, which reduces voltage drop to almost zero.
I'm using Rolls-Serrette flooded lead-acid batteries, with about 600Ah of capacity. They are performing very well.
I"m using a Schneider SW4024 sine-wave inverter in my workshop for my 24V system. It makes split-phase 120V/240V AC that gets wired into a standard American main electrical panel. It also has a generator input circuit, so if really cloudy weather presents itself, I simply plug the generator directly into the inverter to charge the batteries in the absence of solar. But years go by now before I actually have to run the generator. I still start the generator a couple of times per year, not because I need it, but to keep it from rusting up.
I've itemized what you need to create a 24V cabin system in another post. Total cost for a beginner's system was ~2500$. Total cost of full-scale deluxe system was ~3600$.
BTW, I have a 48V system for the cabin, which is needed primarily to run my 240VAC well-pump. I find I can make the 2000W needed to run the pump from about 8AM till 4:00PM, with the 4500W of panels I installed.