So you use excess S and W power to preheat... Did I understand that correctly?
I don't know if I'd call it "excess". I have sort of designed our system to heat water this way, and the solar and wind power is sized to be able to do it. First priority is battery charging and on a typical day our bank is done with bulk and into absorb by noon. No water heating gets done until the bank is in the last stages of absorb (which typically takes 2-3 hours) and once it drops into float the pre-heat elements are on constantly until the bank either drops into re-bulk, or the stats kick the elements out.
The elements are 240 volt AC, powered by the inverters. So every input power source to the batteries (solar wind and generator) can be used to power the water heater elements.
On a typical day around 7 kWh - fully 1/3 of our daily average power consumption - goes into the water heaters. There has been really good days when the wind blows hard that we have dumped 30+ kWh into the water heaters. On "bad" days we normally have enough hot water stored up so the always-on element never powers up. In the winter time when the days are short, and if we don't get decent wind, then the generator keeps that "emergency" ~30 gallons of water in the top of the primary heater up to 125 degrees so we're never completely out of hot water.
Basically the way I look at it is that the only things you need to live off-grid is a good reliable inverter, battery bank and a generator. All the solar panels and wind turbines do is reduce the amount of fuel that gets burned in the generator. I know a lot of folks don't look at it that way. But we're not about to give up any of our creature comforts, or grope around in the dark, just because there's bad days when the wind don't blow and the sun never comes out.
Edit:
I'm going to add some comments here because many folks have a TOTALLY different idea about all this.
I've seen some (the vast majority) of off-grid folks that I know that consider the generator to be a "necessary evil". They will abuse their batteries, and drastically shorten their life, just to prevent having to run the generator.
Our system is not like that. The generator is designed in as an integral part of the system. Our generator is brought online long before our batteries get abused. We don't consider the generator a "necessary evil" - it's our friend. And we're not afraid to run it. That's what we bought it for.
And like I told Bob the other day, we don't burn a lot fuel in the generator. But we burn one hell of lot more than most folks who live off-grid. But most folks use propane for other things like water heater, and we don't - unless it's thru the generator.
But the point is, if you're looking at living off-grid and want a system where the generator is an integral part of the system and totally automatic - you're going to be almost exclusively looking at an inverter from one of the Big Three. I'm sure some of these other inverters like Exeltech might be good. But they don't have the features for off-grid system automation that ours got.
It all depends on what you want.
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Chris