'WaterFurnace' is a heat-pump. They appear to offer a selection of models. Air-loop, ground-loop, forced air heating and cooling. Capable of radiant floor heat, and some models are used with a gas furnace. All heat-pumps focus on a Freon compressor. One side makes hot, the other side makes cold. Find some thing with warmth and extract warmth from it, to put the warmth where you want it to go. Or remove warmth from a place and put it somewhere else.
Running a compressor takes a lot of power.
Some are saying that Heat-Pumps are finally being tweaked enough to make them usable year-round in Maine. They have certainly been marketed for long enough in Maine. I have heard mostly about units that do not work year-round in Maine. Technology is changing. Maybe this company is among the newer ones that can produce warmth in winter, I simply do not know.
The benefit of heat-pumps is that compared to running an oil furnace, your monthly bill is slightly less. With an oil furnace the biggest part of your expense is the fuel oil. Then you also have the electric portion for the fuel pump, blower, and any fans or circulator pumps. Running a heat-pump has no fuel oil expense, but the electric bill will be much higher, because you are running a high-wattage compressor. Over all a heat-pump electric bill is still less than the oil bill. So the new mini-split heat-pumps with variable speed compressor, are supposed to 'win' in terms of being less expensive, when compared to the most expensive source of heat [oil].
Then we come to the last part of the OP, 'Off-Grid'.
We are on Solar-Power.
For heat, we use a wood-stove that heats water, which circulates through a Thermal-Bank and a radiant floor. There is a power requirement for all of this; two circulator pumps that consume around 1.5 watts each [I have the real number written down somewhere, it is not much]. Without any power, our wood-stove can still heat our house. Our house is not heated as evenly as with the radiant floor, and with woodstove alone we will consume a bit more fuel, but it is not a crisis either way. During day-light we make lots of power, we charge the battery and we can operate every appliance and power tool in the house. At night we operate our house on the power from our battery-bank, so we must conserve power usage.
On a cloudy day, we operate our house on the power from our battery-bank. On the third day of a streak of cloudy days, we are still operating our house on the power left in our battery-bank. 3 watts is not a lot of power. 10 watts is a lot more.
If my battery charge goes down too far, then we shut everything off. Our woodstove still works just fine, to heat our home, but we lose the water heating feature. So our home can stay warm.
If your heat came from running a large compressor, plus an assortment of fans or pumps. I would be concerned about what happens when your battery charge goes down.
You would consume battery charge at a faster rate by powering a heat-pump system, and once you are out of power, then you are out of heat.
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