Look at energy requirements of desal vs. fresh water pollution proliferation.
10-14 kwh required to desal 1000 gallons of water.
50% of world's food supply is powered by proliferation of phosphorus and nitrogen rich fertilizer and their corresponding runoff.
Wait, are those numbers right? That doesn't look that expensive to me. That's a roof full of solar for lots of water per day?!
Edit: nope I checked and that is a correct number.
Regarding your greenhouse: Does it sit flat on the earth? I'm thinking about lowering the greenhouse at my parents house somewhat into the earth, I think 2 feet would already help with keeping it warmer...
A lot of people do something like this-
https://ceresgs.com/environmental-controls/gaht/
I did the math and used a kill a watt to monitor the usage and just doing a little space heater hooked to an Inkbird temperature controller was plenty economical for me. I have cheap electricity where I live though. About 30 cents a night when it is freezing outside. So absolutely worst case scenario would be $9 a month which I can handle for how much joy it gives me to be able to grow plants in the winter.
Sounds great with your energy prices. At that rates, I'd go for electric heating, too. :-D