Aloha we live in Waikoloa at about 1000' and are trying to determine our best bet for heating our yet to be built pool. Last year our condo spent $10K to solar heat our condo pool, and a friend in my neighborhood spent $12K to do the same thing.
A pool guy was very opinionated that propane was the best bang for the buck, but that was in conjunction with a hot tub. It does heat quickly. But I am leaning towards a heat pump now, though I don't know anybody on island that has one. It seems like a heat pump and solar power is possibly the way to go. With a heat pump temperature changes are not quick, however.