Solar cover first, heat pump second.

If you’re on a budget, a solar cover is the only answer. It’s cheap, pays for itself in one season, and reduces evaporation and heat loss. It won’t make your pool tropical in spring, but it’ll add 5–10°F for free after the initial $100–200.

A heat pump is the most efficient long-term option, but it costs $2,000–5,000 installed and needs electricity (or solar panels to be truly cheap). It’ll keep your pool warm all season, but the upfront hurts.

Gas heaters are fast but expensive to run. Unless you need to heat a huge pool quickly for occasional use, skip them. The gas bill will eat your budget alive.

Final thought: A solar cover is a no-brainer. Add a heat pump only if you swim daily and have the cash.