**Solar** has the lowest running cost by a wide margin, since it is essentially free once installed. It needs adequate roof area with a decent northerly or easterly aspect, and it heats when the sun is out — which means it extends your season rather than guaranteeing a temperature on a given day.
**A heat pump** is the choice for genuine year-round swimming. It draws warmth from the air and delivers more heat energy than the electricity it consumes, holds a set temperature reliably, and works overnight and in cloud. Higher purchase price, modest running cost, and much better paired with a blanket.
**Gas** heats fast, on demand. It suits a pool that needs to be warm on short notice rather than kept warm continuously — a holiday house or a spa — because running it around the clock gets expensive quickly.
A common answer here is solar plus a blanket for a long shoulder season, with a heat pump added only if the household really does swim through winter. But that is a starting point, not a recommendation: we ask how you use the pool before suggesting anything.