Yeah, the pool actually did have this type of solar thermal heating originally, when they first bought the house around 15 years ago. But it wasn't very effecti

Yeah, the pool actually did have this type of solar thermal heating originally, when they first bought the house around 15 years ago. But it wasn't very effective at heating outside of the peak summer months. Installing the heat pump meant they could swim around 5-6 months of the year instead of 2-3. But to be fair, the collectors probably weren't big enough for the size of the pool. And the technology has probably also improved a lot since then - it was a pretty ancient set up. PV is certainly more expensive but it has other utility besides just reducing the pool heating costs, as it will be able to generate reasonable energy year round for them (at ~45 deg latitude). If they didn't already have the heat pump then thermal might make more sense!