The most commonly installed thermodynamic sized system is for a four-person household that would need one panel for a 250-litre cylinder. It would cost around £5,500 to install.
I’d personally use that investment for PV with a hot-water heat exchanger that doesn’t burn electricity to run the compressor costing you £120.00 per year.
This is why we don’t promote thermodynamic systems.
Hope this helps.