When considering energy-saving solutions – a solar hot water system, a photovoltaic (PV) system, a heat pump - you should aim for a return-on-investment of 5-7 years. It's wise to invest in a good brand – quality is key because you are looking for a 25-year lifespan.
For example: you want to replace your gas water heating with a thermal solar system. Let's assume your gas bills are around €50-€70 per month, €600-€840 annually, therefore €3000-€4200 over 5-7 years. A quality 2-panel solar system can be installed for around this sum, providing free hot water for many years after the investment has been recovered.
Similarly, for a sizeable home with underfloor heating, gas bills could average around €300 per month, €3600 annually, therefore €18000-€25000 over 5-7 years. This sum would cover the cost of a large (10-panel) solar system or air-to-water heat pump, providing low-cost heating for many years – AND with the added benefit of heating your pool!