Solar Hot water heating is effective and a good money saver, with savings per annum on Hotwater heating cost of between 50 and 70%, A 2 year payback is very optimistic, (for example a LPG Hot water heater in a house with 2 people will use about a bottle of gas every 5/6 weeks, at a cost of 15? per bottle is about 130? per year!! and you may save about only 70% of that) ,
I would be careful if installing some of the cheaper models that are about mainly as they tend to be unpressurised see below, and can suffer badly from corrosion. The Glass vacuum tube type seem to be more efficient, however the seals have been known to cause problems after about 2 to 3 years, So I would go for the flat plate type collector
If you put in an unpressurised system, you could end up having major problems maintaining a steady temperature with the shower mixer or other mixer taps, due to mains pressure fluctuations not being seen by the gravity fed hotwater system!!
The tank size can be critical, and I would never put in a tank less than 150Ltr and 180/200Ltr is probably the safe minimum, as anything less could mean frequently running out of hot water or more importantly the booster heater (usually electric,) will be used more often to make up the heat shortfall, which in turn means that you are not saving money on electric , which was the prime purpose of the system!!
The guarantee is important, and if you put in a quality brand such as Solahart or similar you will find that the unit has a minimum of 2 years on parts and up to 10 years on the tank!! The downside of a quality brand is that a 180ltr unit will cost you about 1500/1700? plus IVA, but you will only install the unit once so get it right as you only get what you pay for!!
Cheap will cost you in the long run when dealing with the particularly harsh water here in Spain… ( look in your Kettle, that will tell you all you need to know!!.
