I have had a 2 panel solar panel with a tank (500 litres) for over 10 years. It has a heater installed for any time I do not get enough solar to heat the tank.
Guess what, I have never had to use the heater in all these years.
I also fitted an outside tap connected to the hot water supply, which allows me to top up my hot tub.
Guess what, I have never turned on the heater in my hot tub.
2nd best investment I have made ever. The first was installing solar panels in 2010 when the UK government offered great rates for exporting electricity to the grid. I make over £2000 a year on that.
We are a 2-person household when we are on the rock in 3-month periods. When we are in Fuerteventura from
January-
April, we can last for 4 days of heavy cloud or rain. During this time, we do not change our normal hot water usage from other times. So far, I have been close to putting on the heater on 2 occasions, but the sun eventually came out on the 5th days of these periods. The rest of the year, I never worry about the solar panels not supplying me with hot water due to the amount of sunshine beaming down from the sky.
The only cost to me besides the initial cost for over 10 days for hot water has been for replacement anodes. These are over a metre in length & including installation, these cost me about 120 euros every 3 or 4 years.
I totally don't regret having installed a thermal solar system. Never worry about the amount of hot water we use.
My system is a Solarhart system.