Solar water heaters can operate in any climate. Performance of solar water heaters may vary depending, in part, on how much solar energy is available at the site, but also on how cold the water coming into the system is.
Only during rainy season you will need a conventional electric backup system for getting hot water.
WHAT HAPPENS ON CLOUDY/RAINY DAY? DO YOU STILL GET HOT WATER?
On cloudy days also, if it is for a day or two, you still get warm water as water gets heated due to diffused radiation available in the atmosphere.
If the sunlight is clear and for more time, the water will be heated more. The technical structure of evacuated tubes is made to get maximum benefit of sunlight.
But if there is no sun at all, the water cannot be heated.
The temperature of incoming water also plays important role on output temperature. If the temperature of incoming water is too less than required, output water will also moderately hot.
To overcome such situations, an optional arrangement of heating coil is recommended.
Generally at most of the places in India, sufficient sunlight is available for nearly 300 days in a year.