At balcony we install the solar system and place a water tank for every single condo. 
( the water tank can be hung on wall or at bathroom). 
Back-up heater can be either electric heater in water tank, 
or a separate electric or gas heater as well as auto switching control to solar power.
There is controller which sets water outlet at a certain temperature.
Hand switching to solar is available.
And there is anti over-heating, anti dry-heating, anti electric leaking.

Characteristic：
1, Good for tall building with many condos without enough empty roof room;
2, Perfectly matching building & beautiful;
3, Special for big city use and saving room, saving investment.