1. How much water do you really need to heat up? - Try to get a consumption report of your showerheads to understand the capacity requirements. Do this by using a pail and stopwatch, then estimate average showertime, and get total consumption accordingly. Regular shower heads use 4.5 gallons per minute, Eco Friendly showerheads use 1.5 gallons per minute. Investing in even a high end eco showerhead is still a lot cheaper than buying a very big solar water heater.
2. What direction is your solar water heater facing? - Get the right angle and direction. North facing is best to get sun from its axis east to west. This will maximize heating time for water.
3. Flat collector panel or evacuated tubes? - Evacuated tubes are cheaper but they are not designed for the local climate. On hot weathers, these pipes burst, causing great inefficiency. You will not notice this problem as your back up electric water heater will run instead. This means, lots of money wasted on electricity that you were trying to save.
4. How's the weather? - Good brand solar water heaters can still get heat from a cloudy day. Invest a little more for a good brand and you will better appreciate your returns.
5. Any other use for hot water? - Do not forget to get a complete idea on where else you can use hot water. A pre heated water will be faster to boil for cooking as well, this point should not be discounted.