GM, I have just had to replace my 20 year old electric hot water service with a new electric unit. Earlier this year I crunched the numbers on replacing the electric unit with a solar unit. With 4 in the household and taking into account what it cost us to run the electric unit it worked out that the breakeven point cost wise was at least 20 years, not that the solar units last anywhere near that long from all reports. On top of that is the considerable extra materials and thus energy consumed by manufacturing and installing of the solar unit over replacing the simple electric unit, I seriously doubt that the solar units are energy positive when every thing is taken into account. When people tell me that the pay back time is a matter of a few years, what I am really hearing is that they must be wasting great quantities of hot water. I have been involved in building camps around the world and the rule of thumb for a water supply, at least in the tropics, was 200 litres per person per day to cover all domestic needs. I know we don't use anywhere that amount here as we are totally dependent on tank water. What is your present daily consumption per head? 30,000 litres works out to enough for 1 person for 150 days at 200l per day which is a reasonable storage capacity, but a family would probably run out of water pretty quickly if it didn't rain regularly. Those people who are installing tiny tanks in their backyards are doing little more than paying lip service to the concept of conserving water
