My previous post on the carbon reduction cost performance of my solar hot water system was unduly pessimistic. I now have the December gas bill and it looks as if the reduction is about 10% for a half year of use, or an estimated 20% for a full year, which is what I had originally figured it would be. That considerably improves the carbon reduction cost performance. Here's what the new chart looks like:
Furthermore, the state of California recently announced a subsidy of up to $1500 for solar hot water systems, on top of the 30% federal tax rebate. The last two bars show the per kg carbon reduction cost for the low end thermosyphon system and my high end system taking into account the subsidy. The subsidy still doesn't make a solar hot water system competitive with a gas fired tank hot water heater, but it helps. Figuring that at gas fired tank system costs maybe $1500 installed, it would still take around 15 years of gas savings, assuming the price of gas stays constant, to make up for the difference between the $3300 subsidized, after tax rebate cost of the low end thermosyphon system. The high end system would take longer.
Of course, if you have an electric hot water heater, the savings could be considerably better, since electricity costs more per joule of energy delivered to the water.