Interesting piece about how solar water systems are more efficient from a $$$$ to kilowatt. It is going to be interesting to rate all energy systems in kilowatts -- in how much you are saving in equivalent electric systems. Evacuated tube water heating systems are plummeting in price while photovoltaic systems (solar electricity) are if anything getting more expensive.

The evacuated tube solar hot water heaters like the one shown in the top picture are getting really cheap now that the Chinese manufacturers are cranking them out- the salesman told me that he could put one on my house for $3,000 and that it could serve a family of four.

That would replace a conventional electric hot water heater that uses roughly 15kWh of energy per day. A natural gas heater probably uses pretty much the same, albeit cheaper, energy. But essentially, putting that solar heater on the roof offsets 15kWh worth of energy per day from other sources.

Under average conditions, to get 15 kWh out of photovoltaics you would need a 3 kw system, batteries and inverters. At about ten bucks per watt that comes to $ 30,000. So cost of energy from the photovoltaic system is ten times as high as that from the solar hot water system.