The Unsung Hero of Solar Energy

By the end of 2010, solar hot water was set to pass wind on an installed power basis (~180GW). Growth in wind power is hampered by transmission and storage challenges. Solar hot water for domestic use is installed on the property and hot water is easily stored in auxlliary tanks.

China leads the world in both installed solar collectors and growth rate. Low cost and modest home requirements In 2010 China installed more solar thermal power than the total installed capacity in the US (20GW). In the US low natural gas prices compete with aggressive tax incentives, but improvements in cost and permitting are needed. Solar hot water will be the dominant renewable energy source until photovoltaic (PV) and utility-scale CSP overtake it later this decade.

US installation rate for solar thermal collectors is currently ~2.0M sq ft. at an average installation cost of $125/sq ft. Growth has slowed due to dramatic drop in natural gas prices but is expected to resume moderate growth. Source SEAI A typical flat plate collector will generate ~320BTU or 90W per day per sq. ft. so the US is currently adding ~144MW in equivalent solar power per year.

US Demand Calculations

Q = c m ΔT / Energy Efficiency

Gas efficiencies range from 52% (before 2004) to 58% (>2004). Electric efficiency is 95%.

Thus a typical gas water heater requires 0.36 Therms to supply 40 gallons of hot water per day.

= 1 BTU/°F x 8.3lb/gal x 40gal x (120-60)° / 0.55

= 36,200 BTUs = 36.2 cubic ft of gas = .36 Therms of gas

Typical home hot water usage


There are ~100M water heaters in American homes, 53% gas and 40% electric with the remainder using liquid fuels, wood or solar. From the chart, showers (17%) and washing (22%) implies hot water usage equal equate to 25% to 30% of the total water usage. Per capita water usage is ~100gal per person per day or 240gal of total water per home, resulting in 60gal of hot water per day.

Thus the total annual energy demand for residential hot water is 1.1Quad BTUs for gas (or ~$15B) plus 140Billion KWh for electric (or another ~$15B)

Thus residential hot water directly costs American consumers $30B per year.

In Israel, 90 percent of homes have solar water heaters, saving 1.6 billion kWh each year, or 21 percent of its domestic electricity use.

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