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Currently my house has a natural gas-burning hot water heater, with a 40 gallon (150 liter) tank. Given the vintage, the efficiency of this heater is likely around 80%. A new, drop-in replacement would be over 95% efficient, based on what's on the market now.

However, as I'm considering this upgrade, I also want to evaluate the possibility of a solar thermal system.

What tools and resources are available for me to understand if a solar hot water system makes sense for my situation? Will the cost savings and/or environmental benefit justify the added expense?

Some notes on my house and needs:

  • Two people live in the home
  • We've never run out of hot water (150 liters is more than sufficient)
  • Hot water is used for showers, hand-washing, and our small dishwasher
  • The south face of our roof is unshaded
  • It seems like this could reasonably be broadened to a consideration of all widely-available domestic hot water systems? For example, what about heat pump hot water heaters? Or resistive electric + PV? – Jean-Paul Calderone Apr 6 '19 at 14:15
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With today's prices, it's cheaper to install PV panels and generate electricity, then just use a standard electric water heater.

This has the advantage that it actually works better in the winter when it's cold out. Plus any surplus energy generated can be used for other purposes. Surplus heat from solar thermal systems can be a problem in some cases requiring additional equipment to get rid of the surplus energy.

In addition to being cheaper initially, it is easier to maintain, no antifreeze to replace and no pumps/controls to wear out. Plus (eventually) the water tank will need to be replaced. Replacing a standard electric tank will typically be significantly cheaper than replacing a special purpose solar thermal tank.

Note: if you live in a tropical area where the temperature never get's below 40 deg F, then you might be able to use a solar "batch" water heater which would eliminate all the issues I listed above.

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