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 at 14:15

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