davewill wrote:Sure, there are reasons to do it, but I don't think anyone can pay for a bigger system and the new water heater with the savings.

You may be right. I don't know what natural gas water heating costs look like, so I can only do the comparison with an electric water heater. Perhaps someone else can give numbers for gas...

**Given:**
- Cost of a new resistive water heater: $400.00

- Electricity consumption of a resistive water heater: 4600 kWh/year

- Cost of a new heat-pump water heater: $900.00

- Electricity consumption of a heat-pump water heater: 1900 kWh/year

- Life of water heaters is assumed to be the same with no maintenance.

- Annual production of a 1-W PV module (on my house): 1.4 kWh/year

- Cost per kW of PV (installed): $3.00

- Cost of grid electricity: $0.12

- Cost of money: zero

**Calculations:**
- Initial outlay for resistive approach without solar: $400.00

- Annual outlay for resistive approach without solar: $550.00

- Initial outlay for heat-pump approach with solar: $900 + $3.00*(1900 kWh/year/1.4 kWh/year): $5000.00

- *Minimum* payback time using zero cost of money is: ($5000.00 - $400)/$550.00 = 8.4 years.

All of the above assumes that the price of electricity does not increase over time, but let's assume it is similar to the time value of money. (It's higher than MY TVM, that's for sure!)

Eight-and-a-half year payback is not a no-brainer, but it's a quite reasonable thing to do considering the photovoltaic equipment has a life of about 30 years or so.

So how much does it cost to run a natural gas water heater (50 gallons)? (FWIW, I just used the numbers from the sticker on the side of the water heaters for my calculations.)