on demand water heater - propane vs electric?

Question

We need to replace our (tank) propane water heater. We'd like to switch to a tankless heater, but we're waffling between propane and electric. I saw this question, which was really helpful, but we've got a few other factors that may have an impact on the best solution.

We're unable to do solar panels on the roof at this time, but we're signing up for community solar, which means that 80-90% of our annual electricity will come from solar panels at an off-site grid. (We're in the Northeast US, so this means that in the winter, when we'll probably use more hot water, we'll still be burning natural gas or whatever the electric company uses to produce power, while in the summer, our solar panels will be producing enough for multiple households.)

I've read online that electric heaters are 98% efficient, while propane are only about 85% efficient. Propane being a fossil fuel, and with our electricity (mostly) coming from a renewable resource, it seems like a no-brainer to go with electric.

However - and this is where my confusion comes in - I've gotten multiple in-person recommendations to stick with a propane fuel source. People have said that electric water heaters aren't hardy enough, and will not provide enough hot water for our household (3 people, plus regular guests, and I will admit we like our hot showers).

Why are electric water heaters, despite being 98% efficient, somehow considered less effective than propane? Does anyone have any other factors I should take into account, or any personal experience with tankless water heaters, either propane/natural gas or electric?

Thank you!

Answer 1

The principal question about using electric heaters is, where does the electricity come from. If it must be generated from fossil energies more than 50% of initial input energy gets lost. This is the case when the heat generated from turbines or generators is not used to heat-up water or used for other application. Big power plants frequently waste such energy in big chillers.

If, as in your case, the electric energy is generated directly by solar panels or wind power generators it is different and heating water with electricity may be more efficient. I understand that electricity is generated between 80% and 90% of the time by solar power in your case and fulfils 100% of need.

The other 10% - 20% do not mean that there is no electricity at all, right? There may be additional energy needed to stabilize the network. When it comes from the grid you should not be worried. It is their task to make the grid sustainable. If your network is isolated from the grid your power company must do something to stabilize the network anyway. In most cases he can do that with more efficiency than you can do.

Considering your description and assuming that the grid is stable my first answer from the heart would be: Use the electric heater.

Answer 2

Price

Looking at tankless water heaters on the website for Home Depot, a large home improvement product retailer in the US:

• For gas water heaters (natural gas or propane) in the 4 to 4.5 gallon per minute (gpm) (15 to 17 liters per minute, lpm) supply range, there are two choices with an average price of about $230.
  • This is on the small end -- most of these heaters supply 6 gpm (23 lpm) or greater.
• For electric water heaters with the same flowrate, there are eight choices with an average price of about $620.
  • This is on the large end -- most of these heaters supply 1 to 3 gpm (4 to 11 lpm)

Power

Comparing the two options within one brand, Rheem, provides insight into the power demand differences:

• Rheem electric heaters (pdf) supplying 5 gpm (19 lpm) require a 30 or 60 amp breaker -- at 7 gpm (26 lpm) and above, up to four 40 amp breakers at 240V may be required to supply up to 36 kW.
  • This is significantly more power than a very large whole home air conditioner, which requires a maximum of 50 amps to supply up to 6 kW.
• Rheem Natural gas heaters (pdf) require 120,000 to 200,000 BTU per hour (35 to 60 kW), and have a higher capacity (up to 9 gpm).
  • This is comparable to the range for home heating furnaces (though 200,000 BTU/h / 60kW would be quite large).

Efficiency

As noted in the question, electric tankless water heaters are more efficient. Based on the Rheem units linked above:

Tankless water heater efficiency:
 - Electric:               99.8%
 - Natural gas or propane: 93.0%

Conclusion

Based on the high cost and relative power demand of electric tankless water heaters, it would seem that they are most often used in cases where supplying natural gas or propane is not practical, despite the greater efficiency.

Answer 3

This depends on the amount of CO2 generated by both sources. Propane (LPG) generates around 230g CO2 per kWh used (source https://www.volker-quaschning.de/datserv/CO2-spez/index_e.php ).

Divide this by the efficiency of your water heater:

e.g. 230 / 85% = 271 gCO2 / kWh of water heated.

For the electric heated water, you need to know the carbon intensity of your local electricity grid. Looking at:

https://www.electricitymap.org/

This varies widely between grids e.g. right-now, we have Sweden at 36 g/kWh and Estonia at 984 g/kWh.

So in Sweden the electric water heater is going to be better, and in Estonia, propane wins.

... but you can also do better than straight electric by using a heat pump. This doesn't just use the electricity to heat the water directly, it moves heat from outside the building into the water. Because of this it can achieve an efficiency greater than 100%. e.g. 250% (CoP of 2.5) is common for heat pump water heaters.

For my local grid this would bring the carbon intensity of water heating down from 250 g/kWh (for direct electric heating) to 100 g/kWh (heat pump electric heating).