Calculating electricity costs

waterouzelJune 20, 2011

I'm trying to zero in on a design for a solar system for my 2,000 sq.ft. home that would provide all present and future electric use. I presently use a voyager high efficiency 90,000 btu propane heater that provides dhw as well as the hot water for the radiant floor system that heats the home. The voyager is ten years old and they haven't proved to be the most reliable heater out there though mine ( knock on wood) has been fine since I replaced the motherboard. However, I want to be prepared to make the switch when it does go. The problem is I have no idea how much electricity I will use to provide the hot water for the radiant floor. Even a ballpark figure would be helpful. My thinking is that if I know how much propane I burn every year I can then somehow calculate how much electricity I would need and then I could size my solar panels.

Would it make more sense to separate the two systems and rely on tankless hot water heaters for all dhw? or? If it maters I live in Northern California.

I would greatly appreciate your thoughts.



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you are looking to replace a propane boiler with electric boiler? I assume you do not have a ductwork system for AC and have zero plans to install.

obviously, this will be a serious investment so sizing is key both for boiler, hot water heater, and the solar system to fuel it.

There are calculators to convert propane usage into BTUs.

I would contact a residential specialist at your electric utility for information and good direction for a project like this.

that would be my starting point.


    Bookmark   June 21, 2011 at 10:42AM
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I can't see using an electric boiler (or WH) when solar hot water is available.

You may want to use PV for electricity for other things, but running a WH would be inefficient.

    Bookmark   June 21, 2011 at 3:11PM
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To design a PV system to run electric resistance heat is an expensive and not wise solution.

You are looking at about 10% efficiency of PV compared to solar hot water. I would venture to say that you would need a $150k system to heat a house and water for 2k sqft in "average" NoCA (if that exists).

So you can do a massive solar hot water system with perhaps 10 panels and 500 gallon storage tank(s). You are probably looking at just $30k (I have 2 panels and 80 gallons for about $6k). Generally these things are sized to get about 50-60% of your heating needs. I would think if you house was really tight that might do it. Generally not considered a cost effective solution but about 20% of the cost of PV.

Now driving a heat pump would be roughly 1/3 the KW cost (or PV sizing) but I don't think anyone makes a heat pump radiant heater.

    Bookmark   June 21, 2011 at 7:51PM
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I can see that getting entirely off propane may not be possible in my case. I just heard that there are pv panels that also produce hot water and this might be a way to go.

Thanks for your help.

    Bookmark   June 22, 2011 at 12:15AM
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It would not surprise me if there are solar panels out there that combine PV with water. But they are not using the electricity produced to heat the water. They could be using the water to cool the panel, making it a bit more efficient (PV efficiency drops as temperature rises).

    Bookmark   June 22, 2011 at 4:30PM
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