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Estimate for Electric boiler energy use

mmccabe | Posted in General Questions on

Hi All:
Total noob here. I’m amazed at how much expertise this website/group has. I completely mix up watts/amps/volts/ and have only recently learned that I have a boiler not a furnace, so please take it easy on me 😉

Just getting solar installed in the SF Bay area. We will have a 16 kW system, 42 400W panels. I tried to build as large a system as PG&E would allow in the hope of mostly ridding myself of their bills with electric heat, etc.

Speaking of heat. About November I realized that something was amiss with our boiler. It turns out that our Laars Mini-therm had been poorly installed and the venting had rusted out to a dangerous degree. I hadn’t had it checked on enough and we pulled the plug on it and used a gas fireplace and space heaters to get through the season. Not fun.

We have Slantfin baseboard heating-I think it’s low-pressure, medium heat (?) but definitely partially guessing. The easiest move would be to get a new gas boiler, but I’m really trying to wean us away from PG&E and thought redoing the venting would be a challenge. Enter the idea of plopping in an Electric Boiler…despite not finding much US posting about their use (all over the UK apparently) I thought I’d found some solutions with Thermo2000 and similar products.

I’ve heard since then that these systems are extreme energy hogs and as oversized as I thought the system would be, I’m now learning that it could be hard to run and expensive.

I’ve had the newer Viessmann electric boiler recommended.

I guess the question is, can anyone estimate what an electric boilers use will be like. I think I’d be paying 2-400 at the moment a month in gas heating anyway and would be okay putting that into electricity rather than gas.

Additional detail-we probably have about 15 baseboard heaters. Some are rather small as is. I’ve been told that my other dream, a Air to Water heat pump would not have enough baseboard to heat the heavily windowed, not very insulated spaces. It’s about a 3,000 sq. ft house.

One other question-will I probably need a larger than 200 amp panel? The solar installer mentioned that putting the solar back into the grid will take up quite a bit of the panel’s capacity as is.

Thanks and sorry for the babbling 😉


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  1. Expert Member
    DCcontrarian | | #1

    Every house is different, there really aren't rules of thumb that work.

    This article explains how to use your fuel bills to estimate the heating load for your house:

    From that we can estimate what it would cost with different kinds of electric heat.

    Most people who choose to go all-electric go with heat pumps because they use less electricity for the same amount of heat than a boiler would. In your climate I would guess about a third as much.

  2. Expert Member
    BILL WICHERS | | #2

    It's easy to remember watts, volts, and amps using a water analogy like this:
    Voltage is like water pressure, forcing the electricity through the circuit.
    Amperage is like the diameter of the pipe, how much water can flow at a given water pressure (voltage)
    Watts are like how fast you can fill buckets at the end of the pipe.
    Watt-hours, using the same analogy, is equivalent to how many gallon buckets you filled in an hour.

    Easy :-) Visualization helps.

    The easiest way to get a close estimate for your boiler's energy use is to add up all the fuel it used in the last heating season (use data from your gas bills), then multiply that by the efficiency of the boiler, which will give you a smaller number of BTUs that the boiler actually put into the home. That will give you an idea of the total energy demand of your heating system over a season, based on real data for your own home. That won't tell you how much heat you needed on any given day though.

    There are some heat pump "boilers" out there, but mostly in Europe -- they are not common in the US. You could use an electric resistance boiler, but that would probably cost you quite a bit more in energy costs compared to heating with gas. If your primary concern is keeping operating costs manageable, your options would be a new gas-fired boiler, or a heat pump boiler, which may be tricky to find. It's unfortunate that heat pump boilers aren't more common here, since heat pumps are the way to get more heat for less electricity, and the US has many hydronically heated homes in the Northeast that could use a heat pump boiler as an upgrade path to save money on energy costs.


  3. moe_wilensky | | #3

    very roughly, if your bills range from 200 to 400 a month, and we assume that your heating energy use accounts for all the variation (i.e., peak heating month has 200 for heating and 200 for hot water and cooking). At 1.6 $/therm, this is 125 therms. Assuming your boiler is 80% efficient, this will work out to about 125 therms * 80% * 29.3 = 3000 kWh or around 750 $/month at 0.25 $/kWh. I would guess that you'd be using about 1/3 of your annual generation for heat.

    Note: this is just a super rough analysis, and i did not really check any of my numbers, but i think the approach could be sound enough for very preliminary decision making if you were to double check all the numbers.

  4. paul_wiedefeld | | #4

    “ I’ve been told that my other dream, a Air to Water heat pump would not have enough baseboard to heat the heavily windowed, not very insulated spaces.”

    You can work around this if you want with a talented installer

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