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  • #16
    Just do the arithmetic and see if it makes sense. Bruce Roe

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    • #17
      A fairly new technology is a air to water heat pump. Combine that with thermal storage tank and low temperature emitters and may be an option. The water in the storage would be heated during the day while you have plenty of sun and then the house is heated with the water in the tank. The air to water units look like a minisplit but all the refrigerant is in the outdoor unit. They also can do cooling with the addition of an air handler. The efficiency of the units vary with outdoor temps but expect worse case of a COP of 2 down near zero heading up to above 3. Once you have thermal storage you can add on a small wood boiler and cover all of the heating demand. The big caveat is sub zero these units just do not work well so you need standby heat (or a wood boiler).

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      • #18
        To answer Bruce's question about how I keep the snow off... I cut the legs off an old pair of sweatpants and wrapped them around the bristles on a push broom then attach a long handle off a roof rake. After it snows I just go out and pull the snow off the panels, which isn't all that easy since the panels are on a hillside and the bottom of the panels start about chest high so the top of the panels are a ways up there.

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        • #19
          Originally posted by EMKETART View Post
          In December 2020 I had a 10.8 kw DC, ground mount system installed. (No batteries)
          To me, it is a no-brainer, but I have read on other forums that it isn't the right move, so now I'm not sure.
          show your calculations used to determine why it is a no brainer?
          basically take the amount of propane used to heat x price and compare to
          amount of electricity educated guess x price and which is higher
          right?

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          • #20
            Originally posted by EMKETART View Post
            why doesn't an electric furnace make sense?
            Because you can supply more heat to the load for less $$ spent by using other fuel sources for heat, including (maybe) a heat pump.

            Or, a perhaps oversimple analogy. You need a paper weight. do you use a rock, a lead bar or a gold bar ?

            If you use nat. gas for heating for example, your PV won't need to be as large because it won't need to meet the winter heating load via electricity stored on the grid in warmer, sunnier weather via net metering. Even with using a heat pump for a good portion of the winter, and nat. gas as a supplement for the depth of winter cold when the heat pump C.O.P falls to 1.0 (or less, considering parasitic energy needs of the pump), you'll likely still be money ahead with with nat. gas.

            However, one possible way to make electric resistance heat work (but the construction costs will probably be prohibitively high) is to have/build what's called a "superinsulated" home that also incorporates a very large amount of thermal storage that's integral to the construction (imagine a solid concrete building wrapped on the outside with 12" of vapor impermeable insulation for starters).Then investigate a T.O.U. metering arrangement with super off peak rates from midnite to, say 6 A.M. and only operate an electrical resistance heating system between those times. The house will be so well insulated and so have a heat loss so low, and have such large a thermal mass, that it will take a long time, maybe days for the building interior temp. to drop a degree or two. So, the heating system will only need to operate at off super off peak times, and not much even then because of the very low heat loss even in the coldest depths of winter. However, heating with something other than electric resistance will still be less expensive, just that the heat load for such a building will be so low that the relative differences in cost of heating via the various methods will also be low.

            There's also the energy quality argument. Some say that's a philosophical waste of time but it has real consequences for situations like those your above question applies to. Read on if your interested, but I'd suggest reading thoughtfully if you do.
            .
            Electricity is a high quality energy source. That means it's a very smart and very versatile fuel and so can do things other energy sources can't do. It can provide light. It can run motors and do work like providing motive power for machines that make our life easier and better like cooling things like food. It can power information electronics. Try running an MRI machine on coal. Electricity is powering the screen your looking at. Electricity can do all those things rather more easily that other fuel sources struggle too do, or can't do at all because it has a low amount of a quality called entropy, which is like saying it's a high quality energy source. That high quality/low entropy characteristic is what makes it very versatile. But that high quality and the versatility that comes with it comes at a price.

            Somewhere along the line down the grid, there's a power station where power is made by turning a generator with power provided by some other fuel source - like natural gas for example. If so, say it puts 100 units of nat. gas heating energy into heating water to steam to turn a steam turbine and the turbine's output turns a generator that makes the electricity that goes into the grid (and so to your computer screen BTW). All that effort and the inherent losses will cost energy so that those 100 units of electricity input are only able to produce maybe 35 to 40 units of energy output in the form of electricity and push it out the door and down the grid to your home. Those energy losses are a good part of the reason why electricity is so expensive to use as a fuel, but you wind up with a very versatile tool for the effort.

            So, I'll try to answer your question with another question: Why would you use 100 units nat. gas energy to produce 40 units of electrical energy ? Answer: that's the price of adding versatility to an energy source. And while it makes sense to use that electricity for tasks that other forms of energy cannot do directly (here's the question at last) how much sense does it make to then take that very versatile, high quality energy that costs a lot to make and then use it to perform a dumb task like space heat that is way below it's capabilities while paying about 2 or 3 times as much for the heat by the way, as you would by getting maybe 80-90 units of heat energy out of the same quantity of 100 units of nat. gas when burned in a high quality furnace ?

            The heat pump makes some sense in the scenario but it gets a bit complicated because a heat pump's efficiency (as measured by the Coefficient of Performance = C.O.P. varies and drops as the heat source temp. drops. Long story.

            Back to the simple analogy. The nat. gas is the lead bar. Electricity is the gold bar. Both are fit for purpose as paper weights. Which one is the more cost effective ?

            Take what you want of the above. Scrap the rest.

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            • #21
              Simply sizing your backup generator will give you a clue..
              Gas furnace needs 700W to run the motor. Easy with a moderate generator, and you get lights and keep the beer cold in the fridge..
              Electric furnace needs 700w to run the motor , plus another 4500w for the glow coil.

              Now you need a much larger generator. A Heat pump is in-between those two extremes
              Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
              || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
              || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

              solar: http://tinyurl.com/LMR-Solar
              gen: http://tinyurl.com/LMR-Lister

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