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Maybe a dumb idea, but can I just put a PV panel on my roof and run wires into a room to a power resistor to generate heat? I have just one room that I was thinking of adding this additional heat to. I suppose I could prototype the idea by running a heater of the desired wattage (maybe 200 watts on average) and seeing if it makes much of a difference in the temp of the room. But I dont see anyone who has done this on the web. Also, you would want to maximize the power transfer from the PV to the power resistor, so that suggests trying to figure out the resistance of the panel and supply lines and trying to match your power resistor to that same resistance?

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Works (while the sun is shining), but is a very expensive option and not overly efficient.

Solar PV panels of a remotely affordable price are about 10% efficient in terms of converting sunlight to electricity. Most of the rest of the sun's energy that hits them is converted to heat.

So, from a sun conversion point of view, direct solar thermal collection is more efficient.

However, for a grid-tied house, you might be better off (as relative prices of PV and thermal systems shift) putting up some grid-tied panels and using the grid they are tied to run a heat pump (depending on climate, a low-tmperature heat pump, which are relatively new on the scene and can operate in air temperatures below 0° F.)

200W is a tiny amount of heat in any case - about 600 BTU/hr.

PV panels tend to have two specified numbers that will be somewhat relevant for choosing a resistor - the open circuit voltage (no current, no power) and the short circuit current (no voltage, no power) - a sophisticated system tries to shoot for the "maximum power point" which is somewhere in the middle. If you are lucky that might also be specified. You are not trying to "match resistance" (whatever you mean by that) - rather you are looking for a resistor that will drop a voltage somewhat less than the open circuit voltage at a current somewhat less than the short circuit current.

  • Yup, I realize that 200W is pretty small, as my little plug in electric heater is about 1000W, so to prototype the idea I would have to run that at about a 20% duty cycle just to see how much of a difference it would make to the room. – DavidN Dec 18 '15 at 3:59
  • ...but only at a 20% duty cycle when the sun is shining. – Ecnerwal Dec 18 '15 at 4:00
  • Well, Im just saying I would like to see how much of an effect this makes before investing in the PV panel, so to try the idea out (given that I dont have a 200 watt electric heater), Im thinking of running my 1KW heater at a 20% duty cycle. If it can make a good difference, then it might be worth looking into this. I was thinking of making a passive solar collector, but i have run run the air ducts up and back /inside to outside, and thats just harder to do, so I hatched the PV idea, but maybe the passive solar idea is just the way to go. – DavidN Dec 18 '15 at 4:06
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    Got any old-tech 100 watt incandescent (or halogen) lightbulbs? Two of those are pretty much exactly the same as a 200W heater. – Ecnerwal Dec 18 '15 at 4:27
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    FYI most commercial solar panels sold today should be at least 15% efficient and some of the better ones will be close to 20% even without talking about exotic panels for satellites or something. Also, the maximum power point is not a single number but varies with insolation, so I don't think it's typically listed (except maybe at reference sun conditions?). MPP controllers are sophisticated because they find it. – Hank Dec 18 '15 at 5:18
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The short answer is that electricity is not a particularly cost-effective way of generating heat, and solar panels are not a particularly cost-effective way of generating electricity, so this is not really a smart financial decision.

If you are trying to generate heat from the sun you are much better off getting solar thermal panels. A PV panel only gets about 15-20% of the sun's power converted but a thermal panel can be closer to 80%.

If you are the DIY type you can even make your own thermal panel for cheap. There are a few ways to do it but I have seen people make them from old refrigerator coils, aluminum cans, heating ductwork, or other materials.

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I don't think a plain resistor is what you want, I think a resistive load is what you want. Incandescent bulbs as mentioned would work and they're easy to find and work with. Of course you won't want an inverter in this setup, and 12V bulbs are readily available for automotive applications.

But it isn't necessary to reinvent the heater, there are tons of 12VDC heaters out there made for automotive use that are cheap as dirt that would work fine. Most have a fan and the convection will probably make it more effective. In fact they are a little too cheap, I'd worry about safety left unattended. There are also 12vdc and 24vdc heaters made for industrial use inside outdoor cabinets, those would work and are I'd trust them much more, but they're more expensive, sometimes you get what you pay for.

  • Note that not all solar panels are 12V... some of the small ones are but many of the full-size panels are more like 30V. It really depends on how the panel is constructed... how many individual cells and how they're wired together. – Hank Dec 18 '15 at 18:01
  • Well, Im not suggesting a plain resistor, but rather a power resistor - one designed to dissipate such a load and one whose resistance wouldn't change that much with temp range - Im assuming if you want to maximize the power transfer to the resistive load, you would need to match that to the source resistance, same idea as in matching load impedance of an antenna to source impedance. – DavidN Dec 18 '15 at 18:12

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