A Simpleton's Question about Electricity

First a little set up. I know very little about electricity. I know lots of the words, like volts and amps and watts and AC/DC but the words are close to all I know. I also know there is a thing called resistance, and I believe it is involved in electric heating and in things like stove burners. And I remember reading and it's hopefully true that electric resistance heating is 100% efficient, any inefficiency is in generation and transmission.

I have a cheap hot plate that I use to melt beeswax, and I have a new 100-watt solar panel. And I have some electric outlets out in shed. My question is, if I wire that panel directly to one of the outlets and plug in the hot plate, will the burner get warm, or even hot?

Mark,

If the hot plate's heating elements are directly wired to the plug without some electronics that would disconnect the elements when the voltage drops
-and if the panel has some sufficient voltage that would make the elements conduct sufficient current to generate heat
-and you put a fuse between the panel and the plate to not exceed panel's rated current (just in case) then MAYBE it will get warm.
I'm curious.

Mark, I would either source an inverter or a DC hot plate.

I just looked at my single burner and it is 1100 watts. 1100 watts / 120 volts = 9.17 amps. The resistance of the hot plate is 120 volts / 9.17 amps = 13 ohms. If I work back to amps on 12 volts / 13 ohms = 0.92 amps.

I don't think it'll heat up.

If the label said it was powered by 12 volts AC then it would because at 1100 watts, it would be the same amperage for both ac and dc. But that is 91.7 amps and would not be a real world scenario for a hot plate at 12 vac or vdc and 1100 watts.

I think the 100 watt panels produce 18 volts at optimum conditions. The resistance of the device is the shortfall here even direct wired to the panel at optimum conditions.

The very simple response is: AC and DC systems are different planets. They transmit and utilize electrical energy in very different ways. What works on Planet AC will not work on Planet DC. There are universal translators that allow communication between their different languages, but dangerous misunderstandings are possible.

Yes, converting electricity to heat with a cooking stove (a big resistor) is 100% efficient, because that is how we define 100% in this conversion.

Heat pumps convert electricity to heat with 300% -  500% efficiency, so resistive heating is really not that great.

To make the best use of your 100W (12V?) panel, 12V light bulbs (old car headlights) are probably the best option.
They are pretty well matched to the voltage you have and the amount of power the panel can produce.

To combine and expand on everyone’s posts.

Josh’s numbers would suggest that you would need about 11 of your 100w panels to get the power you need to heat the hot plate. In reality you would need 14, arranged in 7 series groups of 2. This would give you 126 Vdc at peak sunlight exposure. How often do you get peak sunlight exposure?

If you did try to use dc to power your hot plate, you would find that one end gets really hot and one end stays fairly cold. This is because you’re running your heating element on dc. With ac, the polarity changes and half the time the current flows the opposite direction. So one end of the plate gets hot then the other, changing at 60 times per second, averaging out to get an even heat. edit cos I was thinking about flourescent tubes.

The inverter would allow you to run the panel with an even heat at a lower temperature, assuming it doesn’t have a low current cut out.

The car bulb would definitely be more effective than the hot plate and would make the wiring much easier. However I would put 5 20w (brake light) bulbs in parallel. This would give you more spread out heat rather than a single point. You could also put a switch on each bulb for when it gets cloudy, so that you can switch it down to 4 or 3 bulbs when you have less power.


What I would do is to get an old car battery, some 12v leds and a charge controller and use the solar panel to light the shed and charge your phone. And then get a Kelly (storm) kettle and burns twigs, using a double boiler to melt your wax.

Thanks everyone for the replies, now I have some follow up questions. Maybe I should explain a little more about my hair-brained idea. I don't expect the hot plat to operate normally, it is just the resistance heat apparatus I have available for the experiment. I also wonder about those heat tape wraps; I think they might work similarly and perhaps better for the ultimate purpose.

All I'm looking for now is proof of concept. That would be if the hot plate gets warm, even just a little bit, that it doesn't shock me, and nothing gets hot enough to start a fire. Or if one of the second two happens I understand why and can take steps to remedy it.

Cristobal Cristo wrote:Mark,

If the hot plate's heating elements are directly wired to the plug without some electronics that would disconnect the elements when the voltage drops

I'm guessing the voltage drops when sun exposure is reduced?  Are there any safety or fire concerns related to voltage fluctuations? Is damage to the panel possible?

Cristobal Cristo wrote:-and if the panel has some sufficient voltage that would make the elements conduct sufficient current to generate heat

This seems related to the first comment. So, voltage is key? Maybe I need to do some reading and understand better what voltage is.

Cristobal Cristo wrote:-and you put a fuse between the panel and the plate to not exceed panel's rated current (just in case) then MAYBE it will get warm.
I'm curious.

What are the (just in case) possibilities as far as damage to equipment, possibility of shock, or fire? (without a fuse)

Risk of damage: none.

The plate will get a little bit warm.
The voltage is harmless, if you touch both terminals with your tongue it will tickle, but otherwise you will not feel anything.
Solar panels tolerate short circuits.

To get the most out of your panel, you need to match the load to the current the panel can produce at any time. Without active electronics that is not possible. Aiming at 2-3A is probably the most realistic. The heater has a resistance of ~10 Ohm and will create a current of maybe 14 Volt / 10 ohm = 1.4 Ampere depending on the sun.
That means it will heat up with a power of 1.4 Ampere * 14 Volt = 20 Watt

Sebastian Köln wrote:Risk of damage: none.

The plate will get a little bit warm.
The voltage is harmless, if you touch both terminals with your tongue it will tickle, but otherwise you will not feel anything.
Solar panels tolerate short circuits.

Would connecting the wires from the panel to opposite ends of say, a wrench, constitute a short circuit? Would the wrench get hot? Would there be any safety issues?

James Alun wrote:
What I would do is to get an old car battery, some 12v leds and a charge controller and use the solar panel to light the shed and charge your phone....

I do have a charge controller and another panel that I will set up along those lines.  What I'm looking for here is to find out, figure out, if it is possible to generate and store heat, not electricity, directly from the panel without any other electrical components at all. I understand that it will fluctuate drastically and not be available at all when the sun isn't shining.

As long as you don't connect a battery, it is fool-proof.
Yes, connecting the wires is a short circuit, but the panel can only produce a limited amount of current. A short circuit means very little voltage drop and since power is the product of voltage drop and current, it will not generate much heat.

I suspect two hot plates in parallel will be a pretty good match for the solar panel.

Josh Hoffman wrote:Mark, I would either source an inverter or a DC hot plate.

I just looked at my single burner and it is 1100 watts. 1100 watts / 120 volts = 9.17 amps. The resistance of the hot plate is 120 volts / 9.17 amps = 13 ohms. If I work back to amps on 12 volts / 13 ohms = 0.92 amps.

I don't think it'll heat up.

If the label said it was powered by 12 volts AC then it would because at 1100 watts, it would be the same amperage for both ac and dc. But that is 91.7 amps and would not be a real world scenario for a hot plate at 12 vac or vdc and 1100 watts.

I think the 100 watt panels produce 18 volts at optimum conditions. The resistance of the device is the shortfall here even direct wired to the panel at optimum conditions.

Mark, proof of concept will work in this scenario because we are talking about a simple resistor circuit. No motors or other components that are designed for alternating current. ***That I know of, I have never taken one apart to see***

To work somewhat reliably, you need 120 volts DC from your panels. If optimum range is 18 vdc for a 100 watt panel, you'd need 667 watts of solar to power the hot plate at the high level where it draws the most.

Your panels will not usually work at optimum range so you could be set on "high" and only getting 80 volts which may be "medium" on the dial. This is why a reliable 120 vac or 12vdc is the standard and they make appliances to match that.

Mark Reed wrote:So, voltage is key?

Yes, because the current that will go through the heating element is dependent on the voltage:

I = V/R for direct current

Still assuming that there is nothing in between the element and the panel. Other commenters mentioned that assuming 10 Ohms resistance of the plate and the low voltage of the panel, it will produce low amperage that will produce a little heat only. That's why I said "MAYBE" it will heat. But it may be still sufficient for your wax melting application and that's why I even put the comment. Please try and share the observation here.
If you had more panels of the same type you could also connect them in series to produce higher voltage.
Also, some 100 W panels produce more than 18 volts. I'm using 24 V 100 W panels to directly charge my laptops, phones and lights.

Thanks again for the input and advise. it may be that the idea I have is so totally goofy that it will never work. Or maybe they technology for it hasn't been invented yet, especially concerning the resistance heating component. I don't so much want to use the hot plate, as a hot plate. What I want to find out is if it's possible to just heat "something" with the direct output of a panel, regardless of what the output is and regardless of how it changes during the day.

Basically, I want a solar panel to do the same thing as my south facing windows. Solar energy comes in when the sun shines and heats the house and its contents, I store some of it in the thermal mass of the chimney and tile floor. I can't add more windows but what if a resistance heating device imbedded in say a tile floor or brick wall could directly be heated on sunny days, in effect adding the extra warming of more windows. Maybe it will be less efficient that the windows, maybe more so.

Maybe it's not worth the effort but I want to find out. The house is small, well insulated and bermed  on the north and west. Even just a little extra heat stored during a cold but sunny day might be worthwhile. If the hot plate, or some heat tape, or anything else can hot enough to be unpleasant to touch, it might be worth it. It could be buried in the tile floor or anywhere else to save the heat for when the sun goes down.

I hope to get a chance in the coming week to find out with the hot plate.

I've started researching more on how to use a solar panel the way it is intended. I guess I don't need an inverter if I only use 12V devices and I have a controller that came with the panels. I do need a battery though and this is the best one on the market in my estimation, because it is the one I can afford. Battery

Unless someone knows of a still cheaper one that will work.

Mark Reed wrote:Thanks again for the input and advise. it may be that the idea I have is so totally goofy that it will never work. Or maybe they technology for it hasn't been invented yet, especially concerning the resistance heating component. I don't so much want to use the hot plate, as a hot plate. What I want to find out is if it's possible to just heat "something" with the direct output of a panel, regardless of what the output is and regardless of how it changes during the day.

Basically, I want a solar panel to do the same thing as my south facing windows. Solar energy comes in when the sun shines and heats the house and its contents, I store some of it in the thermal mass of the chimney and tile floor. I can't add more windows but what if a resistance heating device imbedded in say a tile floor or brick wall could directly be heated on sunny days, in effect adding the extra warming of more windows. Maybe it will be less efficient that the windows, maybe more so.

Maybe it's not worth the effort but I want to find out. The house is small, well insulated and bermed  on the north and west. Even just a little extra heat stored during a cold but sunny day might be worthwhile. If the hot plate, or some heat tape, or anything else can hot enough to be unpleasant to touch, it might be worth it. It could be buried in the tile floor or anywhere else to save the heat for when the sun goes down.

I hope to get a chance in the coming week to find out with the hot plate.

I've started researching more on how to use a solar panel the way it is intended. I guess I don't need an inverter if I only use 12V devices and I have a controller that came with the panels. I do need a battery though and this is the best one on the market in my estimation, because it is the one I can afford. Battery

Unless someone knows of a still cheaper one that will work.

Mark, all the info I provided is theoretical. I will be interested to hear how it works out in practice!

Oohhhh. I thought this was about melting beeswax.

I think you might want to look at the efficiency of water solar collecter panels vs solar (p/v) panels. While the heating side of electric is much more efficient than hydronic, it doesn't really matter when the solar collector is potentially over twice as efficient as an electric solar panel.

A potentially far more simple system would be a Trombe wall. You could paint your existing wall black and install a glass wall maybe half a foot away from it. The glass wall stops the outside air moving and cooling off the wall. You put up some blinds to reduce heat loss at night and excessive heat gain in the summer.

Sorry if it sounds like I'm pushing you away from using solar pv. My experience of playing with a 100w panel has shown me that I get a lot less power to play with than I hoped.

James Alun wrote:Oohhhh. I thought this was about melting beeswax.

Sorry if it sounds like I'm pushing you away from using solar pv.

Not at all, I love hearing different ideas and opinions. I had never heard of a Trombe wall before, I'm going to look into a bit but expect it wouldn't be appropriate due to how the house is built and situated, and the cost. Collecting the heat in water has the problem of, how do I get the heat inside the house, that is, with little to no expense. Wit the panel I can just drill a small hole in the wall and insert the wire.

James Alun wrote:My experience of playing with a 100w panel has shown me that I get a lot less power to play with than I hoped.

Yea, I figured I'm not going to collect a lot of energy from a 100 w panel, but if just one captures just a little bit of heat, that would be interesting. Seems like the panels are about the cheapest part of the system. If they were the only part I really need, I can just get more of them. And I think, other than degrading with age or getting hit by hail there isn't much that can go wrong with them.

Mark Reed wrote:I guess I don't need an inverter if I only use 12V devices and I have a controller that came with the panels. I do need a battery though and this is the best one on the market in my estimation, because it is the one I can afford.

I'm using 100 W 24 V panels to directly charge our 4 laptops, 2 phones, camera, reachargeable lights. No charge controllers, no batteries and no inverters.
The system consists of:
4 Renogy 100 W 24 V panels
2 "4 to 1 splitters" for MC4 connectors so I can connect all 4 panels in parallel
50 feet 10 gauge cable with MC4 connectors
"MC4 to double car outlet" connector
universal USB charger 12/24 V with 6 USB ports
dedicated laptop 12/24 V charger with USB-C output for my work laptop
universal laptop 12/24 V charger with various connectors

It took me while to find which laptop chargers will work with my computers. I had to return some - as they did not work at all or started overheating my computer.
In theory 1 panel would charge one laptop on a sunny summer day, but because the panel is not following the sun so I got the second one and when the rainy season started I have realized I need 2 more. On a very dark overcast day, I would need probably 6 panels total to charge one laptop. Today was very foggy day and I did not have problems to charge laptop and at the same time all phones and lights.
When I was looking for solution for direct panel to device charging I could not find anything. - only Chinese integrated controller/battery/inverter combos, so it made me angry - why would I want to charge the battery first and then convert to 110 V and then again my power supply would reconvert to DC again. Especially if I already have a batteries in my devices.

I also have direct to panel fridge. I like it this way and these panels satisfy all my basic electrical needs as I'm a minimalist. To run well pump or power tools I use a generator.
The only battery I use is for the solar freezer that I still need to set up.

So I'm hearing that you could do the experiment without harming your panel(s). What are the odds of harming the hot plate? How expendable is it? I'd be tempted to try it, unless the cost of damaged components was more than I was willing to pay. The consensus seems to be that it won't work, or won't work very well. Even if it doesn't, you gain the knowledge.

You've reminded me of a few other projects I've stumbled onto online. A guy built an electric 12 volt heater for instant heat because his engine warmed up too slowly. It used more power than the ones you plug into the cigarette lighter, he powered it straight from the battery. I wouldn't suggest that for a beginner. (I wouldn't suggest that for me.) There are a lot of ways for that project to go wrong. So why do I bring it up? His heating element was designed for 120v AC, and salvaged for free. It came from a toaster. He ran it on either 12v or 24v DC. (He doubled up the voltage for the fan, it's unclear which voltage the element got.) His website is gone, but I found it on the wayback machine. I wouldn't count on him still answering the listed email, but I guess you could try.

Sean Dembrosky of edible acres bought a cheap water heater element designed for AC and hooked it direct to a solar panel. It produced heat, in fact it was around 200°F while surrounded by 4°F air. Keep in mind, it was only heating itself, very little mass. For storage, you'll want to heat a mass, so expect it to reach a much lower temp, most likely. I'd note: Those are designed to be water cooled. Without submersion in water, it's hard to say how long it would last. So design to put it in water, or at least design for it to be easy to replace. If you don't know how to make a sealed hot water system safe, remember it doesn't have to be sealed. (Especially as you're probably going to omit the thermostat.) For more on that see anything Paul's ever said or written that contains "Boom-Squish"™. Sean's a youtuber, so I'll embed the relevant video. He starts talking about this specific project around 6:50.



He mentions another channel or 2 that he learned from. I don't know them, but he found them helpful, so watch for that if you want to go a little more in depth.

Wow, that is very interesting. This other video seems to show almost exactly what I'm thinking about.

In my situation a problem I thought of is I don't need a lot of extra heat while the sun is shining because on a cold sunny day the windows keep the house pretty comfortable on their own. Some kind of insulated container might solve that problem. Maybe a double wall metal container with dead air space so the sand or whatever gets even hotter while the sun is shining. Then at night, take the insulated lid off, or maybe some type of heat-resistant blanket that could be put on and off as required. Ideally it might be buried deep in a thermal mass, where the core is heated to quite high temperature, but it takes a long time for the surface to get warm, and to cool off.

I liked how he did the math too; I'm a bit light on that as it applies to electricity and although I want things to be as simple as possible a bit of understanding exactly what is going on is in order.

This could have all sorts of applications, heating an outdoor pet bed, keeping a garden pond or one where you're raising fish just a few degrees warmer, heating the ground under a greenhouse or cold frame. Lots of things where a constant air temperature isn't required, or where you are willing to tolerate the fluctuations. If it resulted in just a few percentage points reduction in heating costs, or firewood use, it might be worth it.

The hot plate cost around five dollars at a thrift store and is now covered in spilled beeswax, if it blows up or catches fire, no big deal but I will do my experiment outside.

Mark Reed wrote:[...] keeping a garden pond or one where you're raising fish just a few degrees warmer,

You are going to need a LOT more power for that.
Solar radiation is 1000 watt per square meter. You need 5 times the surface area of solar panels than pond area to match just the direct sunshine (assuming your panels are 20% efficient).

Cristobal Cristo, there are cheap USB-C PD (power delivery) chargers that take up to 35V on aliexpress. They work just fine with my new laptop and phone.
The problem is that my battery setup is 48V nominal, up to 56V when charging, so it needs an additional DC-DC converter to run them.

My next setup will be 24V, just so it works with most of the readily available electronics. Then all you really need is a 8 Lithium Iron Phosphate cells and a BMS.

It depends on the scale and intent. My little pond for example is deep enough that it doesn't freeze to the bottom, and it has a pump that keeps the water moving. The little stream portion only very rarely freezes to the point that the animals can't drink. Just a very slight boost might be all it needs to eliminate me having to occasionally break up the ice. I'm not expecting to keep something a nice constant warm temperature but, in my climate, there are situations where just a degree or two, maybe even a fraction of a degree might make a big difference. The pond and especially my cold frames are what come to mind right now.

With the pond the depth and circulation by the pump is what already keeps the surface open, most of the time. Without that circulation it would take a lot of solar to keep it open. If I want to power the pond pump with solar, which I would ultimately like to do, I know that is going to take a lot more as well. I'll need all the equipment for that. I imagine though, keeping the pump working would take less than heating the pond without it.

Actually, the energy the pump adds to the moving water will convert to heat, so you get two benefits at once. The electrical losses in the pump will mostly also go the water as I assume it is build to be cooled by the water itself.