Have you looked at soil heating cables? For example: https://www.amazon.com/JumpStart-Soil-Heating-Cable-48-Feet/dp/B00P218EFA

Shop around for best price.

Then there is the good old 'manure bed' for soil heating. https://www.groworganic.com/organic-gardening/articles/use-manure-to-turn-a-cold-frame-into-a-hotbed

One thing to keep in mind with peppers is that season-extending in the Fall is far more effective with them than is spring extension. They require that mid-summer heat to kick into production so early spring extension gains you some plant growth but little in terms of production.

Dave

A string of lights is an excellent idea, especially if you can easily remove bulbs. That way, you can actually do some designed-in temperature regulation. If it's too hot, just pull some bulbs. I think a 100-count string of incandescent lights is worth about 40 watts. Heating cables run something like 5-40 watts/foot, so if you need lots of heat, that may be the only solution.

Lights are a good idea - but larger scale my guess is they use more electricity than a heater because they are designed for lights. That said if you use other bulbs that can help growth it could be a double whammy. Never knew about the heating cable thanks! Going to do some math know and figure out heat produced vs a matt and order one thanks!

I looked at a 48' heating cable and it uses 140 watts, the 48" x 20" heating matt I use takes 107 watts. I think this means the cable will produce about 30% more heat do you agree?

Heating mats are not really designed for use on the ground. Heating cables are designed for that very purpose. The heat from a mat is diffused over the area of the mat. The cables produce heat in concentrations along the length of the cable. These are engineered for 2 different purposes. If you wish to use one in place of the purpose of other, keep that in mind, and also realize that you may be voiding any warranty and insurance claim.

I did the math and $ for $ both produce the same watts per dollar. In other words if I buy a 48 x 20 matt or a 48" cable I get the same "value". My next question is for semi-permanent use which is better? I want to put it in a large cold frame I will build around pepper plants. It will be in use spring summer and fall - I will likely leave it running all summer - which is the only advantage I can see for the cable - the thermostat will kick it off if it gets to hot. That said is that likely for pepper? What is to hot?

You can't really compare soil heating cables and heating mats in terms of heat production, effectiveness, or per watt output. As rgreen said they work entirely differently so any attempt to compare is very misleading. The temp in cables is preset - mats are adjustable.

Since you want to use it in a coldframe then heating cable is the only way to go. The cables are the only thing intended for permanent use. But I can't conceive of any reason why running it all summer would be good and several reasons why that could be detrimental to plants.

I think you must be confusing soil heat and air heat. Soil heat - in either form - is primarily used for germination only and can be death to plant roots once the young plants are established. Once seedlings become plants it is air temps that are vital to production and soil heat becomes something to be eliminated as much as possible. Pepper plantss don't want hot soil, they want warm summer air.

Dave

a 48" soil warming cable doesn't cover as much area as a 20x48 seedling heat mat. the cable is used in a zigzag pattern.

shocks 48' foot cable! My bad at 3" apart - I will make it closer to 6" because it is t heat not germinate it will cover :

2 rows 3" then a cable, then 6" then a cable then 6" then 3" to end of bed. 18" wide and I can go 24' so that is 36 sf, or at 3" 12sf

Don, looking back at an earlier comment of yours, watts are watts, whether they come out as heat or light. If you block the light so it can't get out, that blocked light will produce heat in the shade that blocks it. So you wrap a light in blackpaper or aluminum foil, and it's all going to be turned into heat. You package a string of lights between black paper, and it's all going to come out as heat. True, that if you don't block the light, and the light travels away, and lights something up, that's watts that are traveling away, and not turning into heat where you want it.

If you don't let the light escape, 40 watts of light will produce 40 watts of heat. Pretty simple.

That being said, strings of lights are not suited to being buried in moist soil. So if that's what you want to do with them, it won't work. But as a plain old (low capacity) heating mat, they'll work fine.

Thanks! Was not sure about watts vs light vs heat

A good way to think about it is sunlight. Sunlight hits the ground just as watts of light. About 1400 watts/square meter, if the sun is straight overhead. The ground absorbs some of that light, and heats up. If half the light is absorbed by the ground, you've got 700 watts of heat in a square meter of ground. If all the sunlight is absorbed by the ground, it gets a lot hotter.

Okay, I don't want to nit pick, but I really think some clarification would help here...

I don't think it was intended, but a reader may get the impression that light and heat are almost interchangeable. They are not, in any way, the same thing. They are each forms of energy, but watts converted to heat are not equal, or even equivalent to watts converted to light. In fact, neither can be measured in watts. Light (especially visible light) is measured in lumens, and heat is measured in calories. When delivering energy to a source that produces either, or both, light and heat, you can make conversions based upon the efficiency and output of the source, but using watts to measure light and heat, then using a shorthand to say that light heats up an area of ground based upon a measure in watts is wholly a misapplication of terms.

The radiation of light can cause excitement of molecules (soil), but the vast amount of heat from the sun is in the form of radiation not described with the term 'light'. Brightness of light does not equate to warmer radiation of heat. You can have an extremely bright light (as measured by lumens) with little heat (as measured by calories) at all. And vice versa... you can have a light source that produces a relatively dim light (again, as measured by lumens,) and a vast amount of heat (yep, measured by calories.) To understand the difference, think of a 60 watt incandescent light bulb, a fluorescent light bulb that produces the equivalent amount of lumens as a 60 watt incandescent bulb, and an equivalent LED bulb. Because of the manner in which these operate and convert watts of electricity to light, they each produce a certain number of lumens, and a certain amount of calories/heat.

The incandescent produces lots of radiation in a spectrum that is not visible light, nor should it be confused with what we call 'light'. It is not measured in lumens. On the other hand, florescents and LED's each produce amounts of light and radiated heat relative to their efficiency in converting energy to radiation. In short, the most efficient source (in this case, LED's) will produce more lumens of effective light than it will heat.

So, when thinking about sunlight, one can think in terms of brightness, but using a term like an equivalency of watts can open a door to a lot of misconception. I'm sorry if I am confusing the issue in any way, but really, this needed to be said.

rgreen48, I don't want to nitpick, but we should be precise. A watt is a unit of power Power applied over time is energy. A 100 watt light bub produces 100 watt-hours of energy every hour, but we just shorten it to "watts" for convenience. Light and heat are, in fact, interchangeable. They both are forms of energy, and are measured the same way. Calories? Just another measure of energy. Think centimeters and inches. I could measure light in calories, if I wanted to. Lumens? Just a measure of the amount of *visible* light. Lumens are of some interest when if comes to how much of the light coming out of a bulb a plant can use, but that's not what we're talking about here.

Very definitely, I can turn all the energy of light into energy of heat, by just absorbing that light. That's called "thermalization". A watt is a watt. I can convert heat watts to light watts (that's what a steam generator attached to a light bulb does), and I can convert light watts to heat watts by absorbing it. No sweat.

For the purpose of this conversation, the light produced by a light bulb (in watt-hours) can be absorbed, and it turns in to heat -- the same watt-hours. My 100 watt light bulb, if stuffed in a bag, will heat up that bag *precisely* as much as a 100 watt piece of heat tape would. The light energy that comes out of the bulb - visible light as well as infrared light, and a tiny bit of ultraviolet light, totals up to be 100 watt-hours. Very true that not all of that 100 watts is visible light, but again, that's not what we're talking about here.

I'm at a loss for words. I'll just add that personally, I don't believe we've yet achieved perpetual motion.

To add to what I said about the wattage of sunlight, if I had a black bag a meter across, with sunlight shining on it and being completely absorbed in it, the sunlight absorbed in the bag would heat it up as much as 13 100W light bulbs would. The sun is bright, and it does a good job making heat!

"The sun is bright, and it does a good job making heat!"

Yes, the sun is bright, and it does do a good job of radiating heat. However lol, sunlight is NOT heat, nor can it be converted to heat in a one to one ratio. One cannot speak of heat and light in terms of watts as if they are interchangeable. That is what you did, and I'm sorry dan, it's just wrong...

"If you block the light so it can't get out, that blocked light will
produce heat in the shade that blocks it. So you wrap a light in
blackpaper or aluminum foil, and it's all going to be turned into heat."

Like I said, I really didn't want to nit pick, but you now have doubled down on that mistaken notion.

After your first mistaken comment, you then said... "Sunlight hits the ground just as watts of light."

That statement is so wrong that it has no meaning. There is no such thing as 'watts of light.' There can be an equivalence spoken of in broad terms, but even the equivalency needs to be defined by the efficiency of the conversion!

Okay... I really do not wish to argue. I'll try to leave it there.

Sorry, wrong. Light, shining for a certain length of time, can be converted to heat in a one-to-one ratio. That's a fact. Ask a physicist. Yes indeed, sunlight is not heat. Over the time that it's shining, it's energy, but heat is matter with energy. And yes, you measure energy in watt-hours, or joules or ergs, or calories, and light can be measured the same.

The "solar constant", which is the magic number about how much energy sunlight has, is measured in watts-per-square meter. Yep, watts (or kilowatts) hitting a square meter. You can look it up here.

Dan, you are conflating issues. The fact that watts can be used to measure the energy irradiated from the sun has NOTHING to do with light being equal to heat! Nothing at all! Watts is a measure of energy output, but the effects of different types of energy is completely different! In fact, the true measure of your solar constant is not watts, but watts per sq. meter. This is not a linear measurement of the energy delivered by the sun, nor can it be used to determine how much light or heat is available for engineering calculations without a whole lot of ifs, whens, and efficiency conditions are met.

The solar constant, for example, is a measure of all solar radiation, not just light and heat. Much of it cannot be converted for any use... light or heat. To use that as some defense of the irrational conclusion that because light and heat are both forms of energy, and are thus interchangeable in engineering calculations of how many watts of electricity are needed to create light, heat, or both, is absurd.

Can light be converted to heat? Yes... but not very efficiently (or you'd be rich and have already put the petrochemical companies out of business.)

Can heat be converted to light? Yes. But again, not very efficiently.

If the conversions were 1 to 1, then you'd be able to convert energy into action, and action immediately back into energy. This is the concept of perpetual motion and it does not work that way. Let's not even get into atrophy...

Sorry rgreen48. No conflating going on here. Light is energy. Heat is energy. No one is saying that one is equal to the other. What I'm saying is that you can convert one into the other readily, and the amount of each is measured in the same way. Let's use watts x time as a measure of that energy. Works for both light and heat.

Solar radiation is not just light and heat, you say? Maybe some fairie dust mixed in? It's energy, and it is *all* light (unless you're outside the atmosphere, and you're getting rained on with solar protons). It is ultraviolet light, visible light, and infrared light. A tiny bit of x-rays and radio waves. The solar constant is all light. Many flavors of light. When that light shines on something, and gets absorbed, it heats it up. You say that much of that radiation cannot be converted for any use, light or heat? Nope. It's *all* light. And if it is all absorbed, it is all converted into heat. That which is reflected is not absorbed and doesn't heat things up. That may or may not be of use to you.

By the way, "heat" isn't radiation. Heat doesn't shine. Heat is a property of a material. If you put your hand next to something hot, you feel it because that something hot is shining infrared light on your hand, heating up your hand, or it is heating up the air by conduction, which flows over and hits your hand.

Now, I'm smelling a problem here with the word "light". I fear that to you, "light" is just what our eyes can detect. Physically, that's incorrect. Our eyes are cable of only seeing a certain flavor of light. When it is absorbed by material, an ultraviolet or infrared photon works just as well as a visible photon in heating things up. About 30% of sunlight is visible light. About 70% of it is not.

Converting energy into action and action into energy isn't called perpetual motion. It's called conservation of energy. If light doesn't hit something and get absorbed, it stays as light, perpetually. If action doesn't encounter any friction (which converts energy of motion into heat) it stays as motion, perpetually.

Can light be converted to heat? You bet. With 100% efficiency. Every solar photon that lands on your house that is absorbed is heating up your house, and the petrochemical companies can't do anything about it! That being said, you have to paint your house pretty black to absorb every photon of light.

To reiterate. A watt is a watt. I can take a watt of light and have it absorbed, wherein it is converted into a watt of heat. That's just a fact. A 100 watt light bulb, stuck in a bag, produces 100 watts of heat. Conservation of energy.

We can go on and on here, but I recommend you do some reading about thermal physics or talk to a thermal engineer who you trust.

P.S. One calorie is the amount of energy you get out of 4.184 watts in one second. Just another unit of energy.

Dan, sorry, but I quit reading your comment when you classified all energy as forms of light.

Listen... light consists of BOTH... waves AND particles. Those particles are specific ONLY to energy expressed in the form of light. Those particles are calls PHOTONS. The word comes from the Greek word for... you guessed it... LIGHT.

Heat, X-rays, Gamma rays, radio waves, and every other type of electromagnetic form of energy there is does not contain, nor emit photons. Are they all forms of energy? Yes, but so is matter. My goodness, it's just plain ignorant to say that these conversions are made readily! In some cases they may be made with a piece of paper and a pencil, or even a calculator, but this thread is about the reality of practical engineering. You're a smart fellow Dan, but this time you've gone far off the deep end. If you are sincere, then you are misleading yourself, and everyone who may read this thread.

Unfortunately, at this point, you're just digging a deeper hole. The only reason I've continued to offer specifics is for those who might read what you've written and misunderstand the entirety of wave and particle physics... as well as the engineering questions in the OP.

I can't imagine why the U.S. has fallen so far behind in education and engineering.

And with the speed of heat... oh, wait, not a thing... I'm out.

I am an engineer (civil) and this is over my head!

Yes, the hole is just getting deeper. Please, rgreen48, do some reading. I very much appreciate your gardening expertise, but you're in way over your head here.

But I can't help but respond to this one.

"Heat, X-rays, Gamma rays, radio waves, and every other type of
electromagnetic form of energy there is does not contain, nor emit
photons"

Some flat-out wrongness here. X-rays and gamma rays sure don't emit photons. They ARE photons. Electromagnetic energy also includes electric and magnetic fields, and those are indeed not photons. Heat is NOT an "electromagnetic form of energy". It is kinetic energy in a substance (whether solid, liquid, or gas). As in, constituent particles moving or bouncing around. The warmer it is, the faster they move. That substance does emit photons, and emission of those photons help cool it. In fact, in a vacuum, that's the only way the substance can cool. You are emitting infrared photons, because your atoms are bouncing around, such that you are warm. An ice cube is emitting infrared photons too, but not as many. "Warm" here means anything with a temperature above absolute zero.

I think you need to know a lot more before you can criticize my science education.

LOL... Yep... very good. So, first... I admit that I wrote a few inaccuracies into my comment.

See how easy that was?

Second... I want to thank you for agreeing with me that energy in the form of heat is much different than energy in the form of light. That's why when engineering producers of heat, and producers of light, efficiency of source is so important, and why watts of light do not equate to watts of heat.

Easy peazy.

I'll send you a PM, rgreen48. I think it's time we relieve the rest of the forum from all this.

Some greenhouses use radiant heat in the form of heated water in pex or epdm tubing or aluminum fins under their benches to heat the rooting zone. It isn't cheap to heat water but at least it holds it''s heat.