Recommendations for Low-Power (DC) Heater

I'd like to dip my toes into the field of photovoltaics (PV).
I'd like to start with a small PV panel that can be folded up and put away when moving; nothing major like replacing a roof or a whole grid that needs to be wired into the power utility's system.

I was thinking about connecting it to a battery and then (controlled by a timer) to a very low-power heater. It would be nice to have the heater in the garage. The patio is above the garage, and there are several plants growing on the patio. IF I could place a small heater directly below these plants, hopefully the rising heat would keep the plants (esp. the roots) a bit warmer through the winter. And being on a timer, I could set the timer to come on during the coldest parts of the nights, say, 3 AM - 6 AM.

Being paranoid about safety, and since nobody would be watching it, it should not come with a risk of fire.

Can anybody here suggest an extremely low power heater, ideally, DC powered?

Unfortunately it seems unlikely to me that you'd get a heater of low enough power usage to run from a small solar panel and appropriately sized battery that would produce enough heat to warm through the roof of your garage to the patio above in 3 hours use a night.

How small do you mean when you say a "small folding panel"? They're usually rated in watts, a backpack sized folding panel might be 20 or 30 watts, while a bigger luggable folding panel might be 60 or 100 watts. Then the battery to store the energy produced needs to be sized appropriately.

If your main concern is warming plants, USB heat mats for starting seedlings or for small animal use placed on the patio itself directly under the plants might do the job, provided they're designed to be waterproof.

Heating is generally quite power hungry and would probably need more than a small foldable panel to provide enough charge to your battery bank, even if you live somewhere with sunny winters. My experience is that the 20 and 30 watt panels are good enough even in winter for charging lights, phones, or small battery banks, but probably won't produce enough charge to run a heater bigger than a hand warmer or small heating pad.

Of course, the bigger the solar panel and battery, the more options you have.

Your best bet is to research sand battery heater.

Basically a bucket of sand with an electrical heater element that heats during the day from the solar panel and releases the heat overnight.

I don’t know what scale you need for your needs.

I hadn't heard of sand batteries! What a great idea for heat storage.

Though looking at the DIY videos on this, they're using big solar panels. One I watched used two 300 watt panels. It got a bucket of sand up to an excellent heat, but he didn't show much of how long that heat was retained - I got the impression it cooled within a few hours. I'm not sure the smaller panel that David wanted to start with would give much of an effect using a sand battery. An excellent idea, but maybe one that works better at a larger scale?

Well you could look at a fridge or freezer as a (relatively) low wattage heater that incidentally keeps a small compartment cool as a bonus...however as already noted the amount of heat produced isn't going to do much inside a large ambient room. Might be better off looking into solar thermal collection and careful airflow regulation?

A lot of people don't understand the capabilities and limitations of solar, because they don't have a basic understanding of what a Watt is, and what things consume how many watts.  In it's most basic form, a Watt is one Ampere X one Volt.  Does anyone here understand why I'm capitalizing the words Watt, Amp, and Volt?

How many Watts is consumed over time is recorded as the Wh, or more likely on your electrical bill a kWh.  That means the number of Watts X the number of hours running.  So, a 100W light bulb, left on for 5 hours will consume 500Wh, or 0.5kWh.  Why is the W capitalized but the hour isn't?

So, understanding the consumption of various items is the key to understanding whether or not they can be powered by solar.  If you don't begin here, you will fail.

Let's look at a few common items and see how many watts they consume.  Your cellphone is likely charging with 1A at 5V.  What works out to be 5W.  A little 12V TV might consume 5A at 12V, or about 60Watts.  A 100W light bulb, of course is consuming 100W.

Once you get into bigger items that either heat or cool, the energy requirements leap greatly.  You might find a little tabletop heater that consumes 250W; a big floor model might be 1500W.   A little freezer might consume just 100W, but it is on maybe 25% of the entire day, so consumes 100W X 6 hours of run time = 600Wh, or 0.6kWh.  A little frig might consume about the same.  My big 18cuft kitchen frig consumes ~1.2kWh over a 24 hour period though at any moment, it's never drawing more than 200W.

So, this is why a little foldable solar panel is not going to be successful.  Anything making maybe 20-30W per hour is not going to make the hundreds or thousands of Wh per day needed to run a heater or cooler.  But, solar can power each and every one of these items mentioned, it's just that the number of Watts of solar panels needs to be in the thousands to make electricity in the range of thousands of Watthours.

So, for this project to be successful, you need to start thinking in terms of Wh or kWh, the number of sunhours your location gets, and what the unit you are trying to power is going to consume.  Then you can start to plan on building a system that is going to work for you.

I want to build on both Ben and Michaels answers.

A human at rest puts out about 100w of heat. That should give you an idea of your scale problem.

However, a heatpump (aka fridges and a/c units) has a Coefficient Of Performance between 3-5. This means that for every 100w of electricity put into the heatpump, 300-500w of heat can be moved from one place to another. In the case of the fridge, the heatpump moves the heat from inside the fridge to outside the fridge and for the a/c from inside the house to outside the house.

I’m curious to see what it would look like to hook an old car a/c unit up to a starter motor on a solar panel, 12V battery, and charge controller with a timer to turn it on at night.

Good comments above.

12V automotive bulbs are an inexpensive heat source. Another option is 12V RV water heater elements.

Generally, using PV panels and batteries to create heat is extremely inefficient. Running small fans, small pumps, and control circuits is more practical, in order to move direct solar heat where you need it.

Look at RV water tank heater pads.

They often include temperature controls to only turn on when it gets close to freezing.

I use a small electric heating pad (the kind you'd use for a sore stomach for instance) as a very local heat source when I'm at home and don't want to turn the thermostat up.

I've tested that I can run that pad for at least 24 hours on low on a single charge of my camping backup battery (ROCKSOLAR Portable Power Station 100W Adventurer Plus) , and I can charge that battery on a sunny day with a 60 W solar panel (but haven't tested in the deep of winter yet). With a good sleeping bag, it can keep me toasty all night, and it will last as long as the sun does.

Jean-Martin Fortier who writes about market gardening year long in northern areas suggests, on a larger scale, a double layer of heat protection for plants: a polytunnel for a first layer, and hermetically sealed row covers directly on plants, with hot air being blown just in the innermost layer. He keeps the heat at the bare minimum needed to prevent cold-resistant plants from being damaged, but nothing more.

At a more makeshift level, I assume that a similar setup to my camping gear might work decently for plants: use a USB seed warming pad underneath, and some kind of row cover material to keep the heat close to the plants (their "sleeping bags"). Perhaps a small fan to keep the heat moving a bit in that inner layer.  You could add warmer layers at night in case of extreme cold (but would need to remove them so the plants get some light during the day).