First, what
@sunshine_eggo said. On sunny days, it's hard to beat the effects of sun beating down on a big, preferably dark-colored, slab of stone, concrete, or even large containers of water, for several hours of the day. They absorb that heat and then release it somewhat slowly as the building cools. The problems are: 1) You need a decent amount of that "thermal mass" to absorb that energy (say, a big tile floor over a concrete bed, with lots of insulation under the concrete), and 2) the sun has this nasty tendency to get covered by clouds during winter days, which means much less (sometimes none) of that heating occurs during those days, so there's no stored heat to be released overnight. But those plants are still gonna need to stay warm, even when there's almost no sun for several days, so now what?
Also, most sheds aren't built with walls of glass u less originally designed and built that way, so that makes the whole "absorb thermal mass" idea a bit impractical without major renovations to the shed itself. It takes a lot of energy to generate heat, and that heat is needed at the very times the sun is least available (winter, and night). So, where does that leave you? We can't know for certain without a lot more information, but understand that heating is just about the toughest thing one can ask a solar electricity system to do. Having tried a number of things, here's what I'd suggest:
Before you do anything, in any situation that requires heat in a building, think about air sealing and insulating. A typical shed has no insulation and leaks air like a sieve, so you're going to be losing much of whatever precious heat you generate, from a y source. Much less expensive to hold onto that heat once it's generated or moved into the space, but air sealing and insulating do cost money upfront. In the long run (years and years) you save money, but is it worth it for a few plants? Not our call to make.
Next, as a general rule, try to capture and move heat to where you need it, rather than attempt to generate it. This basically means using a heat pump in most situations, but again, we're talking a few plants in a shed. Worth it? Not our call.
Bottom line: Unless you want to jump into this with a fair amount of money and time, basically as a hobby, forget doing it with solar for a few plants in a shed. You're not going to find some gadget that will do this, and it's not going to be quick, cheap, or easy. If you want that, and if you have electricity already run to the shed, it's hard to beat an oil-filled, radiator style electric heater, which will give you quick and easy, but will cost you in terms of the energy used. If electricity is not available, you might try a small propane heater. But do be very, very careful about fire safety and carbon monoxide, whatever heat source you choose!
Generating heat is, and will continue to be, one of the most expensive uses of energy. Solar can be a big help in that effort, but it's almost never without a fair amount of upfront time and money.
