I suppose it is possible to just hack the software. If solar is connected to batteries which are internet capable I presume it’s possible to run an overload. Even if not connected to the internet, a friendly technician or installer could rig it somehow.
There was the story, made a Netflix show about it, of an Egyptian Jewish Israeli who infiltrated the Syrian government. He generously donated Cyprus trees to help the Syrian soldiers stay in the shade. The IDF then knew exactly where to target.
Excellent show. It stars Sacha Baron Cohen who portrays Eli Cohen.
Titled, “The Spy.”
If you want to get a bit more technical, at least based on residential solar technology in the U.S....
The inverters are connected to the internet. Not the solar panels. Nor the batteries. Thus, they're the part that'd be hacked into from the internet. The inverters are also the charge controllers. Thus, hacking into them can cause overcharging the batteries.
My inverters continuously read the volts and amps coming from the batteries to determine how charged they are. Thus, my inverters slow down the charging when the batteries near full charge. (Very much like the battery charger I use to charge my lawnmower battery.) Plus, the solar batteries have software on them (BMS = Battery Management System) to help throttle down the charging and discharging if necessary. In order to hack into my solar system to cause a fire, you'd have to hack into the inverters, trick them into charging more than the batteries can handle, and hope the batteries' BMS doesn't kick in to protect the batteries.
Of course, the terrorists may have had cheaper batteries with no BMS to protect them. Or Israel was able to intercept them physically during packaging or transportation like they seemed to do with the pagers.