For a question to be on-topic, it would have to be about design considerations, underlying physics or the proper way to do the electronics mounting/assembly (in terms of electrical safety, EMC, wiring, soldering etc). What makes such questions answerable and on-topic might be the amount of details and research put into it, rather than the topic itself.
For example "what solar panel and battery capacity should I pick" might be on-topic if the poster can present numbers regarding peak and average power consumption, that is some manner of simple, technical specification. Or if they are asking how to wire it after posting photos and simple schematics, datasheets etc.
However the same question would be off-topic if not enough details are provided (close as too broad/unclear/use of electronic products) and it would also be off-topic if it is asking which specific brand that is best (shopping recommendations).
Most of these question are more suitable for Home Improvement (DIY). Please note what's on-topic there before recommending someone to post there instead. For example they don't welcome shopping recommendations either.