WHATS THE DIFFERENCE BETWEEN A DIY SOLAR PANEL SYSTEM AND A COMMERCIALLY INSTALLED SYSTEM?

The harbor freight type system is intended to charge a battery, and is low power. It’s good for experimenting, but is not very efficient, and not very powerful. I’ve seen an experimenter say that their 15-watt panel only gave him 5 watts in bright sunlight. If it’s the amorphous silicon thin film type of panel, it wears out quickly, dropping to a fraction of its original output in a few years.

The $10,000 type whole house system usually has no batteries, and instead ties to the electric grid. There is generally a 10-year warranty on anything going wrong, and the panels themselves, the most expensive part of the system, have a performance warranty of 25 years. The nameplate rating on such a system may be 4000 watts, and you can actually expect to get at least 3400 watts, usually.

Neither type of system is easily expandable. The big system can be made expandable later, but that involves spending money up front, and the final system will likely cost more than if the larger system was installed in the first place. An extra panel or two could be added to the small system, since it is generally underpowered anyway, but otherwise expansion means getting a completely new system and running it alongside the first.

For ultimate cost effectiveness, the big system is maybe 1/8 the price of the smaller system, per watt.