There is over 000 watts of power in a square meter of sunshine. A square meter panel converts only about 20% of it into electricity (200 watts). At my workplace, I have a photo-voltaic solar panel electrical system. Its maximum capacity is about 25 kilowatts. On a sunny day it will produce about 80 kilowatt hours of electricity, but we average about 50 kwh/day. At current value, that adds up to a saving of about $4,000 per year, which sounds nice but that's only about a tenth of my total power bill. The system cost $00,000. Pay-back is 25 years (which by coincidence is the length of the warranty). To answer your question more directly: solar panels are not quite efficient enough to be economical.
Commercial monocrystalline panels are typically 8% efficient but inverters can be 65% to 90% efficient and lead acid batteries are 50% efficient at charging and 92% efficient at discharging plus they self-discharge at 3% to 20% per month.
DS's solar panels convert 22% of their available energy into electrical power. This may not sound very good, but it is much better than most solar panels. Most solar panels on people's houses, for example, are fairly inefficient. Less than 4% of the energy that reaches them will be converted to electricity. DS's panels are so efficient, in fact, that unless the ion propulsion drive is running, DS is producing far more power than it needs.
