A friend recently purchased one of the many PV arrays advertised on eBay only to be disappointed.

A common metric used to evaluate cell technologies is conversion efficiency with 1000W/m^2 insolation. Most popular products are monocrystalline silicon technology which achieves 18-25% efficiency on an assumed 1000W/m^2 insolation.

If we look carefully at the above panel advertised as 200W, the active PV area is less than the frame size, probably \(A=0.93 \cdot 0.63=0.59 m^2\). We can calculate efficiency \(\eta=\frac{p_{out}}{1000 A}=\frac{200}{1000 \cdot 0.59}=34\%\), nearly double expected efficiency for monocrystalline cells.

Common consumer advice: if the offer looks unbelievable, it probably is.

This array is unlikely to be much better than 100W (if that).

A simple test of such a panel is whether it achieves the rates short circuit current when pointed at the Sun at midday with clear sky. Don't be surprised it is doesn't make 50%.

The fine print: PV array capacity is often quoted in watts being the product of the open circuit voltage and short circuit current. They cannot deliver that voltage at that current simultaneously, so it is an overstatement, a lie, the maximum power is unlikely to exceed 80-90% of that rating, and then that is only useable with a true MPPT controller which has its own losses, so in reality much over 80% is unlikely even point at the Sun.

An interesting observation is that these ‘green' products need to misrepresent their capability to get sales.