Additional information on the subject of efficiency by Norman.

Photon Labs are a German organisation that tests and rates solar panels on behalf of manufacturers. The results are published quarterly.

This is called Third Party Testing and is important as it verifies the manufacturers claims. Photon Labs are probably the leading 3rd Party Testing Organisation.

This is a direct quote from their home page on the subject of panel efficiency:

“A module’s nominal power is interesting, as is its efficiency – and, depending on the customer’s expertise, so are a few other pieces of technical module data. But the single most important factor for PV system operators is yield: how many kilowatt-hours per kW of installed power flow from the PV system to the inverter?”

Efficiency will govern the size of a panel rather than its output.

i.e. The LG 300 watt panel is 18.60% efficient and the Solarworld 250 watt panel is 15.10% yet they are more or less the same size. Because of its efficiency it needs less surface area to generate the extra power.