Manufacturers stated output and tolerance – when a solar PV panel is manufactured it is given an output rating in Watts (similar to how light bulbs have different wattages.) When the output is stated by the company for their panel they will also state the tolerance as to how much the output may vary by. For this reason when looking at solar PV panels although a higher wattage panel is a good starting point it is not the only measurement to look for.
After reading the stated output look for the manufacturers stated tolerance level, as there can be vast differences. A common tolerance is plus or minus 3% of the stated output wattage but a better panel will have no negative tolerance only a small positive tolerance – this means that with no negative tolerance you are definitely getting the stated output where as a panel with a negative tolerance of say 5% could end up producing a smaller output even though on first examination the stated wattage could have been higher.
To clarify a panel with a stated output of 240 watts but with a tolerance of minus 5%, positive 10% in reality could only be producing 228 watts, this being less than a panel with a stated output of 230 watts with only a positive tolerance!