This post comes from an e-mail dialogue between members of Solar Washington’s Education Committee.
Here, Howard Lamb, Founder and Principle Engineer of Sunergy Systems, clarifies the difference between solar module efficiency and solar module production:
Higher efficiencies don’t necessarily mean higher annual production unless roof space is at a premium. Take, for example, SunPower and Sanyo modules, which have the highest module efficiencies in the industry. Despite their high CEC to STC (power) ratings, both might be lacking in other areas related to production.
For Sanyo modules, not much real world data is available to back up their claims of high annualized kilowatt-hour (production) per kilowatt (power). SunPower does have the data to back this claim up but at the expense of offering a module with +/- 5 percent tolerance rating, meaning their modules could be 5 percent more OR less powerful than their nameplate rating. Because the frequency of real-world PV module testing is increasing, the industry standard has become a -0/+5 percent tolerance rating, meaning modules are guaranteed to be at least as powerful as their nameplate rating, but not more than 5 percent so.
In regards to PV module testing, SolarWorld modules consistently top Photon Magazine’s real world comparative tests, yet their ~13.8 percent efficiency is the norm for monocrystalline modules. This goes to show that a module’s efficiency can have little to do with its actual production. It’s worth noting that SunPower does not participate in Photon’s testing, and Sanyo got yanked years ago due to very inferior results that Sanyo claimed was due to the distributer shipping B stock modules to Photon for the testing.
On a separate but related note, the case can actually be made that polycrystalline modules will outperform monocrystalline modules because they maintain their efficiency ratings better at low light conditions, which sounds like the Puget Sound region. Evergreen Solar has a low efficiency but is at the upper end of the CEC’s (California Energy Commission) list of performing modules. Even if there is a higher production associated with higher efficiencies, I don’t know if it is worth the additional costs that these premium modules can go for, that is, unless roof space is at a premium.
Just my two cents,