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GaryM

Could anybody please explain the relevance of panel efficiency in real terms?

e.g.

Sanyo H series 250W panel - 18% efficiency
Sharp 250W - 15.2% efficiency
Mitsubishi 250W - 15.1% efficiency

16 of either of the above panels would give me a 4kW system so where does efficiency come into play?
Also, would one 4kW system be pretty similar to another or could they differ by quite a bit in terms of yield?

I've had a salesman tell me that efficiency is relative to panel output in that a 220W panel could be 15% efficient and if it was 18% efficient then it would be a 250W panel (numbers are examples and probably wont add up) but that can't be right as Sanyo do a 240W panel which is more efficient than the 250W H series panel I listed above.
 
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Efficiency is only really relevant when related to panel size. And this is only critical if you do not have a large enough roof for the size of system you want.

e.g. Sanyo HIT = 190W per square meter vs Sharp NU 150W per square meter

Response to low light conditions is a more critical factor for most PV systems in the UK.
 
The panel effciency can be useful but it is something that shouldnt be overplayed and sometimes can be misleading or even unimportant. The efficiency of the panel is most important when you need to get a lot out of a small space. If you have a space that is 10 sqm and you want to get as much as possible on then 10 x 16% efficiency panels wuld be better than 10 x 14% panels.
This is about watts per square meter and getting the most energy from a certain amount of space.
But-many other things normally come into play. Such as what if the 14% panels are cheaper? What if the 14% panels work better in low light conditions and you live in the north or they work better in high temps and you live in the tropics. more importantly- what if you dont have such a tight space? You could then do better having lower efficiency panels that work better or cost less than higher efficiency ones.
The sanyos for example-high efficiency is not really the thing that makes it so good- though it is often useful because of its high efficiency if you want as much in a smaller space as you can. They work better in low light and higher temps than other modules and so give more energy (but also cost more)

The salesman is right to an extent, although he probably doesnt know why. many panels are of a similar dimensions (170w-200w or 220w-250w) and if you have 2 the same size but one is 240w and one 250w then the 250 will be more efficient- so you can say efficiency is sometimes an indicator of the power but this is not always the case. The 250 watt panels will be made of higher efficiency cells than the 240 but those same cells could also be in a 60w module that is higher efficiency than the 240w or in the example you give the cells in the 240w sanyo are a bit more effcient than those used for the 250.

hope thats not too confusing
 

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