inverter undersized

[new sparks]

Hi very new to solar pv. just starting to quote a few systems . i have sent out some tenders to wholesalers to get a feel of meterial costs, in some casses it seems to me and sol/pv that the sma inverter should be the 4000, when quoting fora 3.7 kw ground mount system. some wholesalers have come back with a 3000tl, what are the pros and cons of undersizeing

 

[access_renew]

Hi,

I would suggest you download the SMA software for yourself to run the calculation according to the conditions of the project and module. My gut feeling is it's too undersized for your needs (on face value).

And if you want some pricing from us (including SMA) then I'll be happy to send you our current price list.

Sean

 

[whlphil]

In general you want to be undersizing by about 10% although that can vary on choice of panel and location (further north you can undersize by more), remember the output of a panel is based on 1000w/m2 at STC these conditions are very rarely achieved in the UK (unless your in cornwall or on the south coast - even then only on exceptional or around mid day on good days) so your best designing the size of your inverter around the more usual median conditions and output level.

Usually we are below these input levels of energy so the inverter wants to be undersized to allow for the lower panel output. The idea is to size the inverter to be working at its optimum efficiecy levels for the maximum amount of time - not basing the design around the peak possible output as then you wont be making best use of the inverters efficiency. If you get the SMA software you'll see how undersized in percentage terms the inverter is, generally around 120% is about the max you want to go to as at that point you end up clipping the power off the top of the inverters performance curve and maybe damaging the inverter. In general if they over perform their standard conditions they overheat and derate thus loose performance - hence its a balance.

If you look at the nice picture diagrams Power One do for their inverters you will get the idea of where the different models perform best in the overall string voltage range, look at the picture on page 36 the PVI 4.2 OUTD it shows the efficiency of the unit at given percentages of total available input power which makes it a little easier to understand.

http://www.power-one.com/filebin/flipbooks/RE_Brochure_2011_EN/index.html#/36/zoomed

Its a difficult thing to achieve the best possible conditions at all times and thats where micro inverters have an advantage although they arent without their faults either.