Grid tied inverter rated DC input

[MikeG]

I am looking for some experienced advice here, how strict of a limit is the rated DC input for the inverters? Other engineers I am in contact with, and even SMA engineers I talked to, tell me the SB3000 can handle a 4.7 kW system. The supposed DC input is rated at 3750W. I am always leery of a company giving me sound advice about their own product. Sure, the inverter will kick on, the voltages will meet the range, but some power will be lost. Is there anything else I am missing? Has anyone else done anything like this?

[Naptown]

Most inverter manufacturers will tell you that it is OK to put more power in. You want your inverter operating near the top capacity as that is where they are most efficient.
The reality of the situation is that you are judging input on the STC ratings of the modules. It is very rare that the sun will strike the module exactly perpendicular with 1000W per sq meter brightness the temperature is 25C and the air is moving at 1.5MS. Maybe about 15-20 minutes a year depending on location, azimuth, tilt temperature etc. Add in a little dirt on the modules, a little humidity, some wiring losses etc and you lost about 20% of the nameplate output.

[MikeG]

Assuming that 4.7 kW system is giving 85%, that is 3.995 - still exceeding the recommended limit. I am trying to figure out what that recommended limit really means.

[Naptown]

Too much power and the inverter will throttle back. Probably not the most efficient thing
2 questions
Where are you located?
What modules and how many are you using.

[Sunking]

You can oversize, it is done on occasion to squeeze out some more Kwh during the day. As Rich pointed out, the inverter will just limit itself if there is more capacity available than it can handle. So if you do this just besure to get it in writing from the manufacture to protect yourself if warranty becomes an issue.

[Mike90250]

Go the the inverter manufacturers web page, and they will have a "Array Calculator".

You enter your PV panel model, and quantity, and the inverter model, and it will tell you recommended array possibilities.

SMA's tool is here (this week)

http://www.sma-america.com/en_US/products/software/sunny-design.html

[Naptown]

SMA's new string calculator really sucks for lack of a better word. The old online one was much better

[MikeG]

Thanks, everyone, for the responses. I am familiar with the SMA calculator and have talked with an engineer there. Just to illustrate the issue, we are looking at using an SMA SB3000 with a 4.7kW system - 2 strings of 235w modules. The VOC would not suffice for a SB4000, which is why this issue is coming up. I am handcuffed to SMA products, but the SB3800 is not available. I am awaiting a response from SMA for a document in writing stating that they will still honor their full manufacturer's warranty since they are suggesting that this configuration will work.

The installation is in Arizona.

[Mike90250]

You may want to see if you have enough roof space to add panels to get to the higher voltage (when panels are hot ! ) Will the inverter be located in air conditioned space, or outdoors in ambient temp ? The smaller inverter will dissipate heat less efficently than the larger inverter, and in Az, heat is your enemeny. What's the upper thermal limit, and de-rating on the inverters ?