Inverter choice

[GPack]

I would really like to know a little bit more about choosing an inverter. At the moment all I really know is if I have 10 panels at 185W I would need an inverter rated at 1850W or greater. Are some inverters more efficient than others? How is their efficiency measured? Thanks for your help.

 

[pmac10]

bit more info required mate, what do you want to control??

 

[GPack]

I have seen a package put together for a PV system that supplies 10 x 185 W panels with a 1700W inverter. I would have thought this would be undersized. Is there some kind allowance that suggests the panels will never run at 100% efficiency so the inverter can be under the total potential draw from the panels? Is it a percentage?
I also had a customer who said that he had heard some inverters would need a smaller amount of energy created from the panels to start moving the generation meter around. Is this right?

 

[pmac10]

sorry mate, i'm an industrial maintenance engineer so know absolutely nothing about PV systems or how they utilise inverters but there are other members on here i've seen discussing these systems, hopefully some of these members will be able to help but i'll follow this with interest.

 

[TedM]

There is a body of thought that has argued that, in the UK, it is preferable for inverters to be sized to 80% of the max output of the PV. See the MCS recommended EST PV Installation in Buildings guidelines doc for details. As far as I can find this is based on a very slim scientific body of evidence.

It is imperative that the inverter chosen has an input voltage range that matches the voltage level of the PV strings.

 

[Martin Dawes]

When I calculate the inverter size, I try to aim for around 90%. I personally think that that sizing at 80% will cause problems but I do understand the reasoning behind it.

 

[GPack]

Is the reasoning that the panels will never hit their peak in the uk? or that the inverters have been over rated? Is this guidance from the manufacturers or engineers? Thanks for the input.

 

[Martin Dawes]

The reasoning is that they won't hit their peak for long periods, if at all. Therefore, because maximising the yield is considered so important, it considered acceptable to rate the inverter at less than the array.

 

[GPack]

Are some inverters more efficient than others? How is their efficiency measured? I suppose I just need to phone up the manufacturers/suppliers. I'll be on the phone to them on Monday morning. Thanks for your help

 

[Martin Dawes]

Yeah, some inverters are more efficient than others so it pays to look at the specs. Their efficiency is expressed as a percentage.

 

[GPack]

so you know what makes some more efficient than others?

 

[TedM]

It is down to the detail of their design. But you need to note that efficiency is not a single number - it is a range that varies depending on the input so you need a graph to see the effect. Have a look at some of the typical graphs for SMA inverters:



You also need to check how much power the inverter consumes whilst not generating any power.

 

[GPack]

Thanks for the information. What do inverter manufacturers use to try to persuade us that their inverter is better than their competitors? Which inverters do you prefer? Sorry for all the questions but I'm trying to get a handle on it.