Fit and forget - sounds great doesn't it? That's what PV is right? That's what the sales man said!
But eagle eyed owners will note (if they keep an eye on their system performance or have a monitoring system which tells them) that output from the system rarely matches the rating of their system.
All electrical systems are subject to losses - they are unavoidable. Good installation and design leads to lower losses. Nowhere is this better seen than in PV systems, and there are a number of places where you can see losses, some of which can be managed, checked and tested, and others which are unavoidable.
Lets take a look at why your 4kW PV system might only put out 2.5 - 3.5kW most of the time...
The inverter (-5% losses and more for the "sizing factor")
An inverter is generally pretty efficient - usually around 95%. It is rarely the same power rating as the number of PV panels attached to it though... so if you have 4000watts of solar panels the chances are the Inverter is only rated at 3600watts. So whatever the weather - it can only let 3600w (or 3.6kW) onto the grid. This one factor alone means you will never ever see 4kW output. Just to be clear - its perfectly acceptable to have 4000watts of panels connected to a 3600watt inverter, as long as it stays within the manufacturers recommendations. Oversizing by 20% is perfectly normal.
The panels (-1% for good panels or more for damaged or faulty ones)
Solar panels are made of several layers glued together - the longevity of them depends heavily on how well these laminates stay glued together. It also means, over the longer term, that performance will degrade as environmental factors take their toll and the layers begin ever so slowly to separate. Typically panel manufacturers rate their panels with a tolerance factor of +/- 3%, and also give a warranty that shows this "rated" output value (typically 175 - 300watts) will drop over time. However you can certainly expect good panels installed well to last well in excess of 25 years - there are still panels over 50 years old that are working today.
The wiring / electrical installation (2-4% losses are typical)
Poor wiring termination and incorrect cable sizing and so on are all areas where there is increased resistance and therefore losses - on a separate issue this is also where there is the highest danger of fire and electrocution. If you think about it - every single panel has two connectors on it: if you have 16 panels, 3 junction boxes, an inverter, generation meter and consumer unit you are quickly approaching 50 terminations that were made on your installation.
The temperature (-10% or more in high summer)
The sad irony of solar PV is that it works less well the hotter it gets! As any fule no, resistance increases with temperature. In fact when solar panels are given their rating it is at "standard test conditions" - 25 degrees. So any hotter and performance drops. In fact when it is registering 25 degrees on the thermometer, there is every chance the PV panel temperature is closer to 35-40 degrees, and they can easily see much more than that in mid summer.
Dirt can easily add another 5-10% loss to your output, or even more if you live in a dusty area or near certain types of trees / crops. Do yourself a favour and get your window cleaner (or even Specialist Energy) to come and clean your solar panels. For the sake of a relatively small sum you will mitigate the largest possible drop in performance that you can actually do something about!
If you are unsure about whether your system is either safe or performing properly, give us a call and we'll do what we can to help...