Q. I have put a deposit on 5 kW system and now I have been informed by my solar supplier that the local electricity network will only allow a 3.5 kW system. They have suggested that my only option is to add a feed-in limiting device / export limiter. But would this prevent us using all of our 5 kW solar power when we need it?
A. For a start, in my opinion, they should not have taken your deposit before they got permission to connect the system to the grid! And they should give you your deposit back if you are not happy.
But great question about the export limiter.
An export limiter of, for example, 3.5kW on a 5kW inverter will not limit the output of the inverter to 3.5kW. It will (if properly configured) limit exports to the grid to 3.5kW.
How a solar export limiter works
A solar export limiter uses a little sensor called a 'current transformer' to constantly monitor how much power is flowing out the to grid:
If the export limiter is set to 3.5kW and you have a 5kW inverter:
If, at any moment in time your power flow to the grid goes close to 3.5kW, the inverter's export limiter throttles the solar back, to ensure exports to the grid never go over 3.5kW.
Remember the solar electricity will always be used by your house first, with any surplus solar sent to the grid.
Take the example above and assume the house is consuming 5kW (Arrow B). Assume the solar system is at max output of 5kW (Arrow A). All the solar is going into the house. There are no exports (Arrow C), the inverter is not limited.
Simple arithmetic shows that as long as the house is consuming 1.5kW or more the system can never export more than 3.5kW (5kW-1.5kW = 3.5kW), so the export limiter will do nothing with a house consumption more than 1.5kW.
The export limiter only has to kick in if the house consumes less than 1.5kW.
For example if the house consumes only 1kW and the Solar is at full whack (5kW) the system wants to export 4kW. So the export limiter kicks in and reduces the solar inverter's output by 500W to 4.5kW. Then you have 4.5kW generated (Arrow A). 1kW consumed (Arrow B) and 3.5kW exported (Arrow C).