No Export? No problem

January 19, 2021

The mechanics of a solar PV system are simple -

  1. Generate clean energy during daylight hours
  2. Consume the clean energy first before topping-up from the grid if needed
  3. Export the clean energy to the grid if you are generating more than you can consume (or store in a battery)

But what if the grid do not want your surplus power?

Firstly, it’s important to understand why they might not want your clean solar energy:

The electricity distribution network in the UK is designed to both deliver power to homes and businesses as well as take it away. There is, however, a limit as to the amount of power it can carry at any one time.

If your site needs more power delivering to it, it is likely that your District Network Operator (DNO) will charge you for the infrastructural changes needed to deliver that extra power.
 
Exporting power is no different. If the existing infrastructure cannot handle all of the theoretical output of the solar array being exported onto its network then your DNO will not allow you to connect your solar array without first upgrading the equipment. The cost of these upgrades can run into hundreds of thousands of pounds.
 
The good news is that if your site uses a lot of electricity, it may be possible to install some clever hardware and software that links the site demand with the output of the solar array.

Export Limitation

‘Export limitation’ will see the solar inverters monitor the import of power to the site. If the site is consuming more power than the solar system is producing then the array will continue to work as normal. If, however, the balance flips and the solar array is generating more power than the site can consume, the efficiency of the inverters will be changed to reduce the output of the system to match the demand of the site. 
What is a solar inverter?
 
An inverter inverts the DC current generated by the solar panels into AC for use on site.
Subject to what the local network is capable of handling, it may be possible to secure some export capacity so that the limitation process is not activated the moment generation exceeds demand.
 
If the offer to connect to your local network is subject to export limitation then the relationship between generation and consumption must be studied forensically as multiple, consistent, export limitation events will reduce the overall performance of your solar array and the returns you are expecting.
 
Unfortunately limiting export is not always an option. If a DNO has ‘fault levels’ on their network, they may limit the size or refuse the connection of a solar system to their network – even with export limitation.
Fault Level is defined as the potential maximum current that will flow when a fault occurs.
 
Whilst this is similar to the set of circumstances described above, ‘faults’ are a known point of weakness on the network that, if not correctly managed, may result in the loss of power to connected customers. Sadly the export limitation hardware and software cannot react fast enough to prevent exported power from potentially aggravating the fault.