Feed in Tariffs: Guide to Commissioning


Publication date

Scheme name


The Feed In Tariffs (FIT) scheme is closed to new applications.

This page explains how to establish the date that your installation was commissioned for the purposes of the FIT scheme.

The commissioned date is important because it formed part of the criteria for deciding which tariff your installation was eligible to receive.

This is not a definitive legal guide. If you are in doubt over the commissioned date of your installation you should seek your own independent legal or technical advice as appropriate.

Commissioned date definition

“Commissioned” is defined in Standard Condition 33 of the Electricity Supply Licence. The definition has two main criteria:

a) the completion of such procedures and tests that constitutes, at the time they are undertaken, the usual industry standards and practices for commissioning that type of installation

b) such that it is capable of operating at its Declared Net Capacity (assuming that the relevant Eligible Low-Carbon Energy Source was available to it without interruption or limitation) and the installation is connected to Plant such that the whole of its maximum output could be used in a permitted way.

As defined in the Guidance for Renewable Installations, electricity is used “in a permitted way” if it is:

  1. consumed by the FIT Generator or (if different) the operator of the installation, or by persons to whom it is supplied by the FIT Generator; or
  2. Exported.

This means that both a) all the usual commissioning tests must have been completed and b) the installation could operate at its maximum output (DNC) without limitation. The electricity generated could be consumed onsite or exported.

The date on which both of these criteria are met is the commissioned date of the installation.

Demonstrating your FIT installation has commissioned

When you applied for the FITs scheme, you were required to produce evidence confirming the date the installation was commissioned. We encouraged you to provide as much information as possible to evidence the commissioning of your installation.

Part A

The relevant procedures, tests and supporting documents for each individual installation will vary depending on the renewable technology being used and whether the installation is grid connected or not.

The following are the minimum documents you were expected to provide:

1. Confirmation of the commissioning date

This should take the form of a commissioning certificate or letter signed by the installer or engineer who carried out the tests and procedures for commissioning.

This should confirm the location of the installation and the date the installation was commissioned.

2. G59/G99 Requirement (for grid connected installations only)

Until 27 April 2019, either a G59 or a G99 test certificate could be provided. If the grid connection tests are completed on or after 27 April 2019, only a G99 test certificate will be accepted, unless otherwise indicated to us by the relevant DNO.

You needed to provide one of the following (this document must be dated and refer to the installation name or address);

  1. The G59/G99 witness test certificate signed and dated by the relevant distribution network operator (DNO). This should be signed by the testing engineer.
  2. The G59/G99 test certificate which the DNO has not witnessed along with correspondence from the DNO stating that they did not wish to witness the test.

3. Timeline and explanation of procedures completed. A timeline of all the tests and procedures that were completed as part of commissioning. You should have also included an explanation to why you believe the commissioned date was as stated. An example for a PV application is provided below.

Part B

Evidence that the installation can operate at its maximum output

You were also required to demonstrate that the installation was capable of operating at its DNC on the commissioned date by showing that all electricity could be consumed either onsite (directly or through a private wire network) or exported. On request, you needed to provide evidence to confirm the grid connection capacity and the scale of any onsite loads.

Where the grid connection capacity is less than the DNC of the installation, this may affect your ability to demonstrate that your installation “commissioned”, or it may result in permanently limiting the capacity of the installation.

How do I produce a timeline of the commissioning tests and procedures?

Example timeline for a solar PV station:

Date Commissioning Test
11 February 2019 DC string test
11 February 2019 Earthing test
26 February 2019 G59/G99 test
26 February 2019 First generation
27 February 2019 Inverter commissioning
28 February 2019 Commissioning certificate

This station would have commissioned on the 28 February 2019.

This timeline clearly shows the dates the relevant tests were completed.

It is important that all tests that relate to the commissioning of the site were included in the timeline. This was to reduce the risk that the commissioned date, and therefore tariffs, may be adjusted in the future if further information comes to light. Make sure you keep copies of all testing documents so that you can provide supporting evidence should we audit your installation.


Ofgem has an ongoing duty as FIT Scheme administrator to monitor compliance with the FIT Orders by operators of installations. This ensures that only eligible installations continue to receive support under the FIT Scheme. Ofgem monitors compliance of scheme participants in a number of ways, including on occasion through an audit programme as highlighted in the installation’s accreditation letter. An audit can be conducted at any point throughout an installation’s accreditation lifespan and so any commissioning documents and/or evidence can be requested at anytime by Ofgem and should be retained by the generator.

As outlined in the accreditation conditions, it is the generator’s responsibility to meet any other evidential requirements and conditions that may be applicable in individual circumstances (this might be dependent on the type of installation). Please note that it is the generator’s responsibility to retain and provide evidence upon request regardless of when they assumed responsibility for an installation. This includes circumstances where an installation has been acquired post commissioning.

 For more information about the auditing and enforcement process see 5.77 – 5.90 of the FIT Guidance for Renewable Installations