It is the period between 15th July 2009 and the start of the RHI scheme.
Qualifying systems installed during this period will be eligible for the RHI.
Qualifying systems installed from 15th July 2009 onwards will be eligible for tariffs.
Chapter 6 of the government's RHI announcement says
Support for non-domestic equipment installed during the transitional period
Non-domestic installations that were completed and first commissioned between 15th July 2009 and the start date of the RHI will be able to receive the RHI, providing they meet the eligibility criteria as set out in this policy document and gain accreditation from Ofgem. If they do not meet these criteria, they will not be eligible. The tariff levels and duration of support will be the same as for those installations commissioned after the start of the scheme. Support will be from the date of accreditation and not backdated to the date of commissioning.
The 15th July was the date when the Renewable Energy Strategy was published, and at the time the Government announced it would be the cut-off date.
Chapter 3 of the government's RHI announcement says
We appreciate the arguments that individuals and organisations have presented for allowing installations complete before 15
It's probably of no comfort to anyone, but we think this was wrong.
The Renewable Heat Incentive is due to come into force in 2011 for non-residential installations and October 2012 for households. See here.
All eligible systems installed from then on will certainly qualify. Many systems installed before then (but after 15th July 2009) should also qualify (but the payments won’t start until the tariffs come into force, and won’t be retrospective). See more details about installation dates.
It's the time between 15th July 2009 and the start of the tariffs in April 2010 (for electricity) or April 2011 (for heat).
New systems where the installation was completed after 15th July 2009 will qualify.
The consultation document says "installation completed". We believe this should therefore be the latest date that the installation was installed and commissioned.
In the case of grid-connected electrical systems it could reasonably be the date it first delivered power to the grid.
In the case of systems registered for the Renewables Obligation, assume that installation must have been completed by the date of RO accreditation.
It is proposed that existing systems will qualify only if they were installed after 15th July 2009 and the Government confirmed on September 16th in Parliament that is has no plans to change this. See more on installation dates.