Under the central deployment scenario set out in the Impact Assessment accompanying the current consultation on solar photovoltaic (PV) cost control, see the following website:
www.decc.gov.uk/assets/decc/Consultations/fits-review/4320-feedin-tariffs-review-phase-2a-draft-impact-asses.pdf we estimate that there could be approximately 22 GW of solar PV by 2020 in the central scenario. The assumptions behind this estimate are set out in the Impact Assessment.
Estimates of future solar PV growth are extremely uncertain, and depend on a number of assumptions such as future PV costs, and the growth of the PV market. Thus, whether or not 22 GW is deployed by 2020 will depend on PV costs coming down.