With around 1-2% annual replacement of the UK's housing stock, housing retrofit must play a major role in reducing future energy use and CO2 emissions. This paper presents a whole-house approach for energy retrofit for five houses located in South Wales. This 'systems based' approach combines reduced energy demand, renewable energy supply and battery storage. The paper describes a combination of energy modelling, using the building energy model HTB2, and field measurements to analyse the performance of the houses before and after retrofit. The results indicate that significant reductions in energy use, CO2 emissions and energy costs can be achieved using a whole house approach, combining energy efficiency with building integrated renewable energy generation and energy storage. CO2 emission reductions are estimated to be in the range of 50-75%, with cost savings of 402 pound to 621 pound per year. The cost of carrying out the retrofitting ranges from 23,852 pound to 30,510 pound. Although retrofits are still relatively expensive in relation to their annual cost savings, there are multiple benefits relating to reducing fuel poverty, reducing electricity grid stress and contributing to national CO2 emission reduction targets. Also, as costs of measures are further reduced and energy prices likely to rise in future, the cost balance will change more in favour of whole house retrofit. The paper demonstrates the advantages in using a combination of energy simulation and field monitoring to investigate the performance of buildings in use, which in this case concerns the impact of carrying out energy retrofits in housing. (C) 2017 Elsevier B.V. All rights reserved.