Installing a solar battery as part of a solar installation is a capital expense. Capital expenses are not allowable and cannot be claimed against rental income.
In terms of capital allowances, expenditure incurred on the provision of plant or machinery for use in a dwelling-house is not qualifying expenditure for an ordinary property business.
The plant or machinery does not necessarily have to be located in the dwelling-house in order to be for use in it. For example, if a landlord installs ground mounted solar panels within the garden of a house he lets and the electricity generated will be used in that house, the expenditure will not qualify for capital allowances.
The Landlord’s Energy Saving Allowance was withdrawn from April 2015.
Please see the guidance at:
Work out your rental income when you let property
CA23060 - Plant and Machinery Allowances (PMA): qualifying expenditure: plant and machinery in dwelling house