Photo-voltaic cells generate electricity from light, in any weather. PV generators come in many forms to suit your property and needs.

Installation will typically set you back around £8,000-£20,000, and the system will supply around half your electricity needs (perhaps around £100 per annum saving on your bill – a long payback period). Grants to help with installation are sometimes available, via the Energy Saving Trust

You can in theory sell surplus electricity back to the grid.

The British Photovoltaic Association (snappy!) has much more information and contacts.