No govt grants for a while now for this, but they pay you for every unit you generate (whether yo use it yourself or not) and the electricity compay also pays you a bit for every unit you export into the Grid.
The rate/unit from the govt was halved during the consultation period without sufficient notice, so was re-instated for a further short period. Anyone who had their system installed beefore the 3rd March got in at the full rate of 43.3p/unit generated, guaranteed for 25 years and index-linked. If you install a system now the pay-out is ~21p/unit.
Companies were and still are installing them 'for free' but then they retain ownership of the system & get the payments, you only benefit from the smaaller electricity bill due to not neeeding to buy so much electricity from your supplier.
I got in just before the rate was reduced back to the current rate (registered the system on 1st March) and paid for it, so I get the payments as well as not buying so much power.