What makes micro-generation worthwhile is the UK government paying a Feed-in Tariff (FIT). Every unit of electricity that we generate we get paid 41p for - this dwarfs the amount that we would save by using it ourselves (around 13p - though this is at a historical high). If we don't use electricity, it is given to the grid. In principle, we are paid 3p per unit for electricity returned to the grid in this way - which is pretty rubbish, but standard. In practice, the electricity companies aren't particularly interested in accounting for little bits and pieces, or metering it separately, so they simply pay 3p for half the units generated.
I sent in my first meter reading for FIT payments on 1st March - a hefty 45 units (kWh)! We certainly won't be getting rich on that. However, things have changed substantially since. In the week following this meter reading, we generated as much as we had in the preceding two and a half months. Yesterday - a clear day all day - was the first day when we generated over 7 kWh. The company that installed the system reckoned that we would expect to generate 1500-1600 units per year; to do this, we would need to generate an average of about 4 per day. The 5 day moving average is currently at around 2.5-3 units per day. As we get closer to the summer solstice, the performance of the system improves markedly - the sun is higher in the sky and therefore more intense; the weather improves so there is more sunshine; the days are longer.
The price of solar electric systems is also falling, as the availability of them rises following an increased demand. Sainsburys, in conjunction with British Gas, are offering a 2.1 kWp system from £10,000 - significantly less than we paid. It also sounds as though the government are interested in continuing to encourage the take-up of micro-generation by households - a report in The Times yesterday (paid for service, no link) suggested that the government was likely to cut the FIT offered for new medium and large installations.