The solar cells we see on the roofs of some houses are Photo Voltaic cells.  That is they convert light directly into electricity.  It is often said that they need sunlight.  Although this is basically true, the implication that they only work when the sun is shining is incorrect.  Even on cloudy days they will still produce useful amounts of electricity.

Right Time

These cells on houses are producing power to replace what we pay retail price for, and if it is used in the house itself there is very little lost in transmission.

In countries like Australia, the highest peak of electricity usage is on hot days in Summer.  This is when there are many air conditioners on. It is also the time when solar power generation is at its peak.  The cells on people’s roofs help to reduce the likelihood of black outs.

Cost

The cost of making these cells has been rapidly falling.  The indications are that they will continue to get cheaper for several more years.

Payback of Energy

When photo voltaic cells were first manufactured the time it would take to repay the considerable energy put into the manufacturing exceeded the likely useful life of the cells.  Unfortunately some people still say that this is true.

The energy used in making these cells has dropped to a fraction of what it used to be.  At present the energy payback time varies between 1 year and 2 years.

Payback of Cost

I will not even attempt to assess how long it will take you to save, by reduced electricity bills, the cost of buying and installing solar cells on your house.  This will vary so much in each situation.  My brother did calculations for his own situation and decided that he would get his money back fairly quickly.

Large Scale

Photo Voltaic cells have been used in municipal installations.   In some cases, mirrors have been used to increase the light falling on the cells.  At present, there would be few situations where this is the cheapest form of municipal power.  This could change very rapidly as the cost of the cells falls.

Sources

Emissions from making photo voltaic cells.

http://pubs.acs.org/doi/full/10.1021/es071763q

Solar power for households.

http://www.coolmob.org/drupal-5.7/files/Information%20on%20Photovoltaic%20Solar%20Power%20Systems%20for%20Households%20Dec%2008.pdf

Origin Energy

http://www.originenergy.com.au/2029/Photovoltaic-cells