The most cost effective solution is 'Grid Tied' where any excess electricity that is generated during the day is fed back to the grid and credited by your electricity provider. During the night you use electricity back from the grid and consume any credits you have generated during the day. If you use more electricity than you have generated then you will still get a bill from your power company but it may be only a very small amount depending on how many panels you have and how much electricity you are generating.
Grid Tied solutions are much more cost effective than Off Grid solutions as there are no expensive batteries to purchase and maintain, and no complicated control systems required.
With Grid Tied systems you still need to stay connected to the mains grid but you will significantly reduce your monthly electricity bill. Check with your electricity provider to find out what rates they will purchase electricity from you as it differs from supplier to supplier.
For more useful information please also see the Sustainable Electricity Association New Zealand (SEANZ) website http://www.seanz.org.nz and http://science.howstuffworks.com/environmental/energy/solar-cell1.htm