Who loves the Sun Solar Calculators (aka Solar Estimators) will estimate the size and calculate the cost to install a solar energy system for your home or building. There are many free ones on the Internet. Just type in “free solar calculator”
They are all based on the number of hours of sunlight you can expect at a given address, and then vary in sophistication after that.  One of the better ones (although it serves the US only) is at http://www.solar-estimate.org.
Many calculators let you choose between options include solar electric (PV), solar water heating, pool or spa heating, and space heating & cooling. The financial analysis provided is based upon energy bill savings you can expect and the net system cost, after tax credits and other incentives are applied. The results are based upon many assumptions and the limited data you will enter. An actual site assessment by a Solar Pro will be needed to determine the actual costs of installing a solar energy system.
In the UK, try the calculators at www.encraft.co.uk. They also offer calculators for wind, heatpumps and energy usage.
In New Zealand try www.gns.cri.nz/solar .