This calculator takes account of the gradual reduction in output of your
solar panels as they age, to determine the total amount of electricity
generated over their working lifetime; and thence the equivalent cost per
kWh. of electricity generated by your solar panel system over its working
lifetime.
If the price of electricity passes this value during the lifetime
of the system, even without the benefit of any feed-in tariff or export
payments, you will end up making a net saving.