Gaz, you are of course right that the purchase cost has to be included. However there is a common misconception about solar power.
Solar is cheaper than the most obvious alternative-diesel generator, by far.
Why?
Well, on the assumption you require electricity 24/7 which as a resort we do, then you have to have a battery bank to store your overnight power, and an inverter to convert it from 12vDC to 120vAC WHICHEVER system you choose.
Now we come to the power generation itself. I paid US$8000 for our solar panels six years ago, and they haven't given us a moments trouble in that time.Its worth noting that those same panels today would be around US$5000 and prices are still falling
As it happens we have a diesel generator too. That cost US$10,000 supplied locally, and in the five years we have had it it has cost (in round numbers) US$3000 in repairs, plus of course the 2 hours per gallon of diesel it consumes, plus filters, plus lube etc.
The fair comparison between solar and diesel (or any other off grid system like wind or hydro) should be between the GENERATING methods ie between solar panels and the diesel motor itself.
On that basis solar comes out way ahead.
I do agree however that when comparing to the grid the cost of the "off grid"sytem has to be amortised out over its lifetime whatever that turns out to be. But then what price do you put on having continuous power with no brown outs and no fridges or washing machines failing as a result.
Electricity prices however are inevitably going to rise in the future while we rely on oil for generation. That too has to be factored in.
Its not a straight forward calculation, and living where we do we had no choice. But personally I would choose solar above grid power all over again if the question arose.