This forum is about wrong numbers in science, politics and the media. It respects good science and good English.
They are usually coy about several parts of the equation.
1. How much the installation cost.
2. How much the installation produced.
3. How much of the maintenance they did.
4. The state of the battery.
5. The state of the interconnect..
People who can do an amortization (anyone with Excel should be able to do one... RIGHT?) should be many.
I am not sure how many can use the amortization as a tool for financial analysis.
I get pinged by my mortgage company regularly to refi. I have about 3 years left. They say they can save me $600 / month.
They are telling the truth.
They fail to express the other side of the terms. They might save me $600 / month for the next 3 years, but they will cost me $300 / month for the subsequent 27.
This is why an accountant DOES have standing when discussing global warming.
In reply to Edward, you can estimate the solar power economics of your friend's location using one of the various online solar power calculator webpages, like this one:
Click on the "Calc by System Size (kWp)" option, then enter your friend's postcode, his roof direction and system size (the default for the latter seems to be 4KW).
The calculator then tells you the "Income from Feed-In Generation Tariff", "Income from exporting energy", "Electricity Saving", "Total Benefit" and "Payback Time". The income figures are based on the subsidy arrangement at the time of writing (12.92p/KWh for generation and 4.85p/KWh for exporting electricity). Your friend is probably on a more generous subsidy arrangement than the current one having taken up solar power in 2012. If there was no solar PV subsidy there would be no income figures, and the only benefit of having the solar PV system would be the electricity saving figure, leading to a substantially longer payback time. I also suspect that if there was no subsidy, then the cost figures for solar PV installation (quoted as £5010 to £7655 for a 4KW system on the calculator webpage) would go up, as in my experience, industries tend to charge more the less busy they are.
There is a government proposal to cut back the feed-in generation subsidy from 12.92p/KWh to 1.63p/KWh for new solar power buyers from January 2016, and the UK Green lobby is currently making a lot of fuss over that.
It doesn't surprise me that your friend acts as though the subsidy is of little importance to him - to admit that it is important would be tantamount to admitting he is some sort of parasite, sponging off the rest of the electricity bill payers. The only owner of a solar PV system in the UK that admits to being a parasite is UKIP's Roger Helmer, but he's only doing that to make a political point.
I noticed that Alex Salmond, the former Scottish National Party (SNP) leader, made some comments the other day in an interview about what would constitute the 'trigger events' for a new Scottish independence referendum:
"Mr Salmond’s set of “triggers” that he feels would justify another vote include the UK Government’s failure to keep its “vow” on maximum devolution for Scotland; George Osborne’s refusal to slow the pace of austerity; Britain voting to leave Europe against Scotland’s wishes in 2017; the renewal of Trident against the will of the Scottish people; and the Labour Party failing to make itself electable before 2020."
I assumed in the starting post of the thread that the UK voting to leave the EU would be the only likely trigger event for a new independence referendum for Scotland in the next few years, but with Salmond's list, the SNP could call for a new independence referendum pretty much whenever they want. The last of Salmond's trigger events, the Labour party failing to make itself electable before 2020, is a bit of a cheek as the SNP has contributed significantly to Labour's reduced electability by taking seats away from them in Scotland and also by raising the disturbing prospect of there being a possible future Labour-SNP coalition.
So the scenario where Scotland tries to embark on independence with the millstone of having the world's largest wind and solar power capacity per capita around its neck is more likely than I thought.