thats misleading quoting like that since it makes it look like an investment with dividends and capital appreciation, with a typical investment the 8,000 euro initial capital can always be sold back to the market reclaiming your capital and usually asset appreciation. With solar panels you cannot get back your investment so you really need to be adding depreciation cost aswell for 10 years, 10% per year probably . Then after 10 years if the panels are still working you can use 17.5% return but probably the system won't be so effective after 10 years in operation.Which is a 17.5% annual return on the €8,000