yes 7% ROI if no depreciation, but neither panels nor inverter are expected to last much beyond ten years. I was actually chatting with him just now since I got into this discussion, and he said he also needs a maintenance/emergency callout and cleaning contract, thats about €250 a year. In spite of this, he was still happy with doing this, good for the planet, even if not for this wallet. He also put up a large screen with the relevant numbers to train others in the house to use appliances at optimal times.