I've done the calculations just now:

10W constant over 10 years would cost me 275 euro. My hard drives (7 ST2000DL003 and one ST2000DL001) are 10-15 years old now. They're all different batches, none failed yet, so I expect it to last at least 5 more years, possibly a lot more.

The current NAS setup I have (router + DAS + 2 USB) is around 50W. Over 20years it will cost me ~2800 euro in today's prices. So you see, the electricity is a very significant portion of TCO. In fact, it's more than half, because I bought everything second-hand.