Why are you paying them for two computers, then? You quoted a price of ~$90/yr. Unless you want to back up to one computer, then send the data to Backblaze, I’m assuming you’ll need to pay for two computers, and they have some means of enforcing that.

40W is also a worst-case estimate for average consumption. 40-50 watts is actually close to the peak usage for an Atom board with two HDDs. Spin down the drives, and if it sits idle, average usage will drop to 5-10W.

Most drives will last 5+ years in stationary applications, anyway.