How does hourly pricing work?
All Droplets are billed hourly, up until a monthly cap. We have estimated that each month has about 672 hours (28 days). If you use your server for less than 672 hours during the month, you will be billed for each hour that you used it. If you use your server for more than 672 hours that month, you will be billed at the monthly cost, which is no greater than 672 hours.

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How DigitalOcean estimate my Droplet usage time? Isn’t it metered if I don’t access to server?
Plus, is data transfer fee charged separately?