There is no difference between monthly and hourly billing. If you only need the droplet for an hour a day, you have described the perfect way to use it.
All droplets are billed hourly, up until a monthly cap. We have estimated that each month has about 720 hours (30 days). If you use your server for less than 720 during the month, you will be billed for each hour that you used it. If you use your server for more than 720 hours that month, you will be billed at the monthly cost, which is no greater than 720 hours.