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.

One more question, are snapshots and snapshots storage free ? When will you delete my snapshots ?

Snapshots are free and do not get deleted.

Does the hourly billing mean when the droplet is powered on? So if i keep it on all day long, but don't use it, it will end up with 720h = monthly cap?

If the virtual server is on or off you are still getting billed because the CPU, RAM, Disk and IP space continue to be reserved for the virtual server.

And yes you are correct once you get past 720 hours you reach the monthly cap and are just charged the flat rate.

We provide a flat rate because it's easier to estimate your monthly costs, as it provides the maximum that you could possibly be charged for running that particular server.

For scaling purposes I think about creating new virtual servers on some specific days and time-frames. As I understand the snapshots are tied to a droplet instance so either I pay fulltime or I'll lose any snapshots?

Do you plan to offer a snapshot functionality that is not tied to a running/reserverd virtual server?

Snapshots are not tied to the virtual server, so after you take one if you destroy the virtual server the snapshot remains.

Backups are tied to the virtual server, so if you destroy the virtual server the backups will also get erased.

can you update info in this article
https://www.digitalocean.com/community/articles/digitalocean-backups-and-snapshots-explained
about costs of backups and snapshots. as i see it obsolete
also i think need new clear article about pricing methods. in control panel such info not visible up-to-date so i cant planning costs for not permanent droplets, didnt know how much cost of backups, snapshots, other things (excepts reading this forum, but i dont know - is this info relevant or obsolete)
so i hope see actual uptodate article about all costs and pricing.
in first look all simple, but after reading articles and faq generate more questions than answers )