Quote from Reddit User: /u/louvetic
How do I calculate the Hashrate to mine 1 block a day?
Use the following easier calculation: If there is a new block every 4 minutes, there are 24 * 60 / 4 = 360 blocks a day. So with 1/360 of the hash power you should get an average of one block per day. Right now we have 2’850’000 H/s, so you’ll need around 7’916 H/s.
The difficulty is just a factor that makes sure the next block will probably be found in four minutes with the current hash power. If the hash power increases, the difficulty will raise as well. And the other way round it gets easier to compute a block’s solution when the available hash power gets lower. So it always leads to a value close to 4 minutes.