How to increase average with a new number added each month?

I am trying to optimize our electricity usage and have started a numbers spreadsheet. I would like there to be a running average, that automatically sums all the column C numbers up until the row where the running average is. So far I have just updated the average function manually, but there has to be a way that the new row automatically gets added to the previous rows?

I hope this makes sense and that someone can help me, if you need more info let me know! Thank you! :D