Energy ROI and countdown to profitability

I just recently installed an energy monitor and solar in my home. I want to use a number helper with my total cost set as the initial value. I will be setting up two cost entities. One that is my actual cost(with solar) and another that would of been the cost if I did not have solar. I will use another helper that will calculate the difference so I can get a daily amount saved. I am able to get to this point easily.

The next step will be to take my number helper with my total cost and deduct the daily amount saved for going solar from it. I am unsure of the best way to achieve this and am looking for advise. I would also like to use 12 months of daily amount saved to get a rolling average amount saved per day so I can use it to estimate how many year/days until I break even.

I tried to set up an automation to decrease a number helper but there is no option to use another entity as the value to decrease it by. Any advise would be greatly appreciated.