Use cumulative sum in Excel, ( linked in loops) , need help .

Dear all I need some support in a complicated excel function , which I was trying hard since a week .

its simply a function to calculate electricity power consumption, including generated power from solar systems I will try to explain it in her and will be happy if someone can help me knocking it out. (prefer in excel not VB) .

So
column A ..Is Value " B" = monthly Net power consumption
column B ..Is Value "MC" = Monthly power consumption from Grid ( consumed from the Grid)
column C ..Is Value " MI" = monthly power feed into the Grid ( generated from the solar system)
column D ..Is Value "R" = total power reserve ( cumulated from extra generated power month by month)

SO simply

B = MC - MI + R
in words ( Net power consumption = monthly power consumed - generated power + reserve which was generated and not used from previous months)

if B is positive then its need to be paid
if B if negative (which means we generate more than consumed) , then the new R = old R + the new B

it looks simple , but the problem that if I use if statement I will have a loop , where I can cumulate the R and use it again in the next month.

how can I get this don !!!

thanks all