Counter/Storage Collector + Matlab

I'm trying to design a storage system where excess energy goes into it. There is a cap of a maximum storage size for the system. I am struggling to work out how to code this in matlab.

Currently im using a function similar to this

max_storage = no_tanks*tank_size

if cumsum(excess) > 0 
   storage = cumsum(excess)
elseif cumsum(excess) < 0

After that I am confused how to continue writing the code. Any help would be greatly appreciated

Attempt at mind-reading, while awaiting an update to the question.

To limit the storage size to max_storage, you need to have some code like

storage = calc_storage(excess);      % or whatever
storage = min(storage, max_storage);

Don't forget to finish your statements with ;, and if you need to use cumsum(excess) lots of times it is better to assign it to a variable rather than calculating it over and over again.