Add values trough time

Hello,

I have got the following problem:

I have a system wich has a certain ammount of energy in it [kJ]. Outside of that system i have another system requesting energy trough time. Each second the second system would take away a part of the first system's energy. When the first system is empty it needs to regain energy.

So basicly i calculated the amount of energy in the first system and i created an energy request with the system builder block. Now i need to know after how many seconds my first system is empty.

How it works: System 1: 10000 kJ System 2: s1: 1000 kJ, s2: 3000 kJ, s3: 5500 kJ, s4: 3500 kJ

So after 3 seconds the first system will be at: 500 kJ wich is too few to deliver at system two in the 4th second.

The answer i would like to get is how many seconds i can supply system two with energy. Simulink needs to stop counting when system 1 is empty

I assumed i could compare the values like this:

If system 1 > system 2 then ...

How do i do this?

Kind regards,

Kris