PlantComponent:UserDefined ems control

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Hi everyone, I'm using the PlantComponent:userdefined input object to model a new storage system. I'm learning about Ems and was wondering what's the ems syntax for updating a variable value based values from previous time steps? Say I have a variable x representing the state of charge, and C representing my rate of charge, i want my ems charging script to translate the following:

X(0)=0 ! Initial state of charge

For t from 1 to 7am

X(t)=X(t-deltat)+C*deltat !current state of charge at time t

Any insights? Thank you,