# Dual variables associated with same equation for different time instants

I have three equations that are essentially the same equation defined for three time instants. The equations are basically calculating the state of energy of an energy storage facility.

\begin{align} e(t) &= e(0)-d(t)+\eta \cdot c(t);&&t=1\\ e(t) &= e(t-1)-d(t)+\eta\cdot c(t);&&1

$$\eta$$ and $$E$$ are parameters; the rest are variables. The equations are defining the value for the same variable $$e(t)$$. My question is do I include a single variable to define the dual variables for all the equations or I need to define three separate variables with one for each equation. Therefore, if I were to write KKT conditions for the complete formulation, only one or three dual variables will show up.

You do not have $$3$$ constraints, you have $$T$$ constraints.
For example, if $$T=5$$, then we have
\begin{align}e(1)&=e(0)-d(1)+\eta\cdot c(0)\tag{1}\\e(2)&=e(1)-d(2)+\eta\cdot c(1)\tag{2}\\e(3)&=e(2)-d(3)+\eta\cdot c(2)\tag{3}\\e(4)&=e(3)-d(4)+\eta\cdot c(3)\tag{4}\\e(4)&=0.5\cdot E\tag{5}\end{align}
You might like to use the same symbol but index it with time $$t$$, for example introduce the dual variables $$y_t$$ for the $$t$$-th constraint or introduce a vector $$y \in \mathbb{R}^{T}$$.