so there are different ways of looking at it. The 2 ways I like to think of it are: a) in an abstract sense simply marking both the implied gas and power legs to their respective market prices and add them together (have to be mindful of the +/-). This is how I look at it just from a P&L movement perspective b) concretely in my case (since I think more about hedging fixed price power in an illiquid forward market), I like to hold the power volume leg constant and allow the implied gas leg to vary with the market heat rate. Simplified example: if I locked in at 7 for 100MWh's and the market HR goes to 8, I like to think about it as having "saved" 100MMbtu's by locking in at 7. Make sense? It turns out that multiplying the "saved" or "wasted" (in the case where the market HR moves lower) gas x that day's gas price is extremely close to the the HR Mtm as calculated in a) above. Hope this helps.