I am trying to make a tariff which applies a charge of $4.10 (US) each month based on the peak demand for the year. I've built it this way:
UtilityCost:Charge:Simple 
UtilityCost:Ratchet 
The model shows a varying "12MonthDemandPeak" charge, not a constant amount. Can you point me toward a better approach? Thanks in advance.
The Source Variable under UtilityCost:Charge:Simple is what EnergyPlus multiplies by the cost to give you the cost for that block of time. Right now your example is saying "Multiply the peakDemand for that month by $4.1 oh and also calculate the ratchet but don't do anything with it" which is why you're getting a varied rate. You want to multiply 4.10 by the Ratchet'd demand so you need to change the source variable to 12MonthDemandPeak (Honestly I'd change the name of the ratchet to something different for less confusion). Here is a quick example:
UtilityCost:Ratchet,
Ratchet1, !- Name
UtilityTarrif, !- Tariff Name
totalDemand, !- Baseline Source Variable
totalDemand, !- Adjustment Source Variable
Annual, !- Season From
Annual, !- Season To
1; !- Multiplier Value or Variable Name
UtilityCost:Charge:Simple,
Demand, !- Utility Cost Charge Simple Name
UtilityTarrif, !- Tariff Name
Ratchet1, !- Source Variable
Annual, !- Season
DemandCharges, !- Category Variable Name
4.1; !- Cost per Unit Value or Variable Name
To answer your second question. The utility costs are calculated after the simulation is completed. So if the peak demand is in July, the ratchet cost in January will use the July peak demand.
Comments
Since I've never used a ratchet before, it also strikes me as unclear how a one year simulation will represent it. If the peak demand occurs in July, what charge is shown for preceding months?
Helpful! The "source variable" wording doesn't quite sound like an object name.. After tinkering,and using:
UtilityCost:Charge:Simple, 12MonthDemandPeak,
Consumers Energy TOU, 12MonthDemandRatchet, Annual,
DemandCharges, 4.1;
UtilityCost:Ratchet, 12MonthDemandRatchet,
Consumers Energy TOU, PeakDemand, PeakDemand, Annual, Annual;
... E+ still produces a different demand charge for each month. I've shown "PeakDemand" above, but also tried "totalDemand" - same results.
Results: Corresponding Sources for Charges
Trying again... Corresponding Sources for Charges
Your UtilityCost:Ratchet is missing a multiplier of 1. It should be:
That works perfectly: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Sum Max Category 12MONTHDEMANDPEAK ($) 277.41 277.41 277.41 277.41 277.41 277.41 277.41 277.41 277.41 277.41 277.41 277.41 3328.97 277.41