Viable logo

Marginal abatement cost (MAC)

A MAC curve shows the cost of each internal action per tonne of carbon dioxide equivalent (tCO₂e). The combined emissions pathway does not change when you change the MAC method. The method only changes how much effect the curve gives to each action.

What the MAC curve shows

The chart uses lifetime totals. The x-axis is lifetime abatement. The y-axis is lifetime MAC in dollars per tCO₂e.

The chart does not use a 2030, 2040, or 2050 year selector. One organisation setting applies to the MAC curve, the action table, and cash-flow assessment.

If only one internal action is in the plan, all methods give the same MAC.

MAC curve axes A chart with lifetime abatement on the x-axis and MAC in dollars per tonne on the y-axis. Two action bubbles sit on the plot. Lifetime abatement (tCO₂e) MAC ($/tCO₂e) LED Sensors
Lifetime abatement versus lifetime MAC. Example bubbles, not your data.

Three allocation methods

One action can decrease the effect of another action on the same activity. The method sets the order of that attribution.

  • Lowest-MAC first (default). Each action is first scored on its own with equivalent annual cost (EAC). Actions then stack from cheapest to dearest. Ranking always uses EAC.
  • Chronological. Actions stack in start-year order. An earlier action keeps the effect it would have on its own. A later action takes the residual. Attributed effects add to the combined plan.
  • Leave-one-out. Each action is the full plan minus the same plan without that action. Other actions stay in place, including later actions. Attributed bars do not always add to the combined plan.

REC, LGC, and PPA purchases are not in the seed ranking. Certificate repurchase is always last. See REC, LGC, and PPA.

Example: LED then sensors

Both actions reduce the same electricity activity. LED reduces load by 30% from 2027. Sensors reduce remaining load by 10% from 2029. Select a method. The combined plan stays at 37 units.

Example, not a calculation from your data

LED attributed
—
Sensors attributed
—
Combined plan
—
Sum of bars
—

Which method to use

  1. Use lowest-MAC first to rank overlapping actions by cost, not by calendar order.
  2. Use chronological to explain a transition plan in time order.
  3. Use leave-one-out to test if an action is still worth doing given the rest of the plan.

Lowest-MAC first requires EAC. Standard net present value (NPV) is not available with that allocation. Read NPV and EAC.