# Calculating selected years

You can run a scenario calculation in NEMO for certain years only by using the calcyears argument for calculatescenario. You can also set this argument in a NEMO configuration file. When you specify a value for calcyears, NEMO restricts the scenario's optimization problem to the years you select (provided they are defined in the scenario's database). The problem's objective is to minimize discounted costs in those years. If you don't supply a value for calcyears, all years in the scenario database are included in the calculation.

Calculating selected years offers a way quickly to generate results from large, complex models. While the results may not be identical to what you would get if you calculated all years, NEMO takes several steps to ensure they are a reasonable approximation. Specifically, when calcyears is invoked, NEMO:

• Calculates discounted investment costs for technologies and storage by distributing the costs over modeled (selected) and non-modeled years. Each modeled year is taken as the endpoint of an interval that starts after the prior modeled year (or at the beginning of the first year in the scenario database [the first scenario year], if there is no prior modeled year). For discounting purposes, investment costs incurred in a modeled year are assumed to be spread equally over the year's interval. NEMO's output variables for discounted technology and storage investment costs (vdiscountedcapitalinvestment and vdiscountedcapitalinvestmentstorage) reflect this adjustment.

• Calculates discounted transmission investment costs in the same way as discounted technology/storage investment costs if the continuoustransmission argument for calculatescenario is true. In this case, the output variable vdiscountedcapitalinvestmenttransmission includes the adjustment.

• Estimates discounted operation and maintenance costs for technologies in non-modeled years. Fixed costs are estimated by assuming that capacity evolves in a linear fashion between modeled years (or, if a modeled year's interval begins with the first scenario year, that there is a linear change from the residual capacity in that year to the capacity in the modeled year). Variable costs are computed by assuming activity changes in a linear fashion between modeled years (or, if a modeled year's interval begins with the first scenario year, that the activity in the modeled year recurs in other years in the interval). The output variable vdiscountedoperatingcost includes these adjustments.

• Similarly estimates discounted transmission operation and maintenance costs in non-modeled years. For exogenously specified lines (where yconstruction is defined), fixed costs in non-modeled years depend on whether the line exists in those years. For candidate lines NEMO may build endogenously (yconstruction is not provided), fixed costs are calculated by assuming that the fraction built of each line evolves linearly between modeled years (the fraction built at the start of the first scenario year is 0). This assumption for candidate lines only applies when continuoustransmission is true. Variable costs in non-modeled years are estimated by assuming transmission activity changes in a linear fashion between modeled years (or, if a modeled year's interval begins with the first scenario year, that the activity in the modeled year also holds in other years in the interval). These adjustments are included in the output variable vdiscountedoperatingcosttransmission.

• Estimates discounted technology emission penalties in non-modeled years. The penalties are computed by assuming that penalty amounts evolve linearly between modeled years (or, if a modeled year's interval begins with the first scenario year, that the penalty in the modeled year recurs in other years in the interval). The output variable vdiscountedtechnologyemissionspenalty includes this adjustment.

All of these adjustments are also incorporated in NEMO's output variables that sum discounted costs: vtotaldiscountedcostbytechnology, vtotaldiscountedstoragecost, vtotaldiscountedtransmissioncostbyregion, and vtotaldiscountedcost. Note that salvage values for storage, transmission, and technologies are still based on asset value remaining in the last year in the scenario database (the last scenario year), so using calcyears does not alter how they are calculated. Whether or not calcyears is invoked, NEMO discounts all costs to the first scenario year.

NEMO also makes a few other changes when you turn on calcyears:

Tip

If you calculate selected years, capacity additions in a modeled year are best construed as additions during the interval corresponding to the year. As noted above, each modeled year's interval begins with the year following the prior modeled year (or the first scenario year, if there is no prior modeled year) and ends with the modeled year. For example, if the first scenario year is 2020 and you calculate 2020, 2030, and 2040, the intervals are as follows: 2020, 2021-2030, and 2031-2040.