What the model does

NemSim answers one question, repeatedly and cheaply:

Given this set of regions, with this demand, these generation assets, these interconnectors and these economics, what happens when you dispatch it hour by hour for a year, how much storage does it take to hold the reliability standard, and what does the whole thing cost per MWh served?

Everything else on this site is either how to ask that question, or how to read the answer.

The pipeline

scenario config (JSON)
        │
        │  ScenarioDerivation.Derive  ── + demand series, + weather traces
        ▼
   PowerSystem            regions, fleets, storage, directed interconnectors
        │
        │  StorageSizingService.Size  ── outer loop, only when the target is missed
        │      └── Dispatcher.DispatchSystem  ── re-dispatched once per candidate
        ▼
   dispatch evidence      per-region and whole-system series, hour by hour
        │
        ├── SystemReliabilityAssessment   did it hold the standard?
        └── PowerSystemCostCalculator     what does it cost per MWh served?
                │
                ▼
        published artifacts (JSON)

Four stages, each with a clean boundary:

Intent. A Scenario is what you asked for. It owns the period, one fleet plan per region, the cost basis and any interconnectors. It knows nothing about files or data.

Realisation. ScenarioDerivation combines that intent with parsed demand and weather to produce a PowerSystem, the actual grid to be dispatched. This is a pure transformation; after it, nothing downstream is scenario-aware.

Dispatch. The Dispatcher walks the year an hour at a time. Within each hour it dispatches generation in merit order, moves surplus between regions over the interconnectors, then operates storage. What demand it cannot meet becomes unserved energy; what generation it cannot use becomes curtailment. See Dispatch.

Assessment. Reliability is measured as unserved energy against demand energy. If a region misses its target, StorageSizingService grows Battery capacity and re-dispatches the entire system, until either every region complies or the search reports what stopped it. See Storage sizing. The final system is then priced, which Economics covers.

The two properties that matter

It is deterministic. No random draws, anywhere. The same inputs at the same commit produce identical results, value for value. There are no confidence intervals because there is no distribution, only the single realisation you asked for.

That sounds like a weakness and is mostly a strength. It means a difference between two runs is attributable: if you changed one input and a number moved, that input moved it. It is what makes a sweep of dozens of runs varying one axis a sensible unit of work rather than a statistical exercise.

It is inspectable. Every assumption is either a value you supplied in the scenario or a constant documented in the assumptions register, and the register is checked against the code by a test. Every published result records the SHA-256 of the exact input bytes it was built from. Nothing about a result requires you to take our word for it.

The vocabulary

A small set of terms carries most of the meaning. They are used precisely throughout.

Term	Means
Region	One NEM region, modelled as a single node. NSW1, QLD1, SA1, TAS1, VIC1.
Fleet	All capacity of one technology in one region, treated as a single unit.
Merit order	Dispatch order: ascending short-run marginal cost, tie-broken by technology.
Residual	Demand less generation, within an interval. Positive is a deficit; negative is a surplus.
Unserved energy (USE)	Demand that could not be met. The binding reliability measure, as a percentage of demand energy.
Curtailment	Available generation that could not be used and was not stored.
Delivered to load	Demand minus unserved. The denominator of every levelised cost.
Sizing	Growing Battery capacity until the reliability target is met, or reporting that it cannot be.
Sweep	A series of runs varying one input, with everything else held constant.

What it deliberately does not do

NemSim is a system-planning model. It is not a market simulator, not a power-flow model, and not a forecast.

No bidding, no prices, no settlement. No unit commitment, so no minimum stable generation, no start-up costs and no ramp rates. No forced outages. No intra-regional network. No frequency control or system strength. No demand response.

Those absences are what make it small enough to run a 25-point sweep over five regions and 8,760 hours in a reasonable time, which is the trade it exists to make. They are also why Limitations is required reading before you quote anything from it.

Where to go next