What Firm Power Means

Firm power is generation you can dispatch on demand; it is a different service from clean energy, and a grid needs both.

“Firm” and “clean” are two independent properties of a generating source. Clean describes emissions: how much carbon and pollution accompany each unit of energy. Firm describes availability: whether the source can deliver power when the operator calls for it. A source can be clean but not firm (solar at night), firm but not clean (a coal unit), or — the goal — both.

Firm capacity versus energy

Planners distinguish energy (total kilowatt-hours over time) from capacity (power available at a specific moment, especially at peak demand). Variable renewables supply large amounts of energy but contribute limited firm capacity, because their output at the moment of system stress is uncertain. Firm resources are credited with capacity close to their nameplate because they can be scheduled.

Why firmness is hard to fake

Short-duration batteries firm renewables over hours, which handles the daily solar cycle well. They do not, by themselves, cover multi-day weather systems or seasonal swings, because the stored energy runs out. Providing firmness across those longer windows requires either very large energy stores or a source that can simply keep generating. Fusion is designed for the latter role.

• Clean ≠ firm: they are separate requirements a reliable low-carbon grid must both meet.
• Capacity credit measures firmness; energy share measures contribution to total supply.
• Storage firms renewables over hours to a day; long gaps need firm generation.
• Fusion targets firm clean output, but its availability is still an open engineering question.

Design-and-simulation framing. The Kronos machines are today design and simulation studies: the breeder (Hyperion) and the burner (Aegis / MetroVolt). No hardware net-gain has been demonstrated. Breeder construction is planned to begin Q2 2027, with first-of-a-kind (FOAK) first tritium targeted around 2030. Comparisons on this page are qualitative and use only public, defensible figures; nothing here is a performance guarantee.

The value of a firm clean source is highest exactly where renewables are strongest, because it removes the last, hardest-to-decarbonize slice of the system — the hours when the weather does not cooperate.