Loss of Power Supply Probability (LPSP)

In plain terms

Loss of power supply probability comes from off-grid power system design. It answers one question: over a year, what share of the demand did the system fail to deliver? It is a standard reliability constraint when sizing hybrid solar-wind systems against cost (Yang et al., 2007).

LPSP = Σ unmet demand / Σ total demand   (over the period)

An LPSP of zero means every hour of demand was met. Higher values mean more shortfall.

Applied to a hydrogen plant, the "demand" can be the electrolyzer's power need, or the hydrogen offtake itself. The second is the one a customer cares about: how many kilograms the plant failed to deliver when the offtaker wanted them.

Some studies count unmet hours instead of unmet energy. The two can differ widely, so state which one is used.

Typical range

The acceptable LPSP is a contract and design choice, set by the offtaker's tolerance for shortfall. Sizing studies run the design against an LPSP target and compare the cost of meeting it (Yang et al., 2007).

Why it moves LCOH

Each step toward zero LPSP costs more than the last. Covering the final rare shortfalls, such as a windless week, needs much larger storage, more renewable capacity or a backup supply, which is used only a few hours a year. Designers therefore choose an LPSP target that balances storage and oversizing cost against the penalty or value of lost offtake.

Common mistake