State of health

The current condition of an asset relative to its as-new condition.

Abbreviated SoH. For a battery it usually means present usable capacity as a fraction of rated capacity. For an electrolyser stack it is expressed through cell voltage at reference conditions, and through crossover behaviour.

It is not directly measurable in either case. It is estimated from measurements that respond to it, which means an SoH figure is only as good as the model behind it and should always be quoted with the conditions it was normalised to.

The aggregate figure also hides the distribution. A battery pack reports state of charge but rarely cell-level health, exactly as a stack reports total voltage but not individual cell voltage, and in both the population spread matters more for decisions than the mean.