STOR is one of National Grid's key reserve purchases

Fast-acting generators are held in readiness so that National Grid's control centre can start them quickly if a large power station fails, or if demand is higher than forecast.

Short Term Operating Reserve (STOR) is one of National Grid's most important tools for securing the national electricity system in real time.  Under the STOR arrangements, National Grid pays a ‘rent’ (termed availability) for STOR capacity, and pays a usage charge (utilisation) when the reserve is needed, such as during demand peaks, or when large power stations fail.

STOR is a year-round service, potentially available to National Grid on a 24-hour basis. For most of Flexitricity’s client businesses relatively low utilisation hours are targeted using the STOR pricing strategy, so that the bulk of the clients’ revenue is normally earned from availability payments. 

Availability is paid during key ‘windows’ set by National Grid. These windows vary seasonally, but currently fall within the period 07:00 to 22:30, and amount to roughly 11 hours per day. Most STOR utilisation occurs within these windows.

The STOR market has been growing in volume and value for several years, due to the reduction of flexible coal and oil electricity generation capacity, and the increase in forms of generation which require balancing, such as renewables.

Under typical terms (which can be tailored) Flexitricity provides National Grid with run times of up to two hours per call, plus response time.  Calls typically last around one hour and amount to approximately 50–60 hours of running per year. 

For standby diesel generators or gas-fired CHP generators, STOR is the largest incremental revenue opportunity, with the lowest relative impact on generator run hours, and the lowest exposure to fuel price risk, of any premium energy activity.