digkam (asker) Oct 3, 2012:

The Misha, thank you for the suggestion. The problem is, firstly, that I frequently need the verb for резервировать - along the lines of requesting/ ensuring the availability of/ paying for / backing up - thank you, Jim - reserve capacity (service) from neighboring grids (while the problem of having no reserve capacity on hand within the system is also described). Secondly, peaking plants (peakers) are a subclass, if you will - capacities that take care of peak loads (as opposed to base-load cases, for instance). Here, should this one plant shut down unexpectedly, its entire capacity is lost to the system. That said, I am indeed looking for synonyms to "reserve capacity" - in the very broadest sense.

The Misha Oct 3, 2012:

This is totally out of my area of expertise but we in the States routinely refer to those "reserve" power sources as peak load generators or plants. Maybe you could use that.