6.3 Determining Non-Contingency 73 over-abundance of power generation, but is generally managed separately from increases or decreases in energy production or consumption. Generators usually provide the reserves needed to balance power systems and maintain reliability criteria. However, the same services could equally be performed (and potentially at lower cost) by energy-consuming equipment--i.e. loads. Loads such as electric heaters and lights may have a broad range of energy inputs at which they may still perform the intended functions acceptably. Expanding the use of energy-consuming equipment to provide reserve services is explored more fully in Chapter 12. In the absence of wind on a power system, the primary need for holding reserves arises from the variability and unpredictability of load, and for sudden power system equipment failures (contingencies). It is possible for reliability criteria such as frequency limits to be maintained even in the event of relatively extreme imbalances between a particular system's demand and generation of electric power. This can occur at the expense of service to consuming equipment (i.e. load is different from demand), energy stored in the power system itself (e.g. kinetic energy of rotating equipment) and neighboring systems that may be unexpectedly absorbing or providing generation. To minimize one system from inad-