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7.2 SYSTEM AVAILABILITY
System availability (A) is defined as the percentage of time that a power system is
capable of meeting the load requirements (Ball & Risser, 1988). For instance, a
system designed for 95% availability is expected to meet the requirements of the load
95% of the time. In stand-alone PV-based systems, availability depends primarily on
battery size. Typically, non-critical stand-alone systems are designed with an
availability of about 95%, whereas critical systems are likely to require 99%
availability.

Telecommunication repeater stations, for example, would be considered critical, a PV
cathodic protection unit may be non-critical, while low availability is acceptable for a
grid-connected system.

In a PV system, weather, failures, system maintenance and excessive demands are the
primary contributors to lowering system availability. However, system costs increase
rapidly in trying to obtain the last few percent of availability. This is illustrated in
Fig. 7.1.

Figure 7.1. Availability as a function of cost for a PV-based power system in the
north-eastern United States (reprinted with permission of Sandia National
Laboratories, 1991).

In designing a system, the requirements of each specific application, solar variability
at the site and financial limitations will determine the appropriate availability figure
to apply. For normal power systems, such as for households, the general approach is
to design a system with non-critical availability, increasing the system components
later if necessary and as more finance becomes available.
As a comparison, very few large-scale power source generators, be they coal fired,
nuclear or hydroelectric, achieve availabilities greater than 80–90%. Importantly, for
photovoltaic systems with availabilities below 80%, there is in general no surplus
capacity, as less electricity is generated even on a cloudless summer day than is

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