Definitions of the main units, terms, standards and measures we use in this website

Calculation of performance ratio from the book

Glossary of units and terms

Station capacity and power ratings

We have made specific recommendations on how the first two measures should be used consistently in the rating of utility-scale solar power plant as detailed here.

Megawatt peak


Output of a solar array, which when operating at its peak power point under Standard Test Conditions (see below), produces one million watts direct current

Megawatt AC

MWAC or MW [read]

One million watts of alternating current. When the array is operating at STC, its MWAC is the MWP x the performance ratio.

Performance ratio


Ratio of actual output of a solar generating system to the rated peak output of its solar array under prevailing solar radiation conditions [ref chapter 14 of the book]

Output figures and energy ratings

Megawatt hours


Energy produced by a power plant delivering one MW for one hour. Almost always rated in terms of AC for utility-scale systems

Megawatt hours per annum


Number of megawatt hours delivered by a generating station over the course of a year (a good comparative measure for solar systems, because seasonal output can vary substantially, but year-to-year variations are relatively small)

Load factor or Capacity utilisation


Ratio of the electrical energy produced by a generating unit relative to the energy that could have been produced at continuous full-power operation during the same period (typically 12% to 28% for PV plant, depending on location)

Solar duty



Number of MWh per annum for each MWP of rated capacity (gives the output and revenue potential of a project). Effectively = load factor ÷ performance ratio.


% or h/y

Percentage of the time that electricity generating plant is available to produce electricity (free from downtime). Not to be confused with capacity factor

Other glossary items

There is a comprehensive and excellent glossary in the book.

Standard Test Conditions


For solar module testing, defined in international standards; insolation intensity of 1kWm-2 normal to the plane of the solar module at ambient temperature of 25°C and light spectrum equivalent to AM1.5


See discussion here

Further details

Some terms need further explanation, and we are working on a series of papers, including:

Utility-scale; what is the cut-off? [here]

Capacity rating; MW, MWP and MWAC [here]

Output rating; MWh/yr; MWh/MWp [follows]

Solar array terminology [follows]

Glossary of acronyms


Alternating current


Clean Development Mechanism (of the UNFCCC)


Concentrated solar power (Wiki-Solar does not cover CSP projects, except as detailed here)


Direct current


Feed-in Tariffs; a support mechanism, where producers are paid at a pre-set level for electricity fed into the grid


The Indian government's Jawaharlal Nehru National Solar Mission

MW, MWh, MWp

See definitions on the left


Power purchase agreement


Photovoltaic (direct conversion of light to electricity)


Renewable portfolio standards; a renewable energy support mechanism used mainly in certain states of the USA


United Nations Framework Convention on Climate Change the administrative body for the Kyoto Protocol