Photo Electric Control Unit Arrays


A photo-electric control unit (PECU) Array is one of the ways which we accurately calculate energy consumption.  The PECU Array is populated with 30 photocells which are representative of the photocells used within the lighting authority. As the PECU Array is located within the lighting authority, it provides the most accurate daily switching times for the photocells, ensuring that the half hourly consumption data is accurate.  The data from the Array is downloaded every night.   Power Data Associates has found that annual energy consumption is typically 1% lower using the switching times provided by the PECU Array compared with a passive calculation.  As a result, many customers save thousands of pounds each year.


A PECU Array is a box about 1 x 0.6 x 0.3m fitted with 30 photocell NEMA sockets, within the box are terminal blocks, a data logger and a cellular modem.  See photo above.  Options include a light meter and blanking plates to install non-NEMA cells.


The following should be considered in determining the location of the PECU Array:

  • Typical location is the flat roof of a local authority building within the lighting authority area, near the centre of the street light load
  • Location should be accessible for safe / easy replacement of photocells
  • Location should not be subject to excessive shadows from trees, buildings, lift shafts, air conditioning units, etc
  • Artificial lighting (e.g. floodlights) should not be allowed to interfere with the Array
  • The location on the roof should be sufficiently elevated to ensure that Array is not flooded if the roof ‘puddles’ due to blocked gutters, drainage, etc
  • Elevation should be consistent with normal photocell locations - the top of a two storey office building is ideal, a 20 storey tower block is not appropriate!
  • Array requires a small electricity supply (13amp socket)
  • From 2022, communication is normally provided by a cellular modem using IP communication
  • Prior to 2022, communication was provided using a cellular modem or via a fixed analogue (like a fax machine) phone line with a ‘direct dial in’ phone number
  • Where communications is via a cellular modem then a good signal strength is required for the chosen service provider
  • The Meter Administrator will need to formally agree the chosen location with the local distribution company (UMSO).


  • The Array is populated with 30 photocells which represent the whole lighting inventory
  • If more than one type of photocell is in use, then the Array is populated with a representative proportion of each
  • This proportion is reviewed at least annually to reflect changes to your inventory
  • The photocells should be representative of the physical construction: NEMA, one-part miniature and two-part miniature, etc
  • The photocells should be representative of the age of the photocell population, a brand new Array should be populated with photocells recovered from a representative sample of lighting columns
  • Power Data Associates interrogate each PECU Array and download the events recorded by the data logger each night
  • Power Data Associates then validate and check the event data for each array to check for faulty photocells and other anomalies

Please contact us with any questions or to obtain prices. Contact us.