Calculating Solar Viability

Recent advances in solar lighting technology offer a number of benefits for lighting engineers, contractors and installation teams. 

As solar street lighting requires no mains power,  it can be quickly deployed without the need for time consuming, disruptive and costly trenching, groundworks and mains connection.

Solar lighting does however present additional challenges when deciding on specification. It’s important that all lighting performs as required, however it can be unclear from a manufacturers specification whether or not this will be the case.

Add in additional variables such as the difference in solar irradiation at different points in the UK and a solution that initially promised to solve problems has instead created additional ones.

Fortunately, there is a fast and straight forward method for determining the performance of a solar lighting product. We’ve laid out the maths and workings below, alongside easy to use online based calculators.

Solar Lighting - A Brief Recap

Before beginning any calculations, it’s worth a brief recap of how solar lighting works.

  1. Light energy from the sun is converted into electrical energy through a process known as the photovoltaic effect.
  2. The electricity generated is stored in the units battery
  3. At the required time, the luminaire powers on and the battery begins to be discharged.

Whilst the units batteries will allow light to be emitted for a number of days with insufficient power, it’s crucial to remember that even with a large capacity battery, a product that consistently generates less power then is required will eventually suffer from outages.

Calculating Solar Viability

Here are  the measurements needed to calculate the viability of a solar product.

Watt Hours – Output of a luminaire in Watts x number of hours light required. E.g – a 20 Watt luminaire alight for 10 hours per night gives a figure of 200 Watt Hours

Panel Size – The output of a solar panel, measured in Watts

Solar Irradiance  – Solar irradiance is a measure of the amount of energy hitting the earth from the sun at a given point measured in kWh/m2/day. The daily irradiance data for your location can be found by:

    1. Visit
    2. Select “United Kingdom” and then the town or city nearest to your location
    3. Select any panel direction
    4. Choose “flat surface” –
    5. Select the lowest number, this is usually January or December. We recommend always working from a worst case scenario

Alternatively – feel free to use the January irradiance data for your nearest city listed below. Cities are listed from North to South

Aberdeen – 0.32 kWh/m2/day | Edinburgh – 0.38 kWh/m2/day |

Newcastle – 0.44 kWh/m2/day | Manchester – 0.5 kWh/m2/day |

Nottingham – 0.51 kWh/m2/day | Cambridge – 0.57 kWh/m2/day |

Cardiff – 0.59 kWh/m2/day | London – 0.6 kWh/m2/day |

Bournemouth – 0.67 kWh/m2/day | Plymouth – 0.65 kWh/m2/day

Calculating a Product's Output

If the panel size of a product is known, it’s possible to estimate the light levels in Watts that you will achieve in a certain location. The formula to use is.

Watt Hours Generated = Solar Irradiance x Panel Wattage x 0.9*

Watt Hours Generated can then be divided by the number of hours of illumination to determine the products potential sustainable output in Watts.

* The multiplier of 0.9 is included to account for any potential system losses and to err on the side of caution.


Solar CPD Training

This online CPD course is designed for lighting engineers, contractors, designers and architects. You will leave the course with a firm grasp on the different components of a solar street light, how they work and how best to make use of solar technology.

Solar Install Savings Calculator

This tool can be used to compare installation costs of  wired and solar lighting schemes and to calculate the installation cost savings available by using solar technology.

Solar Yearly Savings Calculator

This tool makes the business case for solar lighting by estimating the yearly and lifetime cost and emissions savings permitted by choosing solar lighting units instead of mains wired units.