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.
Before beginning any calculations, it’s worth a brief recap of how solar lighting works.
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.
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:
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
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.
Units 4 & 5