Shining a light on solar panels

Published: 01 September, 2015
Shining a light on solar panels

As a self-confessed environmentalist the chance to install solar panels (or Photovoltaic Panels – PVs) on the roof of our office was an opportunity too good to miss - but where do you start?

I approached this project from a standing start and thought it might be useful to share some basic information I have picked up along the way.

The most useful (and obvious) advice is that solar panels particularly favour commercial property such as office buildings as they are occupied during the day WHEN THE SUN IS OUT!!

In order to work out whether investment in solar panels stacks up you need to understand how this affects your bills and what the available subsidies actually mean in practice.  This breaks down into three areas of finance to consider.

  1. Savings on your existing energy bills – the electricity your solar panels generate is fed directly into your building to power your equipment this means that you don’t have to draw as much electricity from the grid. This directly offsets the cost of this electricity.
  2. Generation tariff – Once your system has been registered, your energy supplier will pay you a set rate for each unit (kWh) of electricity you generate. This payment is for up to 20 years and is index linked.  The rate of this payment tariff is linked to two things these are: -
    1. The EPC (Energy Performance Certificate) rating of your property where higher rated properties receive a greater amount per kWh.
    2. The size of the installation more detailed information from the Energy Saving Trust here. (
  3. Export tariff – you will get a further payment for every kWh (4.85p/kWh at the time of writing) generated by the panels. In practice this is difficult and expensive for the electricity providers to administer, so a growing number have declared that they will pay an amount equivalent to half the yearly generation being sent back to the grid (only systems above 30kWp need to have an export meter fitted).

PV installation companies will, if asked, be able to make an assessment of your property and depending on the direction the roof faces, the inclination (angle) of the roof, where you are in the country and the size of the roof (and consequently the estimated size of the array) should be able to give you an anticipated generating yield and provide financial calculations for you as to the estimated “payback” time of the investment.

Clearly installation companies are in the business of selling the gear so it is in their interest to make the prospect as appealing as possible so my advice would be to run the figures yourself with some sensible assumptions.

Don’t forget to make an allowance for the output of the panels to degrade; manufactures expect that their panels last at least 20 years, and that the efficiency decreases by no more than 1% per year.

All of our calculations mean that we are able to accommodate a 30kW array on a (roughly) south facing roof and that we estimate that the payback, given all of the above, will be in the region of 6 years.  I intend to monitor the output and will update regularly on whether our estimates were accurate.

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Written by: Neil Cartwright

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